[Audi20V_Kruemmer] FW: [s-cars] I don't understand lowering CR for boost

Trevor Frank tfrank at symyx.com
Thu Mar 11 12:39:47 EST 2004

I will my ignorance with my own observations.

There must be some magic number for compression ratio and boost pressure
for a motor with a gas, to yield the right egt's at X boost with optimum
advance, or more importantly I think the right cylinder pressure peak at
the right crank angle.  I believe that everything else, i.e. timing
related, knock and egt's are just an artifact of this.  So when tuning
and bracketing yourself around egt temps, and knock you are really
trying to infer optimum cylinder pressures at X crank angle ATDC..  So
it is possible I suspect that generically for a given set up you could
say you need a given CR to yield the right EGT's that would result from
an optimized timing and fuel map.  Free up the exhaust and you could
maybe up the CR and keep the same timing and fuel map and egt's. 

>From talking Mihnea, and working with him on chips, I have found and
been told of something with our turbo cars that puzzles me, less advance
=higher egt's, more advance =lower, to a point I suspect if you go far
enough either way both would reverse.  On an NA car from what I recall
more advance =higher egt's/cht's  less =less again to a piont.  To me
this makes sense higher combustion pressures from more advance = higher
egt's, so when you are tuning an NA car you can effectively either lean
out the mixture, advance the timing or put in a hotter plug and to
varied degree's you have a similar outcome.  I wonder that since we have
so little advance, around 20 deg that we are on the side where things
reverse, i.e. we always's have some degree of incomplete combustion, so
egt's are an artifact of the combustion cycle completeing outside the
motor, that way more advance means more complete combustion so less
external combustion so lower egt's?  any ideas. .  I guess this would
explain why turbo motors like so much fuel.

  I really should read up on this before I expose my ignorance to the

-----Original Message-----
From: MLP [mailto:mlped at qwest.net]
Sent: Thursday, March 11, 2004 7:50 AM
To: s-car-list at audifans.com; CaptMagu at aol.com; richard at tanimuras.com;
Audi20V_Kruemmer at yahoogroups.com
Subject: [Audi20V_Kruemmer] FW: [s-cars] I don't understand lowering CR
for boost

Well, Ok, this ought to expose my ignorance in trying to work
to justify the faith / conventional wisdom for arguing on should
dropping stock CR's slightly for added boosted engine performance.  What
you think of this approach:

                CR                MaxBoost           Effective CR (the
applets say)
Stock        9.3 to 1          20.0  psi   ~~~>  @ 21.95 to 1

I know everyone is/wants to run more, in fact a lot more than a paltry
20psi, but just for arguments sake, lets suppose the above represented
practically knock/no-knock limits of your fuel/motor.  Can one then
that, to get to the same CR limit if you started off with, oh for

               8.0 to 1          20.0 psi   ~~~>   @ 18.88 to 1
               8.0   "            25.6 psi   ~~~>    @ 21.93 to 1

So, with an 8.0 CR, and an assumed VE of say @ 85% "fill" and ignoring
the moment (lets say you just have an awesomely powerfully efficient
any significant temperature differences at the intake manifold in the
arriving 20psi in the 9.3 CR motor vs. temperature of the arriving 25.6
for the 8 to 1 CR motor, then

in Case #1, for the 9.3 to 1 engine, you get to jam some 378 cc's (being
85% of a swept cylinder volume of 445 cc's) of 20 psi charge vs. Case
378 cc's of 25.6 psi air/fuel in the 8 to 1 motor.  Somewhere there's
got to
be a conversion for psi of air to mass or lbs/air, and assuming the
temperatures are roughly equivalent, there's got to be a lot more
combustible O2 in the 26psi air charge vs. the 20psi charge on a cc for

Alternatively, one could play with and compare the Ray Hall boost applet
results at http://www.turbofast.com.au/TFcompB.html  which suggests that
run 20 psi of boost in a motor with a standard CR of 9.3 to one (81mm
86.4mm stroke & a VE of 85%) the applet suggests one should be using
octane gas.  With a standard CR of 8 to one, at 20 psi the octane
recommendation drops to 110 gas.  One has to push the boost up to 26+
psi in
the 8 to 1 Cr motor to get the anti detonation gas requirement back up
the 9.3 to 1 @ 20 psi's 118 recommendation.   Isn't 110 octane vs. 118
another way of looking at ignition advance?   So again, with the 8 to 1
one gets to run an additional 6psi of boost vs. the stock 9.3 to 1 CR

So, assuming you get your chip map(s) adjusted to your revised base CR,
get to run a lot more boost, with perhaps a bit more advance to make up
power lost in off boost, naturally aspirated mode by dropping the
compression ratio?  My guess is the major re-mapping must be in the
timing area of the ECU's triumvirate, boost v. timing v fuel at

Bob P.?
Anyone else got an opinion?


-----Original Message-----
From: CaptMagu at aol.com [mailto:CaptMagu at aol.com]
Sent: Tuesday, March 09, 2004 8:11 PM
To: mlped at qwest.net; richard at tanimuras.com; s-car-list at audifans.com
Subject: Re: [s-cars] I don't understand


I want the reeeeaaaallllyyyy long answer.

Hap, dreamin in Evahboost, Maguire

[Non-text portions of this message have been removed]

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