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Compression Test

To: Tiger List <tigers@autox.team.net>
Subject: Compression Test
From: Bob Palmer <rpalmer@ames.ucsd.edu>
Date: Tue, 17 Aug 1999 22:13:53 -0700
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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