Supercharger Performance and Engine Performance Parts

A friend of mine is building an intake manifold for a naturally aspirated Toyota Celica. The car is equipped with a 2.2 liter 4 cylinder engine that generates around 135hp @ 5500 with 6200 rpm redline. With a lot of modifications, the engine can achieve 180 crank hp @ 6200 rpms and turbocharged and supercharged versions of the same car have broken the 320 wheel hp mark @ 21psi of boost @ 6200rpms.

Now that we have our parameters defined:

Displacement: 2200 cc

Peak RPM: 6200 RPMs

Target hp: 180 hp corresponding to about 270 CFM of flow

Number of runners: 4

Number of throttle bodies: 1

Plugging these parameters into my power calculator I get the following dimensions for the intake manifold that I would build. For comparison here, we have the dimensions chosen by Mr. Turrani for his application.

In general, when doing research for the power calculator, I found that typically intake manifolds have a volume that is 50 to 80% the displacement of engine. Obviously proper intake manifold design is much more involved than that as dynamic fluid flow modeling shows us that sometimes very large yet appropriately designed intake manifold shapes can maintain peak velocity while still having a decently sized plenum volume to promote top end power.

The compromise in plenum volume is as follows:

A larger volume leaves more available air to the engine within its reach, and so long as this air can be replenished in time through the intake system, then the engine never has to work hard to get intake air because there’s always enough of it sitting there in the larger plenum.

As the plenum volume gets smaller, it becomes easier for the engine to rapidly consume all of the air in the plenum and thus it would have to spend a lot of effort (after the initial draw of air) trying to suck air in all the way through the entire intake system to stay alive.

The problem with a larger plenum is that it hurts throttle response. Throttle response is very much affected by throttle pressure (or in other words how fast the engine can consume all the air in the plenum and create a significant amount of vacuum in the manifold to draw in fresh air). The smaller the plenum (or smaller the runners), the higher the gas velocity, the faster the pressure drop, the sooner the new air rushes in, the faster the throttle response.

This usually leads to an oddball design by most OEM’s of an oversized plenum wit h a smaller throttle body and runners to try to boost gas velocity, or an undersized plenum (that will be consumed faster for better response) but with a larger throttle body that will not bottle neck the engine as it tries to pull in more air from the outside to stay alive at higher flow demands at higher rpms.

Either way, shifting peak power from 5500 to 6200 has a potential increase of 12% especially coupled with a properly designed exhaust manifold, appropriate camshafts, and a proper tune (all of which Mr. Turrani already has on his car).

As far as superchargers are concerned, intake manifolds have lower diameter requirements for the throttle body and the intake runners because the air is compressed. At the same time, runner length and resonance calculations are not much affected because air in the manifold travels at the speed of sound, and the speed of sound is not drastically affected by a slight increase in temperature and a boost in pressure.

One thing to note is that with something like a roots or screw style supercharger, engine vacuum is not alone responsible for throttle response. As the air is being both sucked in (by the piston stroke) and shoved in (by the supercharger rotation) it becomes easier and faster to fill a larger volume plenum manifold. This allows for an oversized manifold for higher rpm volumetric effeciency while relying on the screw supercharger to take care of the gas velocity, and throttle response.

Technorati Tags: manifold tuning, Power Calculator, supercharger performance, throttle pressure, Throttle response

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