In a message dated 97-04-04 16:27:26 EST, you write:
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> Note: The current head shows about 210# of pressure in each cylinder and
> I have a "calculated" CR of 10:8:1. Does any one have any benchmark
> correlations between the # pressures & Compression Ratios?
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Well Guys...its the semi-factual Nick, with another OPINION...
The Premise:
There is NO relationship between the pounds per square in of
pressure noted on a compression guage, and the actual compression ratio.
The Arguement:
The camshaft is the major governing body on the reading. Think about
it this way...Lets start with a stock motor with a weenie cam with minimal
overlap. Lets say the intake closes at bottom dead center, so the cylinder is
closed as the piston starts back up the bore and at 150 rpm (starter
speed-more or less), the resistance to gas flow thru the intake is minimal
anough to allow complete filing of the cylinder to atmospheric pressure. So,
as the piston rises in the bore, you have a maximum amount of gas being
compressed the maximum amount.
Now lats take that same motor and put a boy-racer cam in that puppy. This cam
is gonna have mondo overlap. Now lets say the intake valve doesn't close til
45 degrees after bottom dead center (yeah, I know thats extreme, but hey, I'm
tryin to make a point here). Now the intake valve doesn't close til the
piston is almost 1/4 of the way up the bore on the compression stroke. Again
assume no resistance in the intake system...where do you think the gas in the
cylinder is gonna go as the piston starts back up ? Yep, right back out of
the intake. We've all seen major amounts of fuel standoff on our racers at
low rpm's - thats the piston pushin the stuff right back out the intake. So,
now back to our scenario...the cylinder now has only about 3/4 as much
compressible gas in the bore when the valve finally shuts. What do you think
that's gonna do to the readings on our Pep Boys compression tester ?
Think about it !
As usual, I will stand firm on this theory - at least until somebody with a
bunch of letters after his name convinces me I'm an idiot
Nick in Nor Cal
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