There is no way to accurately assign a compression ratio based on
compression tests.
The compression ratio is the ratio of the cylinder Volume + comubstion
chamber volume + piston dish volume + gasket opening volume at BDC versus
all the above at TDC. Pressure does not come into it.
Extreme example. If we have a cylinder with a calculated compression
ratio of 10:1 and let us suppose the compression test yields, say,
150psi. Now, remove the piston rings.
The mechanical volume both at BDC and TDC is unchanged. But you can bet
your bippy the pressure went down like a homesick bass.
Another less extreme example. Some racing engines with LARGE
compression ratios (13-15 to 1) will have terrible cranking pressures,
compared to a street car, primarily due to the large amout of valve
overlap. Yet the high c/r's remain.
Too many factors affect compression pressures to definitively say that
XXXpsi is equal to YY to one compression ratio.
Just my £.02 worth
Rick Morrison
72 MGBGT
74 Midget
On Fri, 12 Sep 97 08:27:43 r-james@tamu.edu writes:
>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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