Exhaust temps...

Tue Mar 18 03:04:04 GMT 1997

In regards to the "cheaper nascar engines" with 9:1 compression.  Are you 
sure?  Last I heard they were running 18:1 to compensate for a vacuum 
condition caused in the intake at high RPMs from the restrictors.  Am I 
wrong?

Russ Highton Jr.

----------
From: 	Matthew Beaubien[SMTP:mbeaubie at gpu.srv.ualberta.ca]
Sent: 	Monday, March 17, 1997 8:54 AM
To: 	diy_efi at coulomb.eng.ohio-state.edu
Subject: 	Re: Exhaust temps...

Frank,

> I saw similar 1600 deg temps in a custom turbo Honda vtec I did BUT
> I found the cause was high back pressure from restrictive exhaust and
> cat. At only 5 psi of boost, I had 9 1/2 # of back pressure. When I went
> to bigger dia exhaust and bigger cat, the back pressure went down to 2
> 1/2 psi and boost rose to 9 psi at same wastegate setting. Temps dropped
> 150 deg.

Was this before or after the turbo? It is not-uncommon to have 2x as much
pressure before the turbo as what is in the manifold (ie. 10 psi boost, 20
psi before the turbo turbine), even with a good exhaust system.

In the latest R&T, there's a little article on the Saab top speed
marathon. They mention that EGT's peaked at 1832F or something close to
that... All SAE write-ups of factory turbo engines say they like to keep
the turbo turbine inlet temps to less than 1650F as per Garrett's
recommendations.

> In tests I have done on factory GM race cars with very high ( 16/1)
> compression, exhaust temps are only 1200 deg cause of the better
> combustion.

Higher compression engines have a higher efficiency, and as such, they
reject less heat into the exhaust gasses.

Those "cheaper" 9:1 NASCAR engines are very hard on valves due to their
low CR.

Matt Beaubien
mbeaubie at gpu.srv.ualberta.ca

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