Bob, interesting observation. My experience has always been higher compression
hot.
The two main reasons that compression pressures can't be calculated from
compression ratio (only estimated) are adiabatic considerations (as you pointed
out) and the camshaft timing. On most cams specs, you'll see that the intake
valve opens before TDC and closes after BDC. This means that at cranking speed,
the first part of the upward stroke is pushing air out the intake, reducing the
compression pressure. This is also why hot cams can allow a higher C/R without
detonation on pump gas. Maybe some of the guys at the SAAC meeting tomorrow
will have some ideas on the hot/cold results.
Craig
Bob Palmer wrote:
> Listers,
>
> OK, all you techies out there, here's a question for you. I just recently
> checked my Tiger's cranking compression cold and it was 175 psig (on all
> cylinders, whoopee!). Earlier, I had checked a couple cylinders with the
> engine hot and I recalled them being about 145 psig. I just double-checked
> the hot cranking compression and, yes, it is 145 psig. Now, I carefully
> measured the factors that determine my engine's compression ratio when I
> built it and I'm sure it's 10.9:1. Based on straight compression ratio,
> here at sea level I would have 14.7 psia x 10.9 = 160.2 psia, or 145.5
> psig. But, there's also the adiabatic compression factor which is the ratio
> of the constant pressure to constant volume specific heats, Cp/Cv. I don't
> recall offhand what this is for air, but based on my measured cold cranking
> compression of 175 psig (189.3 psia), it should be about 1.2. I can check
> what Cp/Cv for air is tomorrow, but this may not be the best empirical
> value to use in this case.
>
> I have two questions. First, I don't believe I've ever noticed this
> difference between cold and hot cranking. Is this typical? Second, if this
> is typical, what's the explanation? Or, if it isn't typical, what could be
> the reason in my case?
>
> I'm sure there are several of you out there that can give me a very
> informed answer, for which I will be very grateful.
>
> Thanks,
>
> Bob
> Robert L. Palmer
> UCSD, Dept. of AMES
> 619-822-1037 (o)
> 760-599-9927 (h)
> rpalmer@ucsd.edu
> rpalmer@cts.com
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