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

[John Petersen]

>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

All of the turbo cars I have worked on have had the ECUs control boost,
and none of them have had pre-turbo pressure detection systems involved.
If you set a car to 2 bar absolute of boost, yes it would be a PR of 2
if there is no restriction involved, and 3 if you were around .66 bar
absolute at the mouth of the turbo. If you covered the inlet entirely
the turbo would spin really really fast for a moment, and then blow to
pieces, as they are fragile creatures in the first place, and
centrifugal force acts on them very strongly. 

To run at that increased PR (with the same CFM) you are going to be
using a lot more exhaust energy (the wastegate will remain fully closed
much longer).

If, however, a car was setup so that the PR was monitored, you would
definitely be correct, I just don't know of a situation in which this is
the case.

-John

-----Original Message-----
From: diy_efi-admin at diy-efi.org [mailto:diy_efi-admin at diy-efi.org] On
Behalf Of Andris Skulte
Sent: Thursday, November 21, 2002 3:46 PM
To: diy_efi at diy-efi.org
Cc: 'Geddes, Brian J'
Subject: RE: [Diy_efi] New project- intake effect on VE on turbo car

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. 
(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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