[s-cars] Bottom-End...

[Kirby Smith]

I agree.  With a lower compression ratio, the total mass of fuel/air
mixture at a given pressure/temperature just below detonation is
greater.  This can work to the advantage of the engine designer, if he
doesn't care about efficiency.  The torque vs rpm curve may not be as
flat.

kirby


Charlie Smith wrote:
> 
> 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
> power.
> 
> 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 limit,
> 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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