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.
Of course, on a road car at least, any boosted fuel mixture leaving through
the exhaust valve during the overlap period is going to impact fuel
consumption figures.
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