On Jan 20, 10:36*am, jan olieslagers
wrote:

What's all this with pulling the whole engine apart just to measure
compression?
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You are confusing dynamic pressure with ...can't remember the term...
static or geometric ...compression.

Dynamic compression is for a running engine and is similar to a leak-
down test. It gives you a measure of merit to compare future
measurements against.

Static compression ratio is based on the mechanical dimensions of the
parts built into the engine. Racing class, fuel availability and a
few dozen other things are used to determine the optimum mechanical
compression ratio.

But when you get an engine for which the mechanical compression ratio
is unknown, you need to know it. And that calls for dropping the
engine, pulling the heads and doing a few measurements. This is
especially true when replacing head. I the customer is not sure, you
need to remove BOTH heads in order to verify the chamber volume does
not vary from one bank to the other.

Dynamic compression is supposed to reflect the chamber pressure in a
running engine, typically one that is broken in... and usually just
won a race. The fellow who came in second (or last :-) tells the
judges he thinks the other guy's engine does not match the spec for
that class or whatever. So they can do a dynamic compression test or
-- for a hefty fee -- have the entire engine torn down by the
proctors. (If nothing out-of-class is found, the protester just lost
his $2500 (or whatever). But if they find ANYTHING out of spec in the
winning engine, it's enough to disqualify the winner, who gets his
engine back in a card-board box, and the guy filing the protest gets
to keep his money.)

But if you think about it for a minute you'll see that dynamic
compression has very little to do with the engine's mechanical
compression ratio, which can only be determined ACCURATELY by tearing
it down and measuring it.

-R.S.Hoover