camshaft shootout

CAMSHAFT
SHOOTOUT
Choosing the ultimate
bumpsticks for your Evo,
Part 2
Text and photos by Martin Musial
I
The crank makes two revolutions in this process. For each
revolution, the piston starts at the top, travels down, and then
goes back to the top. The very top position is called top dead
center (TDC), and the very bottom position is called bottom
dead center (BDC). The intake stroke consists of the piston
starting at TDC and the intake valves opening, allowing fresh
air and fuel to enter the engine while the piston is traveling
down toward BDC. Next is the compression cycle where the
intake valves are closing and the piston travels from BDC back
up toward TDC. With the intake charge being compressed, the
ignition sets off the charge near TDC and begins the power
CAMSHAFT BASICS:
stroke. The combustion pressure pushes the piston down from
Most of you are familiar with the terms duration and lift on
TDC back to BDC. The final cycle is the exhaust cycle where the
camshafts. The more duration and the larger the lift the more
exhaust valves are open and the piston is pushing the exhaust
power, right? A lot more goes into cam design than you think
gases out as it travels up to TDC. The cycle repeats itself with
and sometimes a well-designed cam can make power with
smaller advertised measurements. Here’s a quick engine primer the intake valves opening and the piston traveling back down,
drawing in fresh air.
to get you thinking about how an engine makes power. On
Camshafts control when the intake and exhaust valves open
a four stroke engine four events happen to make one power
and close. Camshafts also control how far the valves open.
event: intake, compression, power, and exhaust.
Seems pretty simple: Open the intake valve when the piston
1
2
3
4
is at the top of the intake stroke and close it when the piston
reaches the bottom. Same for the exhaust valve: Open after the
combustion stroke and close it back at the top. However, if we
did this we’d have an engine that wouldn’t make good power
and it would all be at a very low rpm. Realistically the intake
valve opens early (before TDC) and close later, well after the
piston reaches BDC and it’s already on the compression cycle.
The same with the exhaust, it opens early before the power
stroke is fully completed (before BDC) and closes after the
piston reaches TDC of the exhaust stroke. The precise timing
of these events determines how the engine will run through the
rpm range. Certain timing events will benefit high rpm running
Power
Exhaust
Intake
Compression and make more peak horsepower, while others will make more
In this second part of our three-part series on camshafts,
I’m unraveling the inner workings of camshafts. If you
haven’t read the first part, I highly suggest you get a copy
or at least have some basic knowledge about camshafts. In
this month’s issue I’ll talk about the basic power concepts
of camshafts and the detailed terminology used to describe
them. You’ll see that even though two camshafts might both
be advertised at 290 duration, there could be huge differences
between them.
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TURBO & HIGH-TECH PERFORMANCE
CAM DURATION
Cam duration refers to how
many degrees of crankshaft
rotation that the valve is open.
Looking at a lift curve, which
shows valve lift in reference to
crankshaft degrees, you can see
that it resembles a bell curve.
The idea is to gently lift
the valve off the seat, open
it quickly, hold it open, then
close it quickly, and set it
gently back down. At very low
lifts there’s very little airflow
going on so most manufacturers rate their cam’s duration at
a certain lift. For example, the
same cam can be listed as having 300 degrees of crankshaft
duration at 0.004 inch of lift
or it can have 240 degrees
of duration at 0.050 inch of
lift. Most of the cams that I’ll
be testing have advertised
duration ranging from 270
to 280 degrees of duration.
This advertised duration really
must be taken with caution
as it depends on what lift the
manufacturer is specifying,
some specify 0.004-inch lift
while others use zero lift. Let’s
We now know that the intake valve doesn’t close at the
bottom of the intake stroke, but stays open partly into the
compression stroke. This means that as the piston is traveling
upward on its compression stroke the intake valve is still open!
The later we close the intake valve the less compression we’ll
have because we’re letting air back out. Dynamic compression
ratio is the compression ratio taking into account the closing
of the intake valve event. We know that a higher compression
ratio yields more power due to higher combustion pressure on
the power stroke. So now if we close the intake valve later, we
reduce our dynamic compression ratio and also hurt power.
That doesn’t sound good, especially since most performance
camshafts close the intake valve later than stock cams. There’s
some truth to that. Since the dynamic compression ratio goes
down most of the time you’ll see a loss of power at lower rpm
with aggressive camshafts. At the same time though, the power
at higher rpm will increase. Since the dynamic compression is
lower, why isn’t it losing power everywhere? Grab some caffeine and do some mental stretching because here comes the
big one.
Air is a compressible fluid and as it travels at different velocities and pressures through an engine it has momentum. This
concept is why a camshaft that holds the intake valve open longer can make more power at higher rpm. At low engine speeds
the air has less momentum and the cylinder fills in with all the
air it can get and some of it might even be pushed back out as
the piston starts going up on its compression stroke. At high
engine speeds where the piston and air is moving much faster,
we can actually have air still cramming in after the piston is going back up on its compression stroke. A good analogy would
be to imagine a train crashing into a solid wall with the back of
the train keeping momentum and piling up on the wreckage. I
mentioned the intake valve is opening before the piston is all
TURBO & HIGH-TECH PERFORMANCE
57
Camshaft Shootout
15 Unit
Volume
high-rpm power. This leads
into the cam duration and
valve timing events.
Choosing the ultimate bumpsticks for your Evo, Part 2
1 Unit
Volume
the way at TDC and that the
exhaust valve doesn’t close
until after TDC, that means
that both valves are open at
the same time! This is called
overlap, and the same concept
of momentum applies here.
The idea is that the air going
out the exhaust port will help
pull in fresh intake charge as
the intake valve is opening.
Too much overlap can hurt
idle and low speed running.
Not enough overlap and it can
hurt top end horsepower. It’s
a complex interaction that is
determined by various factors
throughout the engine.
The intake manifold design,
head port design, valve size,
valve lift profile, rod-to-stroke
ratio, exhaust port design, and
exhaust manifold design are
all key components in determining the power output of an
engine and how it produces
power across the rpm range.
This subject is very complex
and auto manufacturers and
top-level race programs have
teams of engineers working
on designing better engines.
To keep it simple, let’s just
stick to the concept that
holding the intake valve open
longer will generally reduce
low-rpm power and increase
TECH
power down low at the cost of upper end horsepower. If your
brain hurts already, take a rest, re-read, and get ready for some
more cranial punishment. Ready? Most of you know what your
compression ratio is, but what if I told you that your compression ratio changes when you change your camshaft? All automotive manufacturers and aftermarket companies rate engines
and piston specifications as a static compression ratio. Static
compression ratio is the volume of the cylinder when the piston
is all the way down at BDC, compared to the volume when it’s
all the way up at TDC.
KA24DE cam over bucket
say we have two different
cams rated at 240 degrees of
duration at 0.050 inch, from
the graph you can see how the
same duration cams can have
very different lift profiles.
Cam Lobe Data Collection
As I mentioned in the
previous article I’m using the
Performance Trends Cam
Analyzer to record all of the
cam data. The system consists
If one cam is more aggresof a test fixture that holds
sive on opening and closing
the camshaft and allows it to
the valve, it can have the valve spin concentrically. A rotary
open further for a longer
encoder attaches to the end
period, theoretically supportof the cam to measure the
ing more horsepower. This
rotation and a linear encoder
type of aggressive profile
rides on the lobe to record
will be hard on the valvetrain,
the lobe displacement. All
requiring stiff valvesprings and this data is fed into the Cam
lightweight valves that might
Analyzer software through a
only last 5,000 miles before
data acquisition box.
wearing something out.
To show you how duration
is calculated, let’s look at this
example. Taking an advertised
duration of 0.004 inch lift,
the intake valve would open
roughly 20 degrees before
TDC, stay open throughout
the intake cycle, and then
close 60 degrees after BDC
during the compression
stroke. The advertised duration of this cam would be 20
degrees, plus 180 (the intake
stroke), and 60 degrees, for a
total of 280 degrees.
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TURBO & HIGH-TECH PERFORMANCE
Not to complicate things further, but the valvetrain in the
Evo head is complex. It’s referred to as Cam On Rocker Arm
(CORA). The cam lobe doesn’t act directly on the valve, it transfers the motion through a rocker arm that pivots on one end
and pushes on the valve stem on the other. On the opposite
and easy end of the spectrum, the new Evo X has a cam-onbucket design in which the lobe rides on a bucket lifter that
acts directly on the valve stem. In the case of the Evo VIII, the
trick is getting the cam lobe motion to translate correctly to
valve motion. This procedure requires modeling the dimensions
of the valvetrain in the Cam Analyzer software so it can compute one valve motion to the other. The tricky part is getting
these exact dimensions; luckily, I had access to a coordinate
measuring machine and was able to reverse engineer the head
and valvetrain to get exact dimensions. Now I’m able to compare valve motion from one camshaft to another.
Using the data recorded by the Cam Analyzer we can look
at the valve lift curves. Let’s follow a lift curve and see what’s
going on. The valve lift curve shows crank rotation in degrees
along the bottom (x-axis) and valve lift along the side (y-axis).
Starting from left to right, we see the exhaust valve starting to
open. This is the opening ramp, where it gently starts to lift the
valve off the seat and into the main opening ramp. Near the
peak lift of the cam the valve slows down and starts to head
down the main closing ramp. As the valve gets closer to fully
closing on the seat, the motion slows down and the valve gets
gently set down. If this gently closing ramp wasn’t there, the
valve would get slammed onto its seat, possibly bouncing it
off, causing damage and losing power. You can see that even
though the exhaust valve isn’t fully closed, the intake valve
is starting to open. This is the period of overlap that I talked
about earlier. I’ll discuss overlap in more detail in the next
article. Ideally we’d want to open the valve as fast as possible,
hold it open, and then close it as fast as possible. Physical limitations of the valvetrain simply don’t allow for this, and careful
consideration must be taken to how fast valves open and close.
Aggressive cam profiles require strong valvesprings to control
valve motion and can even cause valvetrain components to
wear out quickly. The design of camshafts is complex and great
consideration must be taken into the power and longevity
requirements of the engine. An endurance motor that has to
run 500 miles of high-rpm running needs different cam profiles
than an all-out drag race motor that only sees very limited run
time. The Performance Trends Cam Analyzer can show the
important data of a cam profile that determines many of the
critical components needed for cam applications. For the average street tuner car this means choosing a cam that’s not overly
aggressive on the valve motions so that the engine lives worry
free for thousands of miles. The Cam Analyzer helps me see the
valve motion, and correlate valve opening and closing events
with power gains on the dyno. The next and final article will
cover valve events for the intake and exhaust, and help bring to
light why some cams might idle better and some just make tons
of power at high rpm.
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59
Forced Performance 4R cams
RESULTS:
Stock Cams
Idle quality is decent, while not as good as the mild HKS
272s, it’s much better than the aggressive Tomei and Crane
offerings. A few pulls later to dial the fuel in and the car made
465 whp at 22 psi, whereas at 30 psi I got 536 whp. After a few
timing changes with the cams gears, the best power curve is
found as per the recommended installed specs.
What can I say, they’re stock. They idle like butter at 900 rpm
and have zero low-speed driveability issues. At 22 psi they pushed
418 whp, quite a bit lower than all the other cams and almost 100
whp down from last month’s test of the Tomei 280 cams. Turning
the boost up to 30 psi, the power climbed to 505 whp.
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TURBO & HIGH-TECH PERFORMANCE
BC Brian Crower Stage 3:
The BC Brian Crower Stage 3 cams have a very similar idle
quality and low-speed driveability to the FP4Rs. On power
pulls, the power came in at 451 whp at 22 psi, and 527 whp at
30 psi. The BC Brian Crower Stage 3’s did require some adjustment on the cam gears to get them to the factory specifications. While most cams were within 1 degree when checking
out the specs on the stand, these cams had to be retarded 3
degrees on the intake and advanced 2 degrees on the exhaust
side to check out. For an experiment I put them at zero, as if
someone installed them without degreeing them, and the idle
and power output suffered. This was a prime example why
checking cam timing is important.
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TURBO & HIGH-TECH PERFORMANCE
Comparing all three horsepower curves at 22 and 30 psi.
AEM
Oliver Rods
AMS Performance
Ross Pistons
BC Brian Crower
Supertech
Crane Cams
TiAL Sport
Forced Performance
Tomei Powered USA
www.aempower.com
www.amsperformance.com
www.briancrower.com
www.cranecams.com
www.forcedperformance.com
www.oliver-rods.com
www.rosspistons.com
www.supertechperformance.com
www.tialsport.com
www.tomeiusa.com
HKS USA
www.hksusa.com
Next month’s issue will bring
a few more new cams, and I’ll
go into each particular valve
timing event and why it affects
the way the engine runs. I’ll
also look at specific cams and
compare lift curves and valve
events to see why they do or
don’t work.