From: cornutt@lambda.msfc.nasa.gov (David Cornutt)
Subject: Re: Chevy A/B catch up with FORD?!
Keywords: Engines, Indy Cars, Tech. 
Organization: NASA/MSFC
Date: Tue, 1 Sep 1992 13:41:38 GMT

yancm@caen.engin.umich.edu (Eugene Harold Mcnay Ii) writes:

>The intake plenum modification - guesses about engine technology.
>I was at a talk in Detroit where one of the designers of the 
>Chevy A was talking very generally about what made the Chevy/Ilmore
>more potent than what was available to date (1989). Aparently
>they had designed the intake plenum such that under operating
>conditions, standing pressure waves (nodes) were set up to coincide with
>the intake valves. The net effect of this was to get better
>efficiency ( ie more real boost than the static plenum pressure -
>the edge). Was this conciously done before this engine? 

Oh, yes.  My understanding is that the idea has been around since the
'40s, and was used on some airplane engines of WWII vintage.  However,
it's apparently extremely difficult to make it work properly simply
by trial-and-error, and so has not been used much in production
until recently (modern CAD systems having made it possible to test
designs without actually building them).  I recall Carroll Shelby
was doing some work with Chrysler, circa 1985, along these lines,
with an idea towards putting it into production, and I believe that
some of the current Dodge Daytona and Ford SHO pieces are based on
this work.  (I'm sure that foreign manufacturers are doing it too;
I just can't think of any specific pieces right off hand.)

>That's
>why I'm trying to start this thread! Anyway, here is a wild guess
>as to why I think the FORD was more powerful. They used the standing
>pressure wave trick and somehow managed to either find a better
>location for the induction of turbo air and or located a low pressure
>wave at the intake to the pop-off valve. Here lack-o-knowledge
>hurts, because I don't know if teh pop-off valve is before the
>plenum or mounted off of it - if anyone knows fore sure, I'd
>like to hear... 

And a good thread it is.  I believe that the valve is mounted on the
fitting between the turbo outlet (compressor side, that is) and the
plenum; there's a T-piece where it mounts.  I'm pretty sure that there
is a rule that specifies a maximum distance between the compressor
outlet and the valve.  (For CART gurus, here's something I don't
understand: the popoff valve doesn't appear to cooperate in any
fashion with the wastegate.  So, how then do they prevent the turbo
from overspeeding when the popoff valve opens?  Anyway...)

There are actually two related, but distinct, things you could do with
manifold tuning here.  The first, as mentioned above, is tuning the
characteristics of the intake runners so the "impedence" (to use an
electrical term) is minimized within the desired RPM band, which
maximizes airflow within that band.  The second, more sneaky, thing
is what Gene hints at, which is creating an artificial low-pressure
standing wave region around the popoff valve, which would allow the
actual inlet pressure at the intake valves to exceed the popoff valve
setting, possibly by 3-4".  NASCAR has been fighting an uphill battle
against this same sort of thing for several years now.  Over there,
they have been using the idea in reverse, tuning intakes so that a
high-pressure standing wave extends through the restrictor plate
holes, yeilding more airflow than a simple pressure-and-hole-size
calculation would indicate.  (Back when the plates were just pieces
of stamped sheet metal, with a thickness of maybe .050", the smart
guys had the intake tuning to the point where it didn't matter if the
plate was there or not!  That's why the "plate" is now a 1" thick
piece machined from a billet, and NASCAR has a mandated "no-touch"
zone in the plenum immediately below the plate.)

>I tend to think it was something like this
>because even teh ESPN anchors picked up teh term plenum as being
>the key mod...

Yep, and this is probably why CART is now working on a "smart"
popoff valve.  They haven't said so, but one of the things the valve
is probably going to do is try to detect these sort of things.

-- 
David Cornutt, New Technology Inc., Huntsville, AL  (205) 461-4517
(cornutt@lambda.msfc.nasa.gov; some insane route applies)
"The opinions expressed herein are not necessarily those of my employer,
not necessarily mine, and probably not necessary."
Newsgroups: rec.autos.sport