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

[serge]

I doubt there is any extra power to be found lowering CR and cranking boost
using formulas. As the saying goes, the engine is just an air pump. Take
some fuel, mix it with air, squash it all down, and fire in the hole.
Compress the air more with the turbo (26psi) and then less (8.0CR) with the
piston, or compress less with the turbo (20psi) and more with the piston
(9.3CR), it becomes a wash in concept. You are still trying to cram as much
air/fuel as possible into the cylinder up to the point of preignition.

So which scenario will have less tendency to preignite? Lowering the CR will
lower the engine's overall tendency to preignite fuel (all other things
being equal), and allow the Motronic to have more control (you can adjust
timing advance with software, CR is not adjustable, aside from that Saab
thing). Avoiding preignition as much as possible, will keep combustion temps
in check, which may allow more boost more often in the higher RPM range.

I guess I'm saying the same thing as Rich WRT squish area. Easier to get a
more complete burn in a slightly more open squish (noun).


Serge Filanovsky

95 S6 Avant

 



> 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
> 
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