Re: highest compression on a cis e

["Bradley Peet" <btpeet@effervescent.com>]

At the risk of pissing you off...

>Date: Fri, 04 May 2001 06:56:22 -0000
>From: "Chris DeLong" <green536@hotmail.com>
>Subject: Re: highest compression on a cis e
>
>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?

Yes!  ;)

>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.

The shape of the piston (flat vs dished) doesn't matter very much because
the swept volume will be exactly the same.  If the pison is "dished" then
the bottom of the dish will reach a lower point inside the cylinder at the
bottom of the stroke (compared to the flat piston), so it all evens out.
You might be able to argue that the top of the piston might be shaped to
encourage better "swirl" in the combustion chamber and therefore better
burning but that doesn't equate to airflow.

>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.

I see your point, and that might be valid for EXTREME differences in the
compression ratio but the difference between 8:1 and 10:1 isn't so much that
it would seriously affect airflow (all other things being equal).  The main
thing a smaller combustion chamber (higher CR) would do for you in terms of
air flow through the engine would be to better evacuate or "flush" the
cylinder on the exhaust stroke.  A total lack of residual gases or pressure
would probably induce a little more airflow on the intake stroke, but how
much is questionable.

Well, that's my $0.02 contribution to the peanut gallery.  All you Einsteins
out there, feel free to prove me wrong.  :P~

Bradley (yes, another freakin' "Net Racer")
88 Scirocco 16V 2.0 lookin' for it's "T"
96 Passat GLX Wagon


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