Forced induction cams, now this is one of those topics I love sinking my
teeth into!
> Don----Cams with higher lift and or longer duration than the
> stock grind adds extra power to the TR engine, even when
> using forced induction. The tradeoffs are the same...less
> torque/power at lower rpms with gains from mid to top end.
Yes, this was the typical thinking twenty five years ago and even just a few
years ago. The problem is forced induction motors do behave differently
from naturally aspirated motors, and they can therefore benefit from
camshafts specifically tailored to suit that setup. Even at that, a
turbocharger cam and a supercharger cam will have different characteristics.
One of the reasons that turbochargers have fallen out of favor with Detriot
auto makers to supercar builders, is that a turbocharger is like sticking a
giant waterwheel in the middle of your exhaust. It virtually destroys any
possibility of gaining horsepower through exhaust optimazation or tuning.
If you have a turbocharged car that is producing 10psi in the inlet tract,
you definitely have 10psi --or more likely 12+psi-- of exhaust back pressure
before the turbo inlet in the exhaust tract. Superchargers do not have this
problem, and this fundamental difference is why the cams are designed
differently.
Supercharged cams benefit from increased exhaust lobe duration and lift.
Whether you choose increased exhaust duration, or increased lift, or both,
depends on what you are trying to achieve and at what RPMs. With a
supercharged cam, you can also reduce the valve overlap period (the period
that both valves are open at the same time). Reducing the overlap period is
only a positive attribute on a supercharged car, otherwise some of the boost
you are putting into the cylinder is being wasted out of the exhaust valve.
The trick here is reducing the overlap to to the point where you are just
wasting a tiny fraction of boost, in order to effectively clear all the
exhaust out and then immediately get that valve shut.
Optimizing a cam in the same way, with that assymetric focus on the exhaust,
for a turbocharged car is not going to do a whole lot for the performance of
the car. In fact, reducing the overlap period of a turbo cam, could have
the undesired effect of causing the turbocharger to spool up more slowly.
The slower the spool up of the turbine wheel, the more measurable and
noticible turbo lag becomes. When you have valve overlap in a turbo car,
you really are not wasting boost, that "wasted boost" is going out of the
exhaust valve and its energy is helping to build pressure in the exhaust
tract that helps drive the turbine. So again, you can tune a lot of
parameters of the cam to acheive performance characteristics different ways
to suit your application.
If you take a stock TR6 cam and you increase the duration or lift and reduce
overlap, you would end up with an optimized supercharged camshaft that made
more power over a broader RPM range. This is one of those wonderful times
in the automotive world where a few key changes allow you to have your cake
and eat it too.
Five years ago when I first became serious about forced induction, is when I
started formulating my ideas about camshafts and how camshaft
characteristics would have an impact on the blown motors. At the same
period of time five years ago, I went to the best British Car cam shop I
know - Integral Cams - and got Steve's input on the matter. The results of
our discussions, and of Steve's hard work and generally vast knowledge,
resulted in the groundwork that eventually became the Integral Cams TR6
Stage 1 Supercharged Cam.
I am as determined as ever to bring my own TR6 supercharger kit to market.
I cannot begin to tally up how much time has gone into the project, or how
much time is left. But when you choose to design something yourself,
different from and better than all else on the market, there are a lot of
details that take quite a bit of sorting out. For instance, I could never
have guessed that I would be trying to find fabrication shops that can
thermoform ABS plastic over foam molds in order to flow bench test different
models of plenum shapes for instance.
--
Kai M. Radicke
Wishbone Classics
* British Car Parts *
www.wbclassics.com
Ph: 215.945.7250
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