Pat Bailey wrote:
> Do you take compression tests at temp or cold?I was working on my B and
> decided to
> check the compression it was #1 145 #2 140 # 3 145 #4 145 but this was
> cold should I take it again hot?
...and Bob Allen replied:
>Yep, motor should be warm, throttle open, jam something under the
>dampner pistons to admit lots of air. It's good to see even pressures
>across the board but those numbers aren't much more than 7 to 1
>compression.
This raises a question I have scratched my head over a time or
two.
(First I think Bob has misplaced his calculator)
The question is what pressure should be seen for a given
compression ratio?
Suppose the compression ratio is 10:1. I assume that means
the compressed volume (TDC) is 0.1 of the uncompressed (BDC) volume.
If so, the ideal gas law says the pressure is 10 times the original
pressure. The original pressure (absolute) is 14.7 psi, so the
pressure at 10:1 is 147 psi (absolute) so the gage, which reads
"gage" pressure should say (147-14.7)=132psi. If all that logic
is correct then the compression gage reading (gage pressure)
will covert to compression ratio as follows:
CR=(Pgage+14.7)/14.7
This is different from what I intuitively though it should
be,( CR=Pgage/14.7psi ), so I'm puzzled.
Either way, the numbers Pat posted are more like 11:1 than 7:1.
I just ran a compression test on a Spitfire 1500 cc engine
that I'm negotiating for. Readings (cold) are (117,117,110,85).
(without propping open the throttle, so these are low).
(Obviously a problem in #4, I suspect rings, and retesting
with oil in the cylinder raised the pressure)
Back to the lecture; using the equation above, this means the
CR (in the good cylinders) is about (117+14.7)/14.7 = 8.9:1
I think this engine is supposed to be 8.9 CR.
How have I gotten so close to the "right" answer
without even propping open the throttle?
Is my theory all wrong? (been a long time since thermodynamics!)
Is my (new Sears Crafstman) gage inaccurate?
Is it not necessary to prop open the throttle?
I let the engine crank through 3-4 compression cycles for
each measurement, until the max. pressure stabilized. Is
this the wrong procedure?
Maybe this engine has old, flat-top pistons, and a higher CR...
Thanks,
Ray
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Ray W. James, P.E., Ph.D.
Texas Transportation Institute
Civil Engineering Department
Texas A&M University
College Station, TX 77843
Phone (409)845-7436; Fax (409)845-3410
E-mail: r-james@tamu.edu
Date: 9/12/97 Time: 8:27:43 AM
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