[s-cars] Question about compression ratios and tuning - More 20vt

[Mihnea Cotet]

Of course Mike, but if you assume (wrongly, I agree) that a motor has lower 
CR than it actually has, static compression ratio at a given boost level 
will affect dynamic CR. I mean, here's an example:

I've tuned several dozens of RS2s with 1.8 Bar relative overboost and 1.1 
boost at 7K. I have a tried and trusted boost/WGFV map for that so I'm 
using it as a baseline on almost all RS2-spec cars.

Now suppose I have to do fully RS2-ed ABY S2, and the owner is assuming he 
has the metal head gasket but it doesn't actually have it. We start off 
with the stock RS2 fuel/timing chip loaded in the emulator, we do several 
WOT runs doing mods on the fly, monitoring boost levels/EGTs/Wideband O2 
and so on. 2 hours or so later, the chip is ready, no knocking is present 
even when accelerating at WOT in 6th gear from 1000 RPM all the way to 7k, 
all the tests are ok so it's ready to be burnt.

Now if I take a look at the timing advance map in the WOT areas and compare 
it to one of the RS2s I've done before, I'll find out between 2 and 4 
degrees less advance in that particular ABY chip than in the RS2 chip. Now 
if I were to ship "off the shelf" RS2 chips for RS2-ed cars abroad assuming 
all AANs/ABYs had a metal gasket from the factory, this could lead to 
serious HP losses in the higher gears on the cars that aren't equipped with 
the metal gasket, and I've already BTDT actually.... but of course, when 
I'm tuning all cars specifically in real-time, static or even dynamic CR 
doesn't matter so much, agreed with you....

That's just one example out of a few dozens I've already had, and no, not 
all AANs/ABYs had the metal gasket from the factory, at least not before 
1994 I think, just because the first car on which this gasket has been 
installed was the RS2 AFAIK....

HTH,

Mihnea

At 08:52 3/02/2004 -0700, M Pederson wrote:
>For tuning purposes, isn't your static CR some what irrelevant?
>You are going to be more affected by the dynamic compression ratio's which
>as I understand, are substantially affected by cam profiles (granted this is
>probably a pretty fixed constant in most of the motors you will see) and
>volumetric efficiency which while one might assume would be fairly constant
>and consistent from factory conditioned motor to motor, might begin to vary
>more than a little in motors with rebuilt heads and valve jobs of varying
>skill and quality.
>
>On a running motor, it sounds to me like you would be tuning for optimal
>timing based on the motors dynamic, not static compression ratio.
>
>Mike
>
> > -----Original Message-----
> > From: Mihnea Cotet [mailto:mik at info.fundp.ac.be]
> >
> > Nope, ADU is 9:1, I can't remember where I've read it but I'm sure it has.
> > My previous example of an ABY running everything ADU except for the head
> > gasket and having to run a few degrees less timing advance is
> > self-speaking, I've BTDT several times, on ABYs and AANs.
> >
> > HTH,
> >
> > Mihnea