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Re: highest compression on a cis e




Forget the shape of the pistons - we are (at least we were) talking
engines of equal *displacement* being filled with the SAME AMOUNT of air
(and hence the same amount of fuel,) regardless of compression ratio.  
That's because the swept cylinder volume is the same - regardless of
wether the combustion chamber's in the head or in the piston.)

Unless you have a turbo/supercharger pushing extra air into the combustion
chamber, atmospheric pressure will try to push the same amount of air into
a 1.8 8.5:1 engine as it will a 1.8 10:1 engine.  The factors that affect
this push are NOT related to compression ratio, but rather intake path (ie
throttle,) valve surface area, exhaust path, engine rpm etc.

The difference with a higher compression engine is into how tight of a
space (and thus to how high of a temperature) the intake charge is
squeezed - it's the definition of compression ratio.  In an 8.5:1 1.8L
engine, the intake charge is squeezed to 1/8.5 of its original 1.8L
volume, in a 10:1 1.8L engine, the intake charge is squeezed to 1/10th of
its original 1.8L volume (assuming 100% cylinder filling, of course.)
Given equal intake paths, valve surface area, throttle plate position,
engine rpm and exhaust path, an 8.5:1 1.8L engine would have the same
volume of intake charge (air/fuel) in it as a 10:1 1.8L engine.
Atmospheric pressure will NOT push more air into the 10:1 engine simply
because it "knows" the engine will be squeezing it harder before it gets
set on fire.

To make a long story short, compression ratio does NOT affect the intake
charge until after it's in the combustion chamber and the valves are
closed.

- Drew (whose experience in compression ratios comes from the diesel
world, where pistons stick out ABOVE the deck at TDC)

On Fri, 4 May 2001, Chris DeLong wrote:

> Just to play the advocate here.... What you are saying is that a flat piston 
> working in a flat combustion chamber will not suck any more air than a 
> piston or combustion chamber with a huge dish in it? I think not. If there 
> is no room in the combustion chamber or piston top for air then it will have 
> to suck more air in to create a stoich ratio for burning fuel.
> 
> Bottom line is that higher compression will increase air flow. No way around 
> it. The engine is a pump; the more "free" space you give it (I.E. lower 
> compression) the less air it will flow (unless it is forced in, I.E. Turbo 
> or s/c). I would have to say that Shawn is closer to reality than you when 
> it comes to air flow and high compression engines.
> 
> This of course is all in my opinion and experience from working on VW's for 
> quite a while for a living.
> 
> I make it my problem to do high performance shit.
> 
> 
> >
> >Absolute Bull. Increasing the compression ratio has nothing to do with
> >mass air flow. How hard you squeeze what's in the combustion chamber has
> >nothing to do with how much air gets in in the first place. Total
> >pressure drop across the intake system, valve size, lift and duration,
> >engine displacement and intake air temperature will determine mass air
> >flow. CR may have some secondary or tertiary effect that is just as
> >likely to decrease mass flow as to increase it.  The amount of fuel
> >required also will not change, except for the possibility of running a
> >little richer to forstall the onset of detonation.
> >
> >Shawn Rader wrote:
> > >
> >Raising the compression has roughly the same effect as
> > > increasing the engines displacement - it increases the amount of air 
> >your
> > > engine flows.  With more air you can burn more fuel which means more 
> >power.
> > > By raising the compression from 11:1 to 12:1 your engine is flowing 9% 
> >more
> > > air at any given rpm- about the same as increasing you engine 
> >displacement
> > > from 2.1L to 2.3L.
> > >
> > > -shawn
> > > '81 S
> > >
> > > On Thu, 3 May 2001 08:35:05 -0400, J Miller wrote:
> > >
> > > >  i am courious what the highest compression rate that the cis e  
> >injection
> > > on
> > > >  a 1.8 16 could handle,
> > > >  i know 11:1 works fine but what above 12:1
> > > >  i need a car and i am just goign to slap the 2.1L in with the stock 
> >EMS
> > > and
> > > >  a few other minor mods
> > > >  Hey mr rust bucket i need an adjustable moduale
> > > >  jonas miller
> > > >  1976 2.0L 8v  scirocco   for sale
> > > >  1987 soon to be a monster 2.1L
> > > >  fzr 600 bleediun carbs
> > > >
> > > >
> > > >
> > > >  --
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> 
> 
> Chris DeLong
> ICQ#45030867
> Seattle, WA USA
> '80 16V 'Rocco ragtop
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> 
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Drew MacPherson, Network Analyst, University of Guelph

drew@dyermaker.cs.uoguelph.ca           |  visit the Massey-Harris page:
http://dyermaker.cs.uoguelph.ca/~drew   |  http://m-h.cs.uoguelph.ca


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