Re: [Amc-list] Compression Ratio: Static, Dynamic

[Greg Taylor <amundaza@xxxxxxxxx>]

Dave,
You can see the difference the cam timing makes on dynamic compression by inputing your engine specs at http://kb-silvolite.com/calc.php?action=comp ;... shows the static vs. dynamic compression ratios.  This has been helpful in using it to determine the camshaft I'll us in my 304 stroker build, so that I can make an 10.25:1 (now 11:1) motor run like a 12:1 motor on E85.  E85 can run a much milder cam than gas, which allows the dynamic ratio to be brought up higher.

Sincerely, 
Greg Taylor :) <>< 
Rochester Hills, MI
1989 Jeep Grand Wagoneer "TRMN8R2"

And the testimony is this, that God has given us eternal life, and this life is in His Son. He who has the Son has the life; he who does not have the Son of God does not have the life. - 1 John 5:11-12


--- On Tue, 10/7/08, David Crooks <david.crooks@xxxxxxxxxxxx> wrote:

From: David Crooks <david.crooks@xxxxxxxxxxxx>
Subject: [Amc-list] Compression Ratio: Static, Dynamic
To: amc-list@xxxxxxxxxxxx
Date: Tuesday, October 7, 2008, 7:43 PM

 > I'm not arguing that the 4.0 head isn't a good swap,
 > just that without changing the cylinder volume, you are not
 > altering the engine's compression ratio.

 > Compression ratio is a measurement comparing the cylinder
 > volume with the piston at BDC vs TDC. The only way to change
 > that is to physically alter the size of the cylinder.

Static compression ratio is the ratio between total volume at BDC
and TDC as you say, but Dynamic compression ratio is a bit different,
and much harder to calculate...  For one thing, you have to take
valve timing into account.  If the intake valve is still open after
BDC (which it commonly is) then you need to consider
where the piston actually is when the valve closes.

I thought that Dynamic compression also included Volumetric
Efficiency, making it a comparison between cylinder pressure
just as the Intake closes, and at TDC.  This would make it a
really good indicator of power potential.  After all, an engine
with a higher VE will achieve higher final cylinder pressures,
for a given compression ratio.

Unfortunately, I can't find a reference to back up my
understanding!  and when I think of it some more, it occurs
to me that the same argument could be made to say a
turbo motor has a really high compression ratio...  So
I guess VE doesn't actually enter into the dynamic
compression ratio calculation, even though it does have
a big influence on final cylinder pressures.....

Cheers,   Dave
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