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

[John Petersen]

Quite a can of worms. Here's a hack at it.

Simply put, yes, the intake tract affects VE... but not a heck of a lot.

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. 

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.
b)spinning the turbo fast generates a lot of heat on the intake, while
it might not make a huge difference if you have an enormous intercooler,
it might cause your engine to ping-ping-ping due to excess heat if your
intercooler isn't dreadfully oversized.

Hope this helps
-John

-----Original Message-----
From: diy_efi-admin at diy-efi.org [mailto:diy_efi-admin at diy-efi.org] On
Behalf Of Geddes, Brian J
Sent: Thursday, November 21, 2002 2:41 PM
To: diy_efi at diy-efi.org
Subject: RE: [Diy_efi] New project

On a turbo car, is VE effected by pre-turbo intake path characteristics?
It seems to me that since the turbos are "pushing" the air at the
throttle body, pre-turbo intake wouldn't matter to VE.  But it would
affect maximum boost level...compressor efficiency as opposed to engine
efficiency, I guess.  Is this the way it actually works?

- Brian

> 
> No, it's much more subtle than that.  Volumetric efficiency 
> captures the
> entire path of the air from the atmosphere through the air filter,
> intake ducting, throttle body, plenum, runners, intake 
> manifold, ports,
> valves, cam timing/overlap, etc.  If air was incompressible and
> inviscid, then you would be correct that simply knowing the 
> displacement
> of the cylinder and air's temperature and pressure would be sufficient
> to determine exactly how many air molecules entered the cylinder.  In
> fact this is the definition of 1.0 or 100% VE, the amount of air that
> would enter the cylinder if the air was some ideal fluid that has no
> compressibilty or resistance to flow or other dynamic effects.  But
> that's not the case, and it's not something that you can calculate
> analytically.  You could get a rough initial map by assuming 
> a constant
> VE (of around .7 or so) for all spots but it will certainly need some
> refinement.
> 
> Brian
> 
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