Intake manifold construction, intercoolers

[Craig Dotson]

> In a N/A your waiting for atmospheric pressure to fill the void as the
> piston moves down.  In a boosted motor, the plenum pressure is helping to
> push the piston down.  In a N/A engine, there are all sorts of wave
> activities, that can help or HURT cylinder filling, and that is about mute
> when in boost.
>   If you think these issues are minor then look at the rod failure
> differences between the two types of motors then
> Bruce

I agree here, and I also agree with Bruce's earlier opinion that you are
thinking in NA rather than turbo terms.

When you tune an intake manifold for an NA car, you're trying to take
advantage of the acoustics generated inside the manifold when you have flow
into the cylinders.  The most you can get out of such tuning is maybe a
handful of pascals.....not even kPa...at the right times that help exhaust
scavenging and cylinder filling at the beginning and end of the intake valve
opening.  Bruce is right, typically at frequencies between the resonances
you actually get a few Pa drop at the beginning and end of the intake valve
opening that tend to empty the cylinder more than fill it.  In a turbo car,
particularly one with 1.5 bar (150 kPa) boost that will likely never see
pressures below atmospheric, a handful of Pa basically amounts to jack and
you're killing yourself for nothing.

The last Formula SAE car my school built with a turbo had approx 30mm
diameter runners to each cylinder and the "plenum" consisted of about a 50mm
diameter "log" connecting the runners...and this is on a 600cc 4 cyl engine.
I don't know what boost levels were, but I doubt they were as high as 1.5
bar.  There was absolutely zero thought into "tuning" the intake, it was the
absolute minimum necessary to deliver the pressurized air to the cylinders.

Additionally, why two plenums and two TBs?  I can see it if you're doing
twin turbo but otherwise I don't see it.  I'd just go for the most direct
means of delivering the air to the cylinder, trying to line up the port in
the head with the runner as well as possible.

Craig Dotson
crdotson at vt.edu
2002 VT FormulaSAE

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