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#12434 07/20/2004 1:54 PM | Joined: Dec 2000 Posts: 506 Shop Shark | | Shop Shark Joined: Dec 2000 Posts: 506 | Anybody know an easy formula to figgure the compression ratio if you know the cranking psi guage pressure reading? I've heard maybe incorrectly that if you have say a 160 psi guage pressure reading divided by 15 will equal compression ratio?
"Drugs are money, money is power, power is security." - Kent Heiner
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#12435 07/20/2004 4:47 PM | Joined: Feb 2000 Posts: 985 'Bolter | | 'Bolter Joined: Feb 2000 Posts: 985 | I dont think that is correct. I may be wrong but I have always thought that compression ratio is the difference between the volume of the piston at top dead center and bottom dead center. Now like I said I might be wrong but I doubt that you can make a correlation between psi and compression ration.
Brian Moore
1949 3100 5 window Deluxe
"Today is better than yesterday, but not as good as tomorrow"
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#12436 07/20/2004 5:02 PM | Joined: Jul 2000 Posts: 962 Shop Shark | | Shop Shark Joined: Jul 2000 Posts: 962 | I had a rough formula going that equated cranking PSI to static compression ratio. accurate to within about .5:1 CR for a stock engine. Take PSI divided by atmospheric pressure of 14.7 times volumetric efficiency of engine.
Say 160 divided by 14.7 times 80 % gives a static compression ratio of 8.7:1.
This is a really crude formula, cuz it doesnt take true atmospheric pressure, VE or cam timing (dynamic) factors into account. But its fun seeing how off you are when comparing a stock motor to book spec. As stated, I've come pretty close with my "fuzzy math"
chip
Preaching the Hot Rod Gospel according to the 4-stroke apostles:
Suck, Squish, Fire and Fumes
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#12437 07/20/2004 6:40 PM | Joined: Jun 2000 Posts: 2,773 'Bolter | | 'Bolter Joined: Jun 2000 Posts: 2,773 | Chips fuzzy math is about as close as you will get, there are a lot of variables to figure. The biggest is the actual size of the combustion chamber, which can vary a lot. Only way to really know that is to measure with a liquid. Saw 2 Guys Garage a while back check a head that was off more than 10% from the advertised.
Fred
52 3600
69 C-10
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