[Amc-list] Dynamic compression ratio (was detonation)

[Frank Swygert <farna@xxxxxxx>]

Okay, this you know: static compression ratio is the ratio between the cylinder volume with the piston at BDC vs. the volume at TDC assuming both valve are closed. Easy! Dynamic compression takes into account that both valves are NEVER closed when an engine is running -- there's always a little "overlap" where both valves are partially open. Since at least one valve is cracked open at TDC the static compression ratio is NEVER achieved in a running engine. In fact, even 11:1 static compression engines actually run at around 8:1 (9:1 tops) dynamic (or "running") compression. Most engines run in the 7-9:1 dynamic range. 

Here's a couple calculators that can figure dynamic compression:
http://www.wallaceracing.com/dynamic-cr.php
http://www.rbracing-rsr.com/comprAdvHD.htm (mainly for Harleys, but this is the one I use most... looks about the same as the other one though). 

You can increase the valve overlap of a cam to reduce dynamic compression. Some of the cylinder pressure is leaked into the exhaust (or is it intake?), reducing cylinder pressure, which has the same effect on timing (and performance) as reducing compression. Some say it's a "band-aid" fix, but I don't buy it. It would be "more proper" to build an engine with the desired static compression, but if you have a high compression engine rebuilding or changing heads can be costly. Changing the cam is much easier/cheaper, but you will be limited on cam choices. 

Tom, have you considered running doubled head gaskets or a thicker copper head gasket? You can easily drop 0.5:1 that way, but I don't know about more. I wouldn't worry much about quench -- that's another non-issue in the long stroke sixes often harped on at the Stroker list. You might get 0.20-0.25 mpg more and 2-5 hp more if it's right in the best quench range, but at a cost of $1500 or so in machine work. Neither of those gains would be noticeable. If the compression is in the 8.5-9.0:1 range a Stroker should run fine with the stock cam, much higher and a higher overlap cam will be needed to reduce cylinder pressure. In these engines compression is more important than quench. The design is supposed to be "quench compliant", but a stock 4.0L piston is around 0.080" down in the bore, "desired" quench is supposed to be around 0.040-0.045" down. 

---------------
Date: Wed, 25 Jun 2008 10:23:36 -0700
From: Tom Jennings <tomj@xxxxxxx>

I don't understand that much about camming and static vs. dynamic 
compression, but it sounds like it would be worth figuring out soon!

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

_______________________________________________
Amc-list mailing list
Amc-list@xxxxxxxxxxxx
http://splatter.wps.com/cgi-bin/mailman/listinfo/amc-list