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

[MLP]

Well, Ok, this ought to expose my ignorance in trying to work
ass-backwards to justify the faith / conventional wisdom for arguing on
should consider dropping stock CR's slightly for added boosted engine
performance.  What do you think of this approach:
 
                CR                MaxBoost           Effective CR (the
java 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
the practically knock/no-knock limits of your fuel/motor.  Can one then
assume that, to get to the same CR limit if you started off with, oh for
example, an
 
               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
for the moment (lets say you just have an awesomely powerfully efficient
FMIC)
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
psi 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
#2's 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 cc basis.
 
Alternatively, one could play with and compare the Ray Hall boost applet
results at http://www.turbofast.com.au/TFcompB.html  which suggests that
to run 20 psi of boost in a motor with a standard CR of 9.3 to one (81mm
bore, 86.4mm stroke & a VE of 85%) the applet suggests one should be
using 118.4 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 to 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 CR one gets to run an additional 6psi of boost
vs. the stock 9.3 to 1 CR motor.
 
So, assuming you get your chip map(s) adjusted to your revised base CR,
you get to run a lot more boost, with perhaps a bit more advance to make
up for power lost in off boost, naturally aspirated mode by dropping the
compression ratio?  My guess is the major re-mapping must be in the
ignition timing area of the ECU's triumvirate, boost v. timing v fuel at
rpm/load?  
 
Minhea?  
Bob P.? 
QShipQ?  
Anyone else got an opinion?
 
mlp
 
 
 -----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



Mike   

I want the reeeeaaaallllyyyy long answer.

Hap, dreamin in Evahboost, Maguire