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

[Frank Swygert <farna@xxxxxxx>]

Here's a good on-line dynamic compression ratio calculator --
http://www.rbracing-rsr.com/comprAdvHD.htm

You're correct -- boosting in any form is roughly equivalent to an increase in compression ratio. Boost (turbo or super charger) as well as NOS both increase cylinder pressure, which has the same effect as a normally aspirated engine with no power adder with a high compression ratio. 

In the case of the 4.0L head, it will breath better than a 258, so the charge can come in faster, which should lead to a little more charge coming in, at least at high rpm -- in theory. So cylinder pressure should go up a little -- as you stated, depending on the cam. On paper the static compression ratio will work out to be the same as the combustion chambers are the same size (258 and 4.0L heads) and the bore/stroke are unchanged. Might be 10 psi more cylinder pressure because the efficiency of the 4.0L ports allow more air to be drawn in, closer to the mathematical maximum for that bore and stroke. That should mean that volumetric efficiency improves also...

--------------
Date: Tue, 7 Oct 2008 19:43:33 -0400
From: David Crooks <david.crooks@xxxxxxxxxxxx>

 > 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

-- 
Frank Swygert
Publisher, "American Motors Cars" 
Magazine (AMC)
For all AMC enthusiasts
http://farna.home.att.net/AMC.html
(free download available!)

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