[Diy_efi] New project- intake effect on VE on turbo car

[Andris Skulte]

On Thu, 21 Nov 2002, John Petersen wrote:
> Let's say that you have an EXTREMELY restrictive intake on your car,
> before the turbo. The turbo will attempt to build boost pressure, and
> will have to spin very fast to get boost to where it should be (the ECU
> should do its best to get it to whatever preset boost level is
> specified, in absolute pressure terms). To spin the turbo "very fast"
> you need to provide exhaust energy. You effectively make an exhaust
> restriction by having an intake restriction. 

Wouldn't the same turbo unit spinning at a higher rpm create less of a
restriction to the exhaust than a turbine spinning at a low rpm? If the
intake is super restrictive (think plugging your shopvac hose w/ a wad of
duct tape), the compressor is in very sparse air (vacuum), so there is
less restriction (less work to be done), and so it speeds up (you hear
your vacuum speed up, and motor HP hasn't changed). If there is less work
being done, there is less energy being taken from the exhaust, so it will
be less restrictive. Granted - you'll be way down on HP since the
centrifugal compressors work across a pressure ratio. No vacuum = ambient
pressure about 14.7 psi, so a 2:1 pressure ratio would give you 14.7 psi
above the ambient intake pressure at the compressor's outlet. If the
intake is choked, the inlet pressure would be a significant vacuum. twice
a significan vacuum = much less boost, which is much less air, which is
less work, which is less of a restriction. If you want the same compressor
outlet pressure, then, yes, you'll have to spin the turbine much faster,
which could be by not opening a wastegate. Since you've got your turbo
bypass closed (wastegate), there is more of a total exhaust restriction
(but the turbo is the same restriction it was before).

 > > There are more important reasons to have an efficient
intake tract. > Primarily that you 
> a)want the turbo to be running at the lowest PR possible. PR is measured
> on both the compressor inlet and outlet, not by the barometer on the
> wall and the manifold boost gauge.

A.


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