camshaft shootout

CAMSHAFT
SHOOTOUT
Choosing the ultimate
bumpsticks for your Evo,
Part 3
Text and photos by Martin Musial
I
n this third installment of the camshaft shootout I’ll start
to look at the details of cam timing and how it can affect
power. The writing of these articles has really re-ignited my
interest in the theory of the internal combustion engine.
I’ve even dusted off the old college engineering books.
Unfortunately my math and thermodynamics skills aren’t what
they used to be and it was quite a challenge to refresh even
some of the basics. I spent nights pouring over books and Society of Automotive Engineers’ papers about internal combustions
engines and realized just how complex the subject really is. It
seems simple—just get the air in the engine and then get it out,
how hard can it be? If you really take a look at how many things
are going on in an engine and how it’s all dynamic, it really poses
a puzzle that I can only barely scratch the surface of. What you’ll
read in this article is just a basic assumption of what I’ve learned
from my research and testing and by no means does it apply
across the board to every engine. If there’s one thing I’ve learned
about camshafts it’s that it’s really a game of sacrifices and complete dependence on the engine combination. Encouraging news
right? So, you may ask, what are you to do to with your naturally
aspirated Nissan VQ35 engine, and what cams do you choose?
Although you can follow some of the same basic principles that I
discuss here, it really comes down to hard science and experimentation to learn what your combination needs.
Timing events and how
they affect performance
Timing events are particular
to the engine setup as a whole,
and one generalization does not
work the same for every engine.
Contrary to popular belief,
retarding an intake cam won’t always make power at higher rpm.
Intake valve opening
Early intake valve opening
is beneficial to both good and
poor flowing intake ports. It
allows more time for the intake
charge to start moving into
the cylinder. The exhaust port
flow and exhaust-closing event
will really determine how early
you can open the intake valve.
An earlier intake valve opening will increase the overlap
period when both the intake and
exhaust valves are open. With
a poor flowing exhaust port an
earlier intake valve opening can
cause the exhaust charge to
pollute the intake charge. This
needs to be taken into consideration in our turbocharged
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TECH
Camshaft Shootout
Choosing the ultimate bumpsticks for your Evo, Part 3
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TURBO & HIGH-TECH PERFORMANCE
application since a turbo is an exhaust restriction that reduces
exhaust flow. Typically, as we near the limits of a particular
turbocharger, the exhaust backpressure rises. On my test car,
I fitted a pressure probe into the collector of the header and
recorded the pressure through the AEM EMS. I found that at 30
psi of boost, I was seeing about 45 psi of exhaust pressure at
redline. This is a good time to really explain the overlap period.
Overlap period is how much time both the intake valve and
exhaust valve are open. While it seems to not make sense to
have both the intake and exhaust valve open at the same time,
there is logic behind it. As the piston nears the top, the exhaust
gas is still moving past the rapidly closing exhaust valve. This
exhaust gas has energy and can actually help to pull in fresh
intake charge as the intake valve starts to open. If the timing is
right the exhaust gas will leave a depression, or low-pressure
area, behind it, literally sucking in the fresh intake charge.
You can actually have intake charge flowing into the cylinder
before the piston starts moving down the bore and drawing
in the intake charge. This overlap tuning effect occurs near a
certain rpm window and, in our example, can be impeded by
the exhaust backpressure. I was running my test car at 20 psi of
boost and I was only seeing 20 psi of exhaust backpressure, as
opposed to the 45psi exhaust pressure at 30 psi of boost. Since
this exhaust-to-intake pressure ratio changes at different boost
levels, the overlap period requirements can change, making
valve timing a little tricky on turbocharged cars.
Intake valve closing
A good flowing intake port can take advantage of later
intake valve closing. At higher rpm, where a good flowing port
is still unrestricted, the later intake valve closing is allowing
the charge air to fill the cylinder and build cylinder pressure.
A poor flowing port that is choked at a lower rpm can benefit
from earlier intake valve closing. It’s important to build as much
cylinder pressure as possible when closing the intake valve. If
the intake port is choked and can’t keep filling the cylinder, it’s
advantageous to close the intake valve earlier so the cylinder pressure is maintained. As you can imagine throwing a
long duration camshaft that closes the intake valve late on a
poor flowing engine combination will most likely hurt low to
midrange power without making any peak power gains. Intake
valve closing is very important and can have a dramatic impact
on power production and the power curve.
Exhaust valve opening
A restrictive exhaust port can benefit from earlier exhaust
valve opening. Opening the exhaust valve early bleeds off the
pressure from the power stroke that is pushing down on the
piston and producing torque, but at the same time it gives
the exhaust more time to evacuate the cylinder through the
exhaust port. It takes power to push the exhaust gases out the
exhaust port, this is called pumping loss. The earlier exhaust
valve opening on a poor flowing exhaust port can reduce this
pumping loss and increase power. The trick is for the reduction in pumping loss to outweigh the power lost in the power
stroke. With a good flowing exhaust port and little backpressure we can open the exhaust valve later because it’s easier for
the exhaust gas to get out of the cylinder (lower pumping loss).
The later exhaust valve opening also keeps more cylinder pressure in the power stroke. My initial thoughts told me that since
the turbocharger is a restriction on the exhaust side, a turbo
car, especially with a stock turbo, can really benefit from opening the exhaust early; conducting a test on a stock turbo car
changed my thinking. I installed an aggressive exhaust camshaft on a stock turbo car and it lost a lot of low-end torque,
and didn’t make much more peak power. I remembered that
a turbocharged engine makes power a little differently than
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a naturally aspirated engine. The cylinder
pressure that pushes down on the piston
from the combustion process last longer,
meaning it pushes down longer over the
rotation of the crank. Imagine giving a naturally aspirated combustion stroke a quick
and violent shove and the turbocharged
a steady long push. This steady long push
can be cut short when you start to open
the exhaust valve too early. Remember that
the exhaust valve starts to open well before the piston reaches the bottom of the
power stroke to give the exhaust gas time
to get out. Well, now we have a dilemma.
If we open the exhaust valve too early we
lose low rpm power but if we open it too
late we lose top-end power due to pumping loss. This led me to another thought.
The restriction (turbine) is after the exhaust
port/valve and at lower mass flow rates
(rpm), the exhaust port to turbine flow ratio. Remember that a turbocharger acts like
a restriction after the exhaust port so the
exhaust valve opening event requirement
will be different than that of a naturally aspirated engine.
Exhaust valve closing
Exhaust valve closing is dependent on intake port flow,
exhaust port flow, and intake valve opening. Poor flowing
exhaust ports with a lot of backpressure need to have the exhaust valve closed earlier in relation to the intake valve open
to reduce overlap. The high exhaust backpressure can pollute
the intake charge if the overlap is too great. Basically an engine with poor flowing exhaust ports won’t benefit from much
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overlap. If the exhaust port flows well and the intake flows
well then it can be beneficial to close the valve later and allow
for more overlap. Another scenario is a good flowing exhaust
port and poor flowing intake port. This combination can benefit from more overlap because the evacuating low-pressure
exhaust gas can help draw in the intake charge and increase
intake port velocity. This is not the case in our turbocharged
application where most likely our intake side flows better than
our exhaust side.
GSC S2
The GSC S2 settled right down to a nice stable idle.
Although not as smooth as the silky HKS 272 or the
GReddy EASY Cam, it needed about 100-200rpm higher
idle to run nicely. Low power and driveability was good
and spool was almost identical to the HKS 272s. Playing
around with cam timing showed that because installed
timing was the best for balanced midrange and top
power. At 22 psi of boost the power curve showed a
peak of 487 whp, almost 60 whp over stock camshafts.
At higher boost, 30 psi, the power checked in a 535 whp.
Overall it’s a nice cam with decent idle quality and good
top-end numbers.
CAMS TESTED
GREDDY EASY CAM
I had to check with my techs to make sure they
didn’t put stock cams in because the idle was like
stock. I could bring it down to 800 rpm and only
a trained ear could pick up any difference. Part
throttle operation was as good as stock and
there was no loss in driveability, a very nice
street-mannered cam. On power runs,
these good-mannered cams gave up a
little in the horsepower department.
Putting down 451 whp at 22 psi
the GReddy EASY Cams made
about the same power as the
HKS 272s. At 30 psi the EASY
Cams put down 526
whp and so far have
the best idling and
driving manners
of all the cams
I tested with
respectable
power
numbers.
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KELFORD 272 CAMS
Kelford claims to have the best 272 cams on the
market. Idle quality wasn’t bad but I had to bump it up
to over 1,000 rpm to stabilize the idle. The part throttle
and street driving manners came in about average with a
few low-rpm sputters in certain conditions. Once under
power they really opened up. Running to redline at 22 psi
of boost gave 498 whp with power climbing to redline. At
30 psi the Kelford 272’s put down 543 whp, putting down
the most power out of the 272 advertised duration cams.
I really liked this cam because it had acceptable street
manners and put down some very impressive power
numbers.
As testing is drawing to a conclusion, I’ve started to see a
pattern in the cam profiles that I can correlate to dyno data
along with what I’ve learned from my research. I can see things
like lift profile and opening/closing events making a
significant impact on performance. Although some cams
are advertised as having the same duration, the rest of the
picture (profile) and the dyno data shows a huge difference. I still have at least one camshaft to test, the TOMEI 280
solid lifter cam, that I’ll compare to its hydraulic counterpart.
In addition, the next and last installment will include a comprehensive camshaft summary along with some of the conclusions
I’ve drawn about the cam designs and how they performed.
CAMS TESTED
AEM
Kelford Cams
AMS Performance
Oliver Rods
BC Brian Crower
Ross Pistons
Crane Cams
Supertech
Forced Performance
TiAL Sport
GSC Power-Division
Tomei Powered USA
www.aempower.com
www.amsperformance.com
www.briancrower.com
www.cranecams.com
www.forcedperformance.com
www.power-division.com
www.kelford.co.nz
www.oliver-rods.com
www.rosspistons.com
www.supertechperformance.com
www.tialsport.com
www.tomeiusa.com
HKS USA
www.hksusa.com
Final Comparison
The next and last installment will include a comprehensive camshaft summary
along with some of the conclusions I’ve drawn about
the cam designs and how
they performed.
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