[s-cars] Bottom-End...

Trevor Frank tfrank at symyx.com
Wed Jan 14 16:55:41 EST 2004

Practically you may be right but....

Hmm, not sure you are right, Indy car 16:1, turbo drag cars around the
same, so if you have unlimited octane and a cooler combustion due to the
amount of fuel and it's characteristics, i.e. methanol, then you can
have nearly unlimited compression ratio.  I think that even at the
extremes our issues with detonation are really with advance more than
compression ratio.  There may be some magic number for advance to get
the best amount of burn time so that our peak pressure is around the
apparent magic 14 deg atdc.  But at the race track with more octane
avail I suspect that the high compression motor will produce not only
better low end, with better throttle response but also more hp and

That is not to say you should't lower your compression ratio, I just
don't like the blanket statement's that lower is somehow better, but I
could be wrong.

-----Original Message-----
From: Charlie Smith [mailto:charlie at elektro.cmhnet.org]
Sent: Wednesday, January 14, 2004 8:55 AM
To: Joseph Pizzimenti
Cc: Charlie Smith; Audi S Car List
Subject: Re: [s-cars] Bottom-End...

I think you guys are way off in left field.

The last thing you want to do on a turbo engine like ours
where you are interested in getting max (whatever that is)
power out of it is to RAISE the compression ratio.

Raising the CR is a good thing on a naturally asperated
engine.  It helps the engine be a better air pump so it can
suck the last little bit of air into the cylinders - thus more

With a turbo'd engine, where you can easily <?> stuff more air
into the cylinders, you are limited by detonation as to how
much air you can stuff in - because the stuffed in air gets
compressed further in rough proportion to the CR as the piston
gets to TDC. 

Lets call the cylinder pressure at just below the detonation
level of pressure, with the piston at TDC, the "safe" pressure.

Give that the timing is the same for all this discussion.  When you
retard the timing you lose power big time.

So the amount of power is really limited by how much air you can
stuff in and still stay below the "safe" pressure.  Remember, it's
the air with the right amount of fuel that really produces the power.

If you LOWER the CR you will be able to stuff more air in before you
again get back to the safe pressure limit.  Likewise, if you RAISE
the CR you won't be able to stuff as much air in before reaching the
safe pressure limit.


Now, on top of all that, when you put those forged lower CR pistons in,
they are also lighter.  Just like the stronger high performance rods
are lighter.  Lighter reciprocating weights mean a higher safe RPM
and that can be a route to more power given you make other changes
to take advantage of the higher red line.

End of SoapBox. 

    - Charlie

Earlier, Joseph Pizzimenti wrote:
> Do it, you wuss.
> Might want to consult mlp on this one, as you're not
> going to have 10.3:1 or 9.3:1 for your compression
> ratio, but somewhere in between because your mix and
> matching heads/pistons. 
> I would swain tech those pistons, too, and keep boost
> down to 15psi...or go with 8.5:1 pistons?
> Pizzo
> --- Cody Payne <cpayne at bconnected.com> wrote:
> > I see that the 20V 7A is 10.3:1 where the 20VT AAN
> > is 9.3:1
> > http://20v.org/specs.htm
> > http://www.s-cars.org/urs4s6/Tech/tech.html
> >
> > A VW Compression Ratio Article:
> > http://www.tunacan.net/t4/tech/cr.htm
> >
> > So I guess this tells me that if I am running 10.3:1
> > that:
> > -1) May have a detonation issue and need to
> > adjust/retard the timing a
> > bit
> > -2) Heat could be an issue
> > -3) Engine Longevity could be an issue?
> > -4) Increase HP and bottom end torque?
> > -5) More potential then normal to bend stock rods w/
> > above increases?
> >
> >
> > Let the fun begin...
> >
> > cp
> >
> > -----Original Message-----
> > From: mlp qwest [mailto:mlped at qwest.net]
> > Sent: Tuesday, January 13, 2004 10:06 PM
> > To: mlped at qwest.net; Charlie Smith; Cody Payne
> > Cc: Audi S Car List
> > Subject: RE: [s-cars] Bottom-End...
> >
> >
> > Whoops -
> > http://www.sae.org/automag/newenginereview/audi.htm
> >
> > ~-----Original Message-----
> > ~From: s-car On Behalf Of mlp qwest
> >
> > ~FWIW, Audi still seems to be doing something like
> > this with its "low"
> > vs. ~"high" output 1.8t motors.  I.e., the high
> > output apparently runs a
> > CR of ~8.9:1 and a K04 turbo to make 225hp/165kW &
> > 280Nm/207 ft-lb while
> > the "low" ~output 1.8t runs a 9.5:1 CR with the
> > smaller K03 to make
> > 180bhp/132kW and ~235Nm/173 ft-lb. ~ ~Why, lower
> > compression? ~ ~Mike ~
> > ~ ~~-----Original Message-----
> > ~~From: s-car On Behalf Of Charlie Smith
> > ~
> > ~snips
> > ~
> > ~~If you need to rebuild the rest later, you absolutely will .....
> > ~~fitting oversize forged lighter, lower compression, pistons ~
> > ~~    - Charlie
> > ~
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