Forced Induction Systems

Forced Induction Systems
By Ian Williams
As the air travels around the scroll, the diameter of
the scroll increases. This slows the velocity of the air,
but further increases its pressure.
Both superchargers and turbochargers are forced
induction systems and thus have the same objective
- to compress air and force more air molecules into
the engine's combustion chambers than would
normally be allowed by atmospheric pressure. The
benefit of forcing more air molecules into the
combustion chambers is that it allows your engine to
burn more fuel per power stroke. With an internal
combustion engine, burning more fuel means that
you convert more fuel into energy and power. For this
reason, supercharged and turbocharged engines
normally produce from 40% to over 100% more
power than normally aspirated engines, and in offboost driving conditions can even improve fuel
economy. Forced induction is the most effective way
to obtain a significant power increase whilst
maintaining a vehicle’s “streetability”.
A. Turbo housing exhaust inlet from motor
B. Internal Wastegate
C. Air inlet to compressor housing
containing impeller blades
D. Compressed air outlet from scroll to inlet
manifold
E. Turbo housing containing exhaust turbine
F. Turbo exhaust outlet to exhaust system
There are three main types of forced induction
system for automotive use:
1. Turbochargers
2. Centrifugal superchargers
3. Positive displacement superchargers
First, let’s look at basically how each of these
technologies works:
Turbochargers
Turbochargers are basically an exhaust powered
centrifugal supercharger in which the compressor
wheel essentially operates like a high speed fan,
sucking air into the centre of the compressor housing
and pushing it to the outside of the rapidly spinning
impeller blades (up to 150,000 rpm).The air naturally
travels to the outside of the blades because of its
centrifugal force created by its rotating inertia. At the
outside of the blades, a "scroll" is waiting to catch the
air molecules. Just before entering the scroll, the air
molecules are forced to travel through a venturi,
which creates the internal compression.
Turbochargers are not 'free' horsepower but they are
way more efficient and cause much less of a parasitic
loss than the belt-driven alternatives. As the turbo
puts more air into the engine, the engine produces
more exhaust, which spins the turbo faster to put
more air into the engine. This process is defined as
the 'spooling up' of the turbo. You can think of the
snowball effect logically in your mind as the above
process continues to build more and more boost but
it actually all happens in a fraction of a second.
Turbochargers can fit on any engine type1 given the
use of the appropriate exhaust manifold. However,
the turbo must be sized properly according to the
application. Smaller turbos mean faster boost
responses, but limited amounts of boost. Larger
sized turbos take more time to build up boost, but
they give a lot of boost. Turbos feed on fresh engine
oil constantly, so oils with high ratings must be used
in the vehicle.
Turbos typically do not produce boost under 2,000
rpm but above that they can spool up and produce
boost. This allows the compressor to go from zero
boost to full boost in a fraction of a second and
sometimes within 100 rpm, depending on what gear
you are in. Due to the efficiency of the system, turbos
typically get very high horsepower per pound of
boost, typically 25 - 35 hp per 1 psi.
1
Standard motors are suitable for this application, but for
reliability, should be in good condition. Decompressed
blocks allow for a higher level of boost, but need to be built
with extreme power levels in mind.
Wastegates
TIAL 38mm External Wastegates
Without a wastegate, the amount of boost that a
turbocharger creates varies with the pressure of the
engine's exhaust. This happens because exhaust
pressure varies with relation to the engine's speed.
This implies that as an engine reaches higher RPM's,
increasing amounts of boost will be created by the
turbocharger. The problem with this is that an engine
can only accommodate a given amount of boost.
Most stock engines can only take about 10 psi if not
less. In order to regulate the amount of boost that
comes into the engine, a wastegate acts as a “gate”
only allowing a given amount of exhaust to hit the
turbocharger's exhaust turbine. Once the engine
starts producing more exhaust pressure, the
wastegate system will open a flap to redirect excess
exhaust away from the turbine blades. This is where
a wastegate gets its name. It's a gate to carry away
waste exhaust. A boost controller can be used to
regulate the level of boost required before a
wastegate opens. Pictured below are examples of
both manual and electronic boost controllers.
TIAL external wastegate showing fitment detail
(behind an APS turbo on a LS2 V8)
An external wastegate, unlike an internal
wastegate, is separate from the turbo unit and does
not require an actuator. With an external wastegate,
excess exhaust can either be fed into the exhaust
system as in the picture above, or it can be vented
straight out and into the atmosphere. Competition
set-ups typically follow the latter alternative. Most
stock systems come with an internal wastegate as
this set-up is better suited for low boost applications.
However most aftermarket systems perform better
with a separate external wastegate assembly which
is ideal for those generating boost up to 30 psi.
Blow Off Valves
APS twin vent Blow Off Valve showing flow paths
Turbosmart Boost Controllers
Manual (left) / Electronic (right)
A boost controller is capable of turning the boost and
horsepower up for performance needs, and then
winding it back for improved fuel economy and longer
engine/drive train life.
There are two types of wastegates. The first one is
an internal wastegate as pictured in the cutaway
diagram on the previous page. An internal wastegate
is a component on the turbo unit itself. The gate is
opened via an actuator which is a diaphragm type
system. Excess exhaust is then fed directly into the
exhaust system.
A blow off valve is a device that releases inlet charge
pressure from the turbocharger when the throttle is
released. It ensures that the pressure shock load on
the turbocharger compressor is minimized and
encourages each turbocharger rotating group to
continue spinning during gear changes. This ensures
that when the next gear is selected and the throttle
re-applied, the turbocharger can rapidly produce high
boost pressure and strong torque.
With a large or twin turbochargers, releasing
pressure from the turbine compressor wheel when
the throttle is snapped shut is critical not only for
lightning fast turbocharger response, but for
turbocharger longevity. A huge volume of air must be
passed through the blow off valve as soon as the
throttle is closed.
Most blow off valves have limitations in their design
that inhibit the level of performance. Piston/bore
diameter and venting ports are often too small and
surprisingly, most will not fully expose the outlet ports
when fully opened.
With the huge air flow capacity of turbochargers
utilised in a high output twin turbo system, APS
designed a high volume twin vent blow off valve
specifically for their intercooled twin turbo system.
This design delivers excellent venting performance
and driveability with crisp turbocharger response in
on/off throttle applications. Being a state of the art
design, we have used it in this article as the basis for
our discussion on blow off valve operation.
The large primary outlet port is designed to plumb
back into the inlet tract and ensures the best
driveability during cruise conditions. Under spirited
driving conditions with turbochargers running high
boost pressure levels, the twin vent blow off valve will
vent through to the second stage port in order to
expel the maximum amount of charge air
possible. Fine adjustment to blow off valve operation
can be performed through an actuation adjustment
screw.
The following diagrams demonstrate the operation of
a high volume twin vent Blow Off valve:
1. Ports Closed - Under open throttle conditions
where turbocharger boost pressure is required,
the bronze Blow Off Valve piston is held shut in
order to supply the maximum amount of
compressed charge air to the engine.
engine operation and alleviates the rich condition
and subsequent backfire when utilizing a Blow
Off Valve design that vents 100% of charge air to
the atmosphere.
3. Both Ports Open - Under high load conditions
where the throttle has been shut, the piston
travels fully so that both the primary and
secondary ports open to vent the maximum
charge air possible. Air vented through the
primary port is routed back into the inlet tract. In
addition, additional excess charge is air vented
through the secondary port to atmosphere.
The arrows above show excess boost air being vented
back into the inlet tract via the Primary Port and then into
the atmosphere via the Secondary Port in an APS twin vent
blow off valve.
Turbo Liquid Cooling
2. Primary Port Open - Under light load conditions
where the throttle opening has been reduced or
shut, the piston moves partially up the bore travel
and opens the primary port to vent charge air
back into the inlet tract. This ensures smooth
The first generation passenger car turbochargers
were derived directly from commercial diesel
engines. Engine oil was used to provide both
lubrication and cooling. Whilst this was an effective
compromise between cost, durability and
performance, in high engine performance
applications durability suffered through fouling of the
turbocharger bearings through high turbine and
bearing temperatures.
By encasing the turbocharger bearings in intricate
water passages, engine coolant can be used to
significantly reduce turbocharger bearing
temperatures in order to eliminate the coking and
lacquering issues that fouled old fashioned
turbocharger bearings. Non water cooled
turbochargers have no place in a high performance
engine application today and should be avoided.
the power adders, the turbocharger produces the
most power with the least abuse to the engine and
drive train and can typically put more power to the
tires with all other conditions being equal.
Comparisons between the various forced induction
systems are looked at in more detail later in this
article.
Twin Turbochargers
An alternative arrangement utilises two turbochargers
of the same size, known as a twin-turbochargers.
APS Intercooled Twin Turbo system on a Chev LS1
The above chart shows the turbocharger bearing
temperature leading up to engine shutdown and for
20 minutes following shutdown. The temperature is
displayed relative to the coking threshold of a high
quality mineral based oil.
As is clearly evident, the old fashioned non water
cooled turbocharger operates above the coking
threshold when under high load and experiences a
very high temperature increase through heat soak
immediately after engine shutdown. This is the main
reason that turbo timers were popular. They can
reduce the turbo’s temperature prior to shut down,
but do not eliminate this coking problem.
A liquid cooled turbocharger on the other hand
remains cooler than the coking threshold at all times
and the bearing temperature increase through heat
soak immediately after shutdown is reduced
drastically.
Turbo Performance
The torque curve of the turbocharger is what is really
impressive. Turbos typically will have a huge broad
torque curve that gives you massive power and full
boost in the 3,000 to 5,000 rpm range - which is
where you spend most of your time driving.
Turbochargers also typically get better fuel economy
than a stock vehicle so they are a great option with
no real downsides. The 'lag' in the system also brings
the power in just a little smoother than superchargers
do. This small cushion makes the turbocharger much
easier on drive train components and on street
applications typically keeps the tires hooked up
rather than causing them to break loose. Out of all
Twin-turbocharging can make more power than a
single turbo of the same output for two reasons. One
is the lower rotating mass of two smaller turbos
versus one large turbo, which allows the compressor
to spin up to speed (spool) much more quickly. The
second is the increased exhaust outlet area available
for the exhaust gas to flow out of the twin turbo
exhaust manifold. Increased exhaust flow will
increase power in most situations.
Turbochargers for automotive applications in
Australia include: Air Power Systems (APS), AXT,
Billet, CAPA, Garrett, LS1 Gen-TT, KKR, KPM and
Turbonetics.
Centrifugal Superchargers
The centrifugal supercharger is an engine-driven
compressor which is practically identical in operation
to a turbocharger, with the exception that instead of
exhaust gases driving the compressor via a turbine,
the compressor is driven from the crankshaft by a
belt, gear or chain drive.
Because the centrifugal design provides little boost at
low engine speeds it is often employed on nearstandard compression engines. This means that it
can facilitate airflow at higher engine rpm’s (when
most motors tend to have poor volumetric efficiency)
without substantially increasing cylinder pressures at
low to mid rpm operation, causing knock. This
principle makes this type of supercharger ideal for a
"bolt-on" type power adder, with no modification of
the pistons and/or compression ratio necessary.
increase. This causes the compressor to make more
boost as rpm's increase. The higher you rev the
engine, the more boost the compressor will make. So
you may not have any boost at 2,000 rpm, a couple
pounds by 3,000, 4 psi by 4,500 and then the full 6
psi (if that is your max boost) by 6,500 rpm. Then
when you shift the car and the rpm drops back down
to 4,500 the boost also drops back down to 4 psi.
This is what is called "Supercharger Shift Lag"! This
is why typically centrifugal superchargers produce big
peak numbers but don't perform so well in the 2,0004,000 rpm ranges.
Fuel economy is near-stock in the cruise rpm range.
This design is also popular with cars that have a
sufficiently large engine to provide adequate
acceleration from a standing start without boost,
while at the same time helping to avoid wheelspin.
On the other hand at higher engine speeds, this
centrifugal design is more efficient than positive
displacement designs like the Roots type
supercharger and the twin-screw type supercharger,
which in contrast have the advantage of producing
boost at any engine speed.
Denny Terzich’s 1967 Camaro – winner of Hot Rod
Magazine’s “America’s Fastest Street Car 2007”.
7.64 secs at 179 mph after driving it 2,100 kms in 4 days.
Reverse Mounted ProCharger F3 centrifugal supercharger
mounted on a 580 cu in big block Chev blowing into a
single 4 barrel carb. The previous year the car had a Roots
6-71 blower but could not make the drive distance required
due to overheating.
Centrifugal superchargers are also popular where
they may need to be removed at a later time because
the exhaust system is unaffected as it would be with
a turbocharger, and it is a relatively simple bolt on as
compared to a Roots type blower.
Centrifugal type superchargers for automotive
applications in Australia include: Paxton, Powerdyne,
Pro-Charger, Raptor and Vortech.
Centrifugal Supercharger on a VN Group A
However, detractors of the centrifugal-type
supercharger (at least in street-driven automobile
applications) note that it combines what some feel
are the worst qualities of a turbocharger and a
supercharger, since it doesn't develop appreciable
boost at low rpm (boost threshold), but still uses up
engine power to operate. Since it is crankshaft-driven
and cannot benefit from a device like a wastegate on
an exhaust-driven turbocharger to control its
rotational speed, its boost threshold is always within
a thousand or so rpm of redline. As such, the
horsepower rating of the engine can be greatly
increased, but in a small part of the upper rpm range.
By design, the centrifugal supercharger compressor
wheel is not a positive displacement compressor. So
this means that it gets more efficient as the rpms
Positive Displacement Superchargers
Superchargers that fall into this category compress
air by means of counter-rotating rotors driven by the
motor from the crankshaft. During each rotation, a
specific fixed amount of air is trapped and moved to
the outlet port where it is compressed, which is why
they are called fixed, or positive displacement
superchargers.
Positive-displacement superchargers may absorb as
much as a third of the total crankshaft power of the
engine, and are generally less efficient than
turbochargers. In applications for which engine
response and power are more important than any
other consideration, such as top-fuel dragsters,
positive-displacement superchargers are extremely
common.
There are three basic positive displacement
supercharger designs. They are the counter rotating
rotor Roots design such as the GMC blower, the twin
helical rotor design of the Eaton and the twin rotating
screw (Lysholm) design of the Whipple.
type of supercharger has very few moving parts and
spins at low rpms, making it one of the more reliable
and durable supercharger designs.
Depending on the type of Supercharger being
considered, the ease of fitting varies from being
almost impossible, to as easy as fitting an airconditioner pump.
Counter Rotating Rotor (Roots) Design
Superchargers
The Roots design supercharger is a positive
displacement pump which operates by pulling
air through a pair of meshing (outward and counter
rotating) lobes not unlike a set of stretched gears. Air
is trapped in pockets surrounding the lobes and
carried from the intake side to the outlet/discharge
port and into the motor.
Belt driven systems are capable of different boost
levels (within fairly small limits without changing
compressors) but do require a belt/pulley size
change to raise or lower the boost level. These
systems tend to have instant throttle response and
produce great bottom end power, moderate midrange
and lower high rpm performance mainly due to the
heat and parasitic loss. Great for burning the tires at
a stop light, but not the best for nice hard
acceleration on the freeway.
The Roots design supercharger is known for its
ability to produce large amounts of boost while
spinning at very low speeds. On an automotive
application, a Roots design supercharger can often
make its full (peak) boost by 2,000 engine rpm. This
characteristic has contributed to its success and
popularity on the top fuel racing circuit and has made
it ideal for use on smaller 4 and 6 cylinder engines
that traditionally struggle in the lower half of the rpm
range.
Another advantageous characteristic of the Roots
design supercharger is its simplicity of design. This
Roots Superchargers are common with drag racers.
The big disadvantage to the Roots design
supercharger is its thermal inefficiency. Its nature to
produce high discharge temperatures robs power
from the engine. With a Roots design supercharger,
an intercooler is almost always a necessity to bring
the air charge temperatures down to an acceptable
level. This poor thermal efficiency can be attributed to
the fact that it has no internal compression, as
compression is done after the air leaves the
discharge port.
VK Sedan (with VL front), 355 Holden V8, 671 Roots
Blower and 2 Demon 750 carbs. Boost = 9 psi.
Rear wheel power = 328 kilowatts or 440 horsepower
A Roots supercharger sweeps atmospheric air into
the manifold and is compressed in the manifold only.
With manifold pressure, additional heat is created by
compressed (hot) air that leaks backwards past the
rotors and heats up the temperature of the inlet
charge. The Roots design uses Teflon to try and seal
the rotors to cure this, but touching tolerances cause
more frictional heat and greater parasitic losses. This
problem is multiplied when boost levels rise or in
sustained on-boost situations.
Automotive and Marine Roots superchargers have
greater tolerances between the rotors and case so
they live longer, but this causes more leakage back
through the rotors.
Roots blowers tend to be 40–50% efficient at high
boost levels as compared with centrifugal
superchargers which are 70–85% efficient at similar
levels, but produce better power in the lower rev
range – before the centrifugal becomes efficient.
Like the centrifugal design, Roots are crank shaft
driven. They suck more power off the crank than a
centrifugal - so with a Roots blower you can expect
less hp per pound of boost and worse fuel economy.
The Eaton Supercharger
The Eaton supercharger system incorporates a
specially designed bypass valve, which is actuated
by a vacuum motor near the throttle body, and
recirculates the supercharger air flow when boost is
not required. During typical driving conditions, the
engine is under boost around 5% of the time, which
means the remaining 95% of the time the engine is
under vacuum, allowing for better fuel economy and
a quieter ride. The associated ducting and mounting
used in installing the supercharger can also play a
major role in reducing the noise emitted by the
supercharger.
Typically used in larger capacity engines, Eaton
design superchargers are offered as original
equipment on motors such as the Holden Ecotech V6
and also have a significant presence in the
automotive aftermarket.
Walkinshaw, Harrop, PWR and Bullet superchargers
are based on the Eaton supercharger design, and
most often use genuine Eaton rotor groups.
Twin Rotating Screw (Lysholm) Design
Superchargers
The Eaton supercharger is essentially a Roots blower
pump, with one substantial design wrinkle; each rotor
has been twisted 60 degrees to form a helix. The two
counter rotating rotors have three lobes, which
intermesh during operation. These outward rotating
twisted rotors, along with specially designed inlet and
outlet port geometry, help to reduce pressure
variations resulting in a smooth discharge of air and a
low level of noise during operation. This arrangement
also improves efficiency over traditional Roots
superchargers. With helical rotors and an axial inlet
the Eaton supercharger can be spun to up to 14,000
rpm, thereby reducing package size. The Eaton
design is much more efficient than the basic Roots
design and is a far better choice for the street.
Lysholm superchargers are similar to the modified
roots type, but the impellers are dissimilar. One is a
male rotor of three lobes and the other is a female
rotor of five lobes. The rotors in a Lysholm
supercharger inwardly rotate as compared to the
Roots or Eaton design superchargers which
outwardly rotate.
The Lyshlom superchargers are primarily used in
applications where the required boost level exceeds
13 PSI. The reason for this is that the supercharger
compresses the air to a high degree inside the
supercharger itself and it decompresses as it enters
the plenum.
The advantage to this is that the air in the plenum will
not back flow into the supercharger as the pressure
in the supercharger is always more than in the
plenum.
Whipple (Lysholm design) Supercharger
This diagram illustrates the axial air flow through an Eaton
supercharger. Inlet air is drawn in through one end of the
unit by the two helical rotors and discharged through the
base of the unit into the motor’s intake plenum after
passing through the intercooler/aftercooler core (if fitted)
positioned directly underneath the supercharger.
The Lysholm compressor has very tight tolerances
between the rotors. The rotors never touch,
eliminating big parasitic and frictional losses as well
as significantly reducing wear rates.
Lysholm superchargers like the Whipple can be
nearly as efficient as their centrifugal counterparts
over a particular range of load/speed/boost, for which
the system must be specifically designed.
Forced Induction Comparisons
1. Squires Forced Induction Comparison
Comparisons in December 2008 of a twin turbo
system and two different brands of high-output
superchargers by Squires Turbo Systems in Orem
Utah, USA on a LS1 V8 Corvette are shown to the
right. The boost level of 7 psi was chosen by Squires
solely due to the availability of published data for
each system at this boost level.
Unfortunately no information was available on the
superchargers used in this comparison.
At 7 psi of boost, the efficiency of the twin turbo
design is significant, and Squires say that at higher
boost levels, the price per horsepower gain becomes
even more favourable due to the turbo’s higher
horsepower gain per pound of boost.
2. "Battle of the Boost", Hot Rod Magazine
2002 LS1
CORVETTE
TWIN
TURBO
SUPERCHARGER
#1
SUPERCHARGER
#2
345
345
345
7
7
7
After
Flywheel
Horsepower
592
535
494
Horsepower
Gain (%)
72%
55%
43%
Horsepower
Gain (hp)
247
190
149
Torque Gain
(ft-lb)
255
165
123
Torque Gain
(%)
73%
47%
35%
HP Gain per
pound of
boost
(hp/psi)
35
27
21
Price (US$)
$7,995
$6,995
$6,995
$32
$37
$45
Stock
Flywheel
Horsepower
Boost
Pressure
(psi)
Price per HP
Gained ($/hp)
3. LS1 R & D Centre Comparison
HP
The differences between the three main forced
induction designs are apparent in Chart 1 (at the top
when you turn the next page) which appeared in the
"Battle of the Boost", Hot Rod Magazine August 2003
issue.
This Chart tells a better story, not just the one at
peak boost. Note just how much fatter the Turbo
power curve is to that of the centrifugal blower, and
the centrifugals relative lack of power at low rpm.
The Hot Rod Magazine article says that turbo
systems are the clear choice if you are looking to
generate usable horsepower between 2500 and 5000
rpm.
To quote the author, Richard Holdener "Given
equivalent vehicles, the turbo would easily motor
away from the centrifugal in an acceleration
contest......The turbo offered massive midrange
torque production, the only system to exceed 600
lb-ft. Need more convincing? At 4,000 rpm, the
turbo was more than 100 lb-ft. stronger than
either the Roots or centrifugal."
Turbo
8psi @
3300rpm
Centrifugal
PSI
8psi @
5600rpm
The LS1 Turbo Research and Development Centre in
Gillman, SA tested two identical cars featuring
standard LS1 engines with the same transmission
and final drive ratios. One was fitted with a Gen-TT
turbocharger and the other fitted with a Centrifugal
supercharger. All testing was conducted by an
independent workshop to ensure an unbiased result.
This demonstrates just how much quicker the turbo
builds boost, achieving 8psi at 3,300 rpm while it took
the centrifugal 5,600 rpm to reach the same 8psi.
“Battle of the Boost” Hot Rod Magazine
Chart 1.
4. HSV Dyno Comparisons, March 2009
Chart 2.
Ian Williams’ APS Twin Turbo VE HSV
Chart 3.
Chart 1 “Battle of the Boost” on the previous page
does not show the relative efficiency of an Eaton
design Roots supercharger. To do this we have
compared the dyno readings from two HSV Owners
Club vehicles in Chart 2 on that page.
The first vehicle is Ian Williams’ VE HSV with a
factory standard LS3 motor that was dynoed at
258kW at the rear wheels. An APS twin turbo system
with 1,000hp capacity was added together with
upgraded fuel and exhaust systems which increased
peak power by 81% to 467kW at 10psi of boost.
The second vehicle is Dave Blake’s VZ HSV with a
modified LS2 motor that was dynoed at 275kW
before an Eaton design Harrop HTV2300
supercharger was added. This supercharger also has
a 1,000hp capacity and the fuel and exhaust systems
have also received upgrades. The result was a 54%
increase in peak power to 424kW at 10.5psi of boost.
Dave Blake’s Harrop Eaton supercharged VZ HSV
However, just measuring peak power is not the full
story. Chart 2 shows the extremely strong bottom
end performance of the Eaton supercharged motor
with a 62% increase in power compared to the 47%
increase in power of the twin turbo motor. At 90kph
the supercharged motor had 55kW more power than
the twin turbo.
(Left) Underneath Ian Williams’ APS Twin Turbo VE HSV
When looking at a 130kph speed range between
60kph and 190kph, the supercharged motor had
superior power to the twin turbo motor for the first
35%, then the twin turbo exceeded the supercharged
motor’s power (by up to 62kW at 125kph) for the
remaining 65% of the speed range.
Chart 3 on the earlier page shows the difference in
power curves between the Eaton design
supercharger and a Centrifugal supercharger. In this
example both motors peak at an identical 424rwkW,
but how they get there is the interesting thing.
Again, the Eaton design supercharger shows it’s
superiority down low, with 47% or 91kW more power
than the centrifugal supercharger at 105kph.
Remember that these comparisons are about the
shape of each power curve, not the maximum power
attained. For example, in Chart 3, if the LS1 was
dynoed at the same 22 degrees atmospheric
temperature as the LS2, rather than the hot 35
degrees that it was, it may have recorded a higher
maximum power output – but the relative shape of
the power curve would have remained the same.
Chart 3 also shows the linear nature of the power
build up in a centrifugal supercharger which as we
have previously discussed, needs high rpms to
become efficient.
Chart 4 below shows the power curves of each of the
three systems under discussion compared. The very
strong power from mid range of the twin turbo system
is quite apparent here, especially in comparison to
the centrifugal supercharger.
These comparison charts are good illustrations of the
variation in power characteristics of the different
approaches to forced induction that we have been
discussing.
VY Clubsport Vortech V2 centrifugal supercharger.
However, this time the Eaton maintains a relative
power advantage all the way through the rev range
until both motors reach the same maximum power.
Chart 4.
Our Conclusion: If you want instant (and higher)
low end power an Eaton design supercharger
such as the Harrop is your answer. If you want
the greatest power and a broader high end power
band, a twin turbo system is best. A centrifugal
supercharger can deliver a good maximum power
result especially on a standard high compression
motor, but it remains well behind the Eaton
supercharger and twin turbo’s in terms of power
throughout the remainder of the rev range.
5. The Smile Factor
However, power numbers are not the end of the story
in terms of comparing forced induction systems. A
most important consideration is the “smile factor” of
the vehicles driver.
One problem I have had is with the clutch. I have
now replaced it with a triple plate carbon fibre unit.
The best way to understand this is by letting drivers
themselves tell us:
Dave Blake: Supercharged LS2 VZ Toll HSV
“I have had a Harrop HTV2300 Eaton design
supercharger bolted onto my cammed LS2 for 9
months now, and have clocked over 20,000kms of
town, highway and track driving. The power I have
from the instant I put my foot down is incredible and
yet I can drive my car on an everyday basis.
Reliability is no concern and even the fuel economy
is better than standard.
The downside with so much power, particularly down
low, is that new driveline components are required, a
new twin plate clutch being the first.
Ian Williams’ APS twin turbocharged VE HSV.
The other problem is the LS3’s 10.7:1 compression
ratio limiting boost. The LS3’s 6.2 litre alloy block is
being replaced with a GM Performance LQ9 6.7 litre
iron block with a forged bottom end and 8.5:1 blower
pistons to allow us to increase the boost to 15 psi
and hopefully see around 600rwkW’s.
Even though this level of power is not practical on the
street, it is immense fun to have under your control.”
Ian Maloney: Supercharged WH LS1 Grange
“I was looking for a significant performance
improvement from my WH HSV LS1 Grange without
affecting driveability or reliability. After discussions
with Sam’s Performance, I chose a Harrop HH122
supercharger kit with a Harrop intercooler and
transmission cooler for that mix of seamless power
and reliability.
Dave Blake’s Harrop supercharged VZ HSV.
The 10psi boost pulley which is currently producing
424rwkW’s in my car is not standard. The standard
13.5psi pulley was producing a power figure way too
high for my stock LS2 bottom end.
Apart from the dint in my wallet and some minor
noise in the cabin from the supercharger at low rpm, I
am extremely happy with my choice. The Eaton
design supercharger provided a massive increase in
performance without any increase in fuel
consumption. Externally the supercharger is quiet
and looks good in the engine bay.
So I am planning to install a new bottom end with
forged 402cu in internals to give me the strength and
reliability at the anticipated 550rwkW that should
result from running at the superchargers standard
boost of 13.5psi.
I have enjoyed being involved with each step in this
build and would not have done anything different. I
enjoy just sitting back and taking in every sweet note
that this beast of mine produces”
Ian Williams: Twin Turbo LS3 VE Clubsport
“I installed an APS intercooled twin turbo system on
my standard LS3 motor 7 months ago and have
clocked up over 6,000kms. The thing I like about my
twin turbo system is that under 3,000rpm it is just like
driving a strong standard car. It idles like a standard
car, averages 9.9L/100km and can handle heavy
traffic just like any other manual Clubsport.
But when you let the turbo spool up at 3,000 rpm you
need to hang on for the ride of your life! Traction then
becomes the main problem, and even with 285 rear
tyres and the VE Clubsport’s improved rear
suspension and weight distribution, it will still spin the
wheels at 100kph just by pressing the accelerator.
Ian Maloney’s Harrop supercharged WH Grange.
My Grange is a daily driver and a pleasure to drive in
traffic. It has heaps of power for overtaking on the
road and is an extremely quick, comfortable and
enjoyable way to get from A to B.
The supercharger has been on the car for about 9
months now and has not caused any problems. It has
unlocked the beast within the car and me. When I
stomp on that right-hand pedal it makes my inner
child happy and brings a smile to my face. I’m sure it
also brings a smile to the Grange.”
Pros and Cons
Lets now look at the pros and cons of each type of
forced induction system as compared to the other:
Boost
Because they are belt driven from the engine
crankshaft, centrifugal superchargers build boost as
rpm increases in a linear fashion. As engine rpm
increases, the supercharger compressor speed (and
boost level) increases to the point of peak boost
occurring at peak engine rpm. For example, a
centrifugal supercharger designed to produce 8 psi at
6,000 rpm may produce as little as 2.5 lbs. of boost
at 3,000 rpm.
only to the amount that a centrifugal supercharger is
going to produce at that rpm. With a centrifugal
supercharger, stabbing the throttle quickly produces
3 psi at 3,000 rpm (e.g.) but then you have to wait
until the speed of the vehicle picks up and the engine
reaches 6,500 rpm before you see the full 8 psi.
Unlike positive displacement superchargers,
turbochargers can't really produce boost pressure
much above 21psi. Turbochargers are used normally
between 6psi to 17psi.
A Roots or Eaton supercharger is a positive
displacement air pump, and builds boost in a different
way. For example, if you have a 2.3 litre pump and
turn it at 140% of the engine speed, you are pushing
3.22 litres of air past it per engine revolution. With a
6.0 litre LS2 engine, you take in 3.0 litres per
revolution (4 cycle engine). This means the 3.22 litres
is squeezed into the 3.0 litres, creating boost.
Whatever the RPM you are at, except for variations
on volumetric efficiency, the boost is the same
through the whole rev range with a Positive
Displacement supercharger. With an 8 psi set-up,
you will get 8 psi at 2,000 rpm and 8 psi at 6,000
rpm.
When you look at the boost pressure curve of
turbochargers and centrifugal superchargers, the
centrifugals don't look overly impressive in
comparison. This is because they have to be geared
to produce the maximum boost pressure at high rpm.
VP Clubby, 304 V8, single T04Z turbo. Boost = 9psi.
Rear wheel power = 312 kilowatts / 418 horsepower.
Spooling
Turbochargers also suffer (to a greater or lesser
extent) from so-called turbo-spool, in which initial
acceleration from low rpm’s is limited by the lack of
sufficient exhaust gas mass flow (pressure). Once
engine rpm is sufficient to start the turbine spinning,
there is a rapid increase in power, as higher turbo
boost causes more exhaust gas production, which
spins the turbo yet faster, leading to a "surge" of
acceleration.
Turbo-spool is often confused with the term turbo-lag.
Turbo-lag refers to how long it takes to spool the
turbo up to when there is sufficient engine speed to
create boost. This is greatly affected by the
specifications of the turbocharger. If the turbocharger
is too large for the power-band that is desired,
needless time will be wasted trying to spool-up the
turbocharger.
Holden 308 with twin GT 35/40 centrifugal Turbos.
Turbochargers on the other hand are exhaust driven,
and come up to speed very quickly (almost instantly if
properly sized), and may reach the same 8 lb. peak
boost level as low as 2,500 rpm, at which stage the
wastegate will limit the boost irrespective of the
increase in rpm.
The throttle on a supercharged motor is usually very
responsive and boost is nearly instantaneous but
By correctly choosing a turbocharger for its use,
response time can be improved to the point of being
nearly instantaneous. Most well-matched
turbochargers will provide boost at cruising speeds.
Centrifugal superchargers suffer from a form of turbo
spool. Due to the fact that the impeller speed is
directly proportional to the engine rpm, the pressure
and flow output at low rpm is limited, thus it is
possible for the demand to outweigh the supply and a
vacuum to be created in the inlet until the impeller
reaches its compression threshold. Not good.
Lag
This is perhaps the biggest advantage that the
supercharger enjoys over the turbo. Because a
turbocharger is driven by exhaust gasses, the
turbocharger's turbine must first spool up before it
even begins to turn the compressor's impeller.
Turbo lag is less noticeable with automatics or during
manual gear shift, but it can be quite noticeable
during a "standing start".
There is a way to minimise turbo lag and it is called
twin turbo. A twin turbo system has two smaller
turbochargers which can quickly spool, but combined
have the total capacity of a large turbo, giving
maximum boost with minimum lag.
as it as it is driven primarily by potential energy in the
exhaust gasses that would otherwise be lost out the
exhaust, whereas a supercharger draws power from
the crank, which could have been used to turn the
wheels. The turbocharger's impeller is also powered
only under boost conditions, so there is less parasitic
drag while the impeller is not spinning. The
turbocharger, however, is not free of inefficiency as it
does create additional exhaust backpressure and
exhaust flow interruption. Having said this, a twin
turbo system creates far less exhaust back pressure
than a single turbo system.
Also, in terms of fuel efficiency, when driven
normally, a turbocharged car will not consume more
fuel and in fact, fuel economy will usually improve.
VY HSV. Vortech V2 centrifugal s/charger. Boost = 12 psi.
Rear wheel power = 424 kilowatts / 569 horsepower
APS twin turbo system on a Chev LS1
The problem with twin turbo setups is the cost of
building one. People who want big turbo boost and
cannot afford a twin turbo setup often install an
automatic transmission with a high rpm stall torque
convertor which will allow the turbo to spool prior to
launch.
Cost
The prime costs of a positive displacement
supercharger or turbocharger systems for the same
engine are approximately the same for comparable
levels of boost. The cost of actually fitting a
centrifugal supercharger is the lowest of the three as
it is basically a bolt on application that “blows
through” the existing intake system.
The cost of installing a turbo, particularly a twin turbo
system is the highest as the exhaust system needs to
be modified and other components such as boost
controllers fitted and tuned. However, as the Squires
Forced Induction Comparison on an earlier page
shows, the cost per horsepower gained is generally
the lowest on a turbocharged application.
Efficiency
This is the turbo's biggest advantage. The
turbocharger is generally more economical to operate
Just like the air conditioner compressor on a car, all
superchargers, including centrifugal, roots and
screw-type, require horsepower to turn them. This
"parasitic" drag is always present, even when the car
is being driven normally. It will rob power any time
the engine is on, just like running your air
conditioning, so fuel economy must suffer. At full
throttle when the supercharger is working hard,
depending on how much boost you are running, it
can rob anywhere from 50 to 100+ hp on street
applications. The result from this is that you are
putting a substantially large demand on the engine
and it's components to make additional horsepower
that you aren't ever going to see at the tires. For
example, at full throttle, a typical supercharger would
need 12 psi of boost to produce as much horsepower
at the wheels as a turbocharger would at 9 psi.
Controlling Boost
With Superchargers, because of the belt drive, any
change in boost level requires a pulley/belt size
change. Running higher boost most often requires
that you run a 'cogged' type of belt rather than the
standard ribbed belt. This increases the cost and the
noise that the drive produces and is obviously
something that you can't do 'on the fly'. Larger
increases in boost levels also may require changing
the compressor head unit which is going to be very
costly.
In contrast, turbo charged systems can simply
increase boost via a boost controller – provided in
both cases the motor and fuel system are both
capable of handling the additional power and fuel
flow.
Noise
Turbochargers and Superchargers are two of the
best ways to accomplish your goals by producing
more power and faster quarter mile times. In the
writer’s opinion, both have great sounds, the
superchargers with their aggressive whistling, and
the turbo’s with their jet engine type sound.
casing. Because hot air expands (the opposite goal
of a turbo or supercharger), an intercooler becomes
necessary on almost all turbocharged applications to
cool the air charge before it is released into the
engine. This increases the complexity of the
installation.
In addition, turbocharger heat can cause under the
bonnet problems if not installed intelligently, with due
care for ventilation, heat shielding etc. Under the
motor “stealth” applications like the APS twin turbo
system can address the problem of heat quite
effectively, as the turbos are mounted underneath the
motor in an airstream.
VX SS M122H Eaton supercharger. Boost = 12 psi.
Rear wheel power = 380 kilowatts / 510 horsepower
The exhaust note from a turbocharged application is
generally quieter than a supercharged one. Because
the turbo's turbine is in the exhaust flow, the turbo
can substantially reduce exhaust noise.
Some centrifugal superchargers are quite noisy and
constantly whistle - which annoys some drivers. Just
make sure that you want your vehicle to produce this
kind of noise before making a purchase.
Heat
Heat is a major factor in power production. The laws
of physics demand that producing more pressure
creates more heat. Horsepower is lost at a rate of
approximately 1/2 HP per 1 degree F that the intake
temperatures rise. The higher cylinder pressure
created by the added boost pressure, combined with
the added heat in the intake charge of
superchargers, actually lowers the threshold for
detonation (the engine will tend to detonate at higher
cylinder pressures and with higher intake temps) and
detonation is the one factor that will destroy an
engine faster than anything else. Consequently, the
timing must be reduced to avoid the risk of
detonation. Reducing the timing reduces the power
as well as induces more heat into the cylinders and
drives up the exhaust gas temperatures.
With a cooler air charge, you can run more ignition
advance for higher performance, or run lower octane
fuel before risking detonation.
As turbocharger’s are mounted to the exhaust
manifold (which is very hot), turbocharger boost is
subject to additional heating via the turbo's hot
VE Clubsport LS3 APS twin turbo. The “stealth” installation
keeps the engine bay looking standard with the turbo’s
mounted underneath the motor in an airstream.
A centrifugal supercharger on the other hand creates
a cooler air discharge, so an intercooler is often not
necessary at boost levels below 10 psi. That said,
some superchargers (especially roots-type
superchargers) create hotter discharge temperatures,
which also make an intercooler necessary even on
fairly low-boost applications above 6 psi.
Intercoolers are used to cool the air as it comes out
of the compressor, and use either air cooling or water
cooling. For drag racing applications water cooled
aftercoolers packed with ice work very well because
they only need to work for a very short period of time.
For other racing and street applications air/air
(intercoolers) or water/air (aftercoolers) with separate
radiators are more practical as their ability to cool
inlet air is not reduced with time (that is as the
cooling water itself heats up).
Water cooled aftercoolers with a separate radiator
also provide a more consistent inlet air temperature
and a lower inlet air temperature when not
moving, but there is more stuff to break.
On the other hand, air cooled intercoolers have no
heat soak, are cheaper and more reliable. They will
most often give lower inlet air temperatures on a
moving vehicle due to their much larger surface area.
Generally speaking, water cooled aftercoolers are
more commonly used with positive displacement
superchargers as they can be placed in the inlet
manifold underneath the supercharger (which limits
their size). Air cooled intercoolers are most common
with centrifugal superchargers and turbochargers,
and can be made quite large as long as there is
sufficient space in the engine bay.
Turbochargers have no belt and no direct mechanical
connection to the crankshaft, thereby eliminating
these problems. It is interesting to note that many
automobiles and nearly all large long haul trucks use
turbochargers that regularly log in hundreds of
thousands of kilometers of reliable performance.
When the engine is turned off, the turbo usually shuts
down. Stored heat will then cause the turbocharger’s
temperature to rise. If the turbo does not have water
cooled bearings, the residual oil inside the turbo's
bearings can be baked as the units temperature rises
above the coking threshold. For this reason it is
unwise to install a turbo that does not have water
cooling.
VE SSV 6.0L L98, Procharger blower. Boost =8 psi.
Rear wheel power = 332 kilowatts / 446 horsepower.
Finally, exhaust manifolds are crucial in transferring
energy and raising the efficiency of each
turbocharger. Cast iron exhaust manifolds provide a
high thermal inertia and perform this function much
better than a fabricated tubular manifold. Cast iron
manifolds also have the strength to take the weight of
the turbocharger system and to remain rigid without
distortion or fracture even when working up to 1000
degrees C. They also have the wall thickness to
withstand the corrosive effects of running high
temperatures over a long period of time.
Reliability
Both superchargers and turbochargers require high
compressor rpm to compress the inlet air. This
ranges from 30,000-65,000 rpm in superchargers
and can be over 100,000 rpm with turbochargers.
In order to achieve the high rpm levels required to
compress the air to the psi required, superchargers
must have a step-up mechanism (gears, belts,
pulleys or a combination thereof) consisting of
numerous moving parts, to convert engine rpm to the
supercharger rpm necessary to build boost.
Turbochargers need no step-up mechanism and
have only one moving part, the compressor/turbine
wheel assembly. The simplicity of the turbocharger
is therefore less prone to mechanical problems.
VT LS1, single Garrett GT4088 turbo. Boost = 8 psi,
Rear wheel power = 347 kilowatt or 465 horsepower
To increase the reliability of turbochargers that do not
have water cooling, a turbo timer can be installed.
This timer allows the motor to keep running at idle
after the ignition is turned off to allow time for the
circulating engine oil to cool the turbo down.
It is also good practice to cool the turbocharger down
in this manner after it has been driven hard,
irrespective of its cooling system.
Ease of Installation
Superchargers are substantially easier to install than
a turbocharger because they have far fewer
components. Turbo’s are relatively complex and
require manifold and exhaust modifications,
intercoolers, extra oil lines, etc. - most of which is not
generally needed with superchargers. A home
mechanic can install a centrifugal supercharger kit,
while a turbo installation is best left to a turbo expert.
Tunability
Superchargers must have a belt to drive them, and
belt slippage, shrinkage or breakage is not unusual.
More serious problems include crankshaft, bearing
and engine damage caused by belt tension forces on
the crankshaft.
Installation is just one factor. Turbochargers,
because they are complex and rely on exhaust
pressure, usually require a higher level of tuning than
superchargers. The results however are a more
efficient application that provides a higher and
broader power band than can be achieved at similar
boost levels by a supercharger.
Upgradability
Picking any method of forced induction compression
that cannot support the mass of airflow needed for
the engine creates excessive heat in the air/fuel
charge temperatures. This is true with all forms of
supercharging. It is critical to not under-size the
compressor.
Superchargers are generally not upgradeable. When
higher performance is required beyond the
capabilities of a specific supercharger system, the
entire system must be replaced. Turbocharger
systems, however, are usually upgradeable by simply
upgrading or installing a larger turbocharger or
increasing the boost without requiring replacement of
the entire system.
On the other hand, much more homework needs to
be done in selecting the correct supercharger design
and boost pressure to ensure the resultant power
profile across the rev range meets the car owner’s
expectations. It is expensive to get that wrong.
With modern computer systems, most performance
cars have program upgrades anyway, and most
forced induction installers have “out of the box”
computer solutions for the units they are installing,
and then optimise that tune on the dynamometer.
Generally speaking, “old school” carbureted cars are
better suited to a centrifugal supercharger “blow
through” system as the tuning needs are relatively
basic. However, turbocharged applications really
need the sophistication of an electronic engine
management system to safely optimise the engines
power.
VR SS 304 V8, Vortech V1 s/charger. Boost = 14 psi.
Rear wheel power = 312 kilowatts / 281 horsepower.
Maintenance
Importance of Rear Wheel Horsepower
Some superchargers have a separate lubricating
system that must be maintained, but turbochargers
are lubricated by the engine oil and require no
additional maintenance beyond that which is normally
required for a naturally aspirated car.
The question is frequently asked, "How much boost
is safe to run on a stock engine?" This is an
important question that requires more than a simple
answer of how many pounds of boost. Most people
think of boost rather than of horsepower. The
question really should be, "How much horsepower
can I run without damaging the engine?"
Additionally, a supercharger’s drive belt should be
regularly inspected as should the water/air
aftercooler hoses and fittings (if fitted).
Streetability
Superchargers are connected to the engine so they
are always producing some level of boost and cannot
be "turned off". Because turbochargers only produce
boost when under load (as in full throttle
acceleration), performance under normal driving
conditions is not much different than if the engine
were naturally aspirated. Forced induction cars
exhibit excellent drivability characteristics, with
improved power right throughout the rev range, and
are generally much more “streetable” than a naturally
aspirated vehicle that has been modified to achieve
similar gains in power over factory standard.
Because of the efficiency differences from system to
system as well as many other factors involved, the
boost can almost be an irrelevant part of the
equation. Obviously, the more boost pressure
you have to stick into the engine to receive the
desired horsepower output, the harder that is on the
engine itself. The optimal goal is to achieve the most
horsepower output with the least amount of pressure
in the cylinders.
When we talk about horsepower output, there is
another large factor that needs to be considered - the
difference between actual engine horsepower and
wheel horsepower. What is important to the driver of
the vehicle is the wheel horsepower (as measured on
a chassis dyno) because that is what actually gets
your car down the road. Things like drive-train loss
and other 'wasted' or 'lost' horsepower are removed,
leaving you with only the resultant power to the
ground - which is really the only thing that counts for
the cars actual performance.
A Final Word …
Adding a forced induction system is the most
effective way to significantly increase engine power
whilst maintaining a level of “streetability” that cannot
be matched by a comparable naturally aspirated
engine of similar power.
The selection of the correct size and manufacturer of
either Turbocharger or Supercharger is both a
science and an art, which should be left to the
experts. Many technical books have been written
about it and free advice is available from the
manufacturers and installers of Turbochargers or
Superchargers, but at the end of the day you will
mostly always get what you pay for.
VS GTS 355 V8, Vortech V2 s/charger. Boost =8 psi.
Rear wheel power = 255 kilowatts / 342 horsepower.
By better understanding each forced induction
technology and the differences between them, you
should now be able to ask the right questions to be
able to successfully determine the best forced
induction system to increase the performance of your
particular application. IW
On the other hand, what is important to the engine is
the amount of work it has to do in order to produce
the desired wheel horsepower. This actual engine
horsepower can even be broken down farther.
You have the flywheel or crank horsepower (as it
would be measured on an engine dyno) and you
have the 'real' or actual engine horsepower that the
engine is required to produce (which can be
calculated by the fuel consumption). This is the
actual power demand, which creates the forces and
'stress' that are placed on the internal engine
components (rods, pistons, crankshaft, head gaskets,
etc.) that when exceeded, cause engine failures.
So the optimal goal is to get the most wheel
horsepower (actual performance) with the highest
degree of efficiency so that the strain on the engine
stays below the failure point. Choosing the forced
induction system that produces the most horsepower
with the least amount of parasitic loss is the way to
achieve the highest wheel horsepower with the least
amount of boost pressure required.
So what does all this mean in real world terms?
Turbocharged cars produce more wheel horsepower
with less stress on engine components. You can use
this benefit in two ways: 1) you can run the same
wheel horsepower as the supercharged car with less
boost and less risk of engine damage, or 2) you can
run more boost and produce more wheel horsepower
with a similar risk of engine damage.
Turbochargers are by far the best, but often the most
expensive choice when you are looking for the
greatest amount of horsepower per pound of boost.
© HSV Owners Club (Incl. HDT) of NSW 2009
Thanks and acknowledgements to the below sources:
“Turbocharging and Supercharging” by Alan Allard,
first published by Patrick Stephens in 1982.
“Maximum Boost” by Corky Bell,
published by Robert Bentley, 1997
“HSV Dyno Comparisons” data from:
Dave Blake VZ HSV - Harrop HTV2300 supercharged LS2
(ESP Racing Dyno, Queanbeyan ACT (02) 6299 4444)
Ian Williams VE HSV- APS twin turbocharged LS3
(Autotech Engineering Dyno, Granville NSW (02) 9897 1378)
“Grunts” VY HSV - Vortech V2 supercharged LS1
(Monster Performance Dyno, Mackay Qld (07) 4957 7400)
LS1 R&D Centre Turbo v Centrifugal SC Comparison dyno
Morpowa Dyno Centre, Gillman SA (08) 8264 2077
www.airpowersystems.com;
www.associatedcontent.com;
www.automotivearticles.com;
www.birkey.com;
www.bulletcars.com;
www.capa.com.au;
www.enzinearticles.com;
www.harrop.com.au;
www.hot4s.com.au;
www.howstuffworks.com;
www.justcommodores.com.au;
www.ls1turbo.com.au;
www.newcarbuyingguide.com;
www.perfectpower.com;
www.performancemotorresearch.co.nz
www.racespec.com.au;
www.streetcommodores.com.au;
www.ststurbo.com;
www.superchargersonline.com;
www.turbochargedpower.com;
www.whipplesuperchargers.com.
Researched and written for the HSVOC by Ian Williams