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Re: Compression check

To: rgb@exact.com, fot@autox.team.net
Subject: Re: Compression check
From: Malaboge@aol.com
Date: Sun, 14 Sep 1997 13:31:45 -0400 (EDT)
In a message dated 97-09-13 20:22:51 EDT, you write:

>
>Hummmm
>
>I would be tempted to think that the dynamic compression, i.e. run with
>all the plugs out, throttle open (I wedge the SU pistons open as well),
>starter turning about 1,500 RPM, would be a excellent way to evaluate the
>effective engine's compression (and octane required).
>
>For instance, the static compression means little without a cam profile
>and some means to calculate overlap's effect on the engine, BUT a
>compression test would take this into account.  Not quite the same as
>a running engine, but certainly a much better approximation than any
>other bench calculation.
>
>Is this true?  170 psi the limit for street gas...? 
>
>Roger
>

Roger-

Oh, the temptations...
Included here is a little missive I have previously posted (sorry for those
who have read this before...but that's what the delete button is for) <g>
Your idea of figuring overlap into your theory might work at one distinct
rpm, but unfortunately, not consistantly. The big problem, as noted below, is
reversion due to the charge being pumped back out of the intake. The
variables affecting this problem are governed by more than cam profile
(intake length, diameter, port size, chamber design, temperature,
straightness of intake system, valve size, etc etc) and any computation would
have to take all of these into consideration and if sucessful, would yield an
answer which would be good at only one specified rpm, as the reversion
phenomenon changes drastically with rpm, and my guess is that is does not
change linearly.
Unfortunately, depending on the cam, the reversion factor is still at work
until much higher rpms than a starter motor could achieve.

Also, some of the newer designs on a few cars (Jag H.E. and some Hondas, I
believe) are running fairly high compression ratios compared to other
designs, and are all running on pump gas with no ill effects.


                                 The old post follows...
>>
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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