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compression ratio

3

36 A

Member
I'm putting together a 39 B john deere for my 11 year old boy . I have stock stroke 5.5 , 4.625 flat top pistons , and the block is in the tractor . How do I CC the top of the block to figure the ratio . piston seems to be about 13/16 " from the top of the block . I know I need the thickness of the head gasket . All this to figure out how long to make my rods . What is a good ratio 8 to 1 ? just a toy plow day tractor and might take it to a pull just for the heck of it once . any ideas ?? thanks mark
 
enter the information on this calculator make your changes and you'll have what you need . start with your known size and cr .from the nebraska test on your tractor .


http://www.race-cars.net/calculators/compression_calculator.html
 
(quoted from post at 16:53:29 08/13/14) I'm putting together a 39 B john deere for my 11 year old boy . I have stock stroke 5.5 , 4.625 flat top pistons , and the block is in the tractor . How do I CC the top of the block to figure the ratio . piston seems to be about 13/16 " from the top of the block . I know I need the thickness of the head gasket . All this to figure out how long to make my rods . What is a good ratio 8 to 1 ? just a toy plow day tractor and might take it to a pull just for the heck of it once . any ideas ?? thanks mark
Click to expand...

If you have flat top pistons then the volume above the piston to the top of the block (deck height) can be calculated. Volume = ((4.625 x 4.625) x 3.14) /4) x deck height. Multiply the result by 16.387 to convert to CCs. Add this to the CCs of the cylinder head chamber and head gasket; This is the number to enter for chamber volume in the compression calculator. See dblair post.
 
I think the 39-40 have a flathead been awhile since I have worked on one. If you have 13/16 from piston to end of block then it is .8125 from piston to top of block. Figure .060 for gasket totals up to .8725 add 5.5 then divide by .8725 you get 7.3:1 compression ratio. Take off .050 off block and you get 7.7:1 CR. Getting compression to high and you start having to mix high octane with super unleaded. Lynn
 
The piston is 1"3/16 from the top of the block . And yes I don"t want to be running racing fuel . So some thing 87 oc rating friendly .
 
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