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Tractor Pulling Discussion Forum

figuring compression ratio

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AdamA

12-18-2004 15:36:23




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How do you figure the compression ratio? Is there a certain formula to use?




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Marty in Mi.

12-19-2004 18:59:10




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 Re: figuring compression ratio in reply to AdamA, 12-18-2004 15:36:23  
Type in "compression ratio calculator",hit enter, then go down eight titles till you come to the doverusa site. This one is about the easiest one to use. Marty



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OLD -JOE

12-18-2004 18:05:15




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 Re: figuring compression ratio in reply to AdamA, 12-18-2004 15:36:23  
FIRST FIND THE SWEEP VOLUME
radius x radius x stroke x 3.1416=cu. in. x 16.3872= cc (OF ONE CYL)
THEN YOU WILL NEED TO KNOW how far down from the block deck is the flat of the piston ( not the dish or the dome)
radius x radius x distant down x 3.1416=cu. in. x 16.3872= cc
THEN YOU WILL NEED TO KNOW YOUR HEAD VOLUME ( you will need to have the head off then you can use a large syringe to fill the combustion chamber to find the cubic centimeters
THEN YOU WILL NEED TO KNOW YOUR HEAD GASKET VOLUME
radius x radius x thickness x 3.1416=cu. in. x 16.3872= cc THIS IS WHAT YOU NEED IF YOU HAVE A FLAT TOP PISTON ( if you have a dished piston you need the cc of the dish OR if you have a dome piston you need the cc of the dome. EXAMPLE #1 JD 70 gas stock 2.9375 x 2.9375 x 7.000= 60.40 x 3.1416= 189.76cu in x 16.3872= 3109cc

deck cc 215 head cc 285 gasket cc 24
piston dish cc 110
----- ----- ----- ----- ----- total 635
sweep cc 3109 plus 635 divided by 635 = 5.8960 to 1 comp ratio

EXAMPLE #2
IF YOU HAVE A FLAT TOP PISTON IT WOULD LOOK LIKE THIS deck cc 215 head cc 285 gasket cc 24
piston cc 0
----- ----- ----- ----- ----- total 524
sweep cc 3109 plus 524 divided by 524 = 6.9332 to 1 comp ratio

EXAMPLE #3
IF YOU HAVE A DOME PISTON IT WOULD LOOK LIKE THIS deck cc 215 head cc 285 gasket cc 24
piston DOME cc -110
----- ----- ----- ----- ----- total 414
sweep cc 3109 plus 414 divided by 414 = 8.5096 to 1 comp ratio

Hope this helps

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buickanddeere

12-18-2004 18:03:36




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 Re: figuring compression ratio in reply to AdamA, 12-18-2004 15:36:23  
While compression ratio is important. The cam timing and volumetric efficiency at various rpms will vary the compression pressure dramatically.



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DMont

12-18-2004 16:21:09




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 Re: figuring compression ratio in reply to AdamA, 12-18-2004 15:36:23  
I should have mentioned that the formula for the volume of the cylinder at BDC is Pi x r squared x stroke equals the area of the cylinder in cubic inches. There does exist a tool that is filled with a liquid and that is injected into the plug hole and the amount of fluid injected is measured in cc's. You would then need to know the cc.s of the cobustion chamber to get the ratio. I have taken a head, put a plexiglass sealed with oil over the combustion chamber with a small hole to add fluid to the chamber. You can buy a large cc liquid injector that will give the cc's that the chamber holds. Probably a very simple method that I am hot privy to from someone else. DM

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DMont

12-18-2004 15:55:01




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 Re: figuring compression ratio in reply to AdamA, 12-18-2004 15:36:23  
Compression ratio is the volume of the combuston chamber at TDC compared to the volume at BDC. You can fill the combustion chamber of the head and measure the cc.s and then compare that to the
cc's of the cylinder volume at piston bottom dead center.



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