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Old 02-18-2013, 10:53 AM
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how to figure out compression

I getting ready to buy a complete rotating assembly and heads for a project 355 sbc. I want the compression to be between 10 and 11.1, using 64cc chambers. The pistons ive seen are measured in cc. Found a set of domed two valve relef 5cc pistons. Is there a mathmatical way or figuring compression?

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Old 02-18-2013, 11:18 AM
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bore x bore x.7854 x stroke will give you the volume in the cylinder(close enough) do the same with the remaining part of the stroke(the part where the piston does not come to the top of the bore) usually 0 to 30 thou,you have to measure this during the first test fitting.At the same time measure "ALL" piston to wall clearances,and at this time also measure piston to valve at zero and plus and minus 4 cam timing.measure bearing clearance,make all hand fitting adjustments at this time.
use same formula for head gasket,NOTE: bore on head gasket is likely more than the blocks bore,
measure combustion chamber volume with water
add heck height volume,add or subtract volume from dome and or valve reliefs.plus headgasket volume plus cc of combustion chamber,THAT number divided into the cylinder volume
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Old 02-18-2013, 11:24 AM
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Originally Posted by vinniekq2 View Post
bore x bore x.7854 x stroke will give you the volume in the cylinder(close enough) do the same with the remaining part of the stroke(the part where the piston does not come to the top of the bore) usually 0 to 30 thou,you have to measure this during the first test fitting.At the same time measure "ALL" piston to wall clearances,and at this time also measure piston to valve at zero and plus and minus 4 cam timing.measure bearing clearance,make all hand fitting adjustments at this time.
use same formula for head gasket,NOTE: bore on head gasket is likely more than the blocks bore,
measure combustion chamber volume with water
add heck height volume,add or subtract volume from dome and or valve reliefs.plus headgasket volume plus cc of combustion chamber,THAT number divided into the cylinder volume
Ouch! That hurts my head. I might just let my machine shop order the parts for me. Haha
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Old 02-18-2013, 11:45 AM
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you still need to test fit the pistons and measure depth. all the other clearances are done by hand.minimum four test fits to build an engine
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Old 02-18-2013, 11:49 AM
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Look up the advertising info on your pistons, a lot of the time they will give you the compression ratio when used with either 76 or 64 cc heads. Summit or Jegs websites have calculators to help you. It will at least give you a good ball park estimate.
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Old 02-18-2013, 12:01 PM
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Originally Posted by vinniekq2 View Post
you still need to test fit the pistons and measure depth. all the other clearances are done by hand.minimum four test fits to build an engine
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Originally Posted by Unks37 View Post
Look up the advertising info on your pistons, a lot of the time they will give you the compression ratio when used with either 76 or 64 cc heads. Summit or Jegs websites have calculators to help you. It will at least give you a good ball park estimate.
Thanks for the help guys.
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Old 02-18-2013, 01:26 PM
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google search for Maverick and compression calc ...

Download this free engine compression ratio calc and use it .
Be ware of the inputs. flat top piston with valve reliefs is a neg - entry eg(-5)

dome piston is a + entry eg(+4)

You need the assembled piston to deck clearance @ TDC. Ya it matters.

Do not buy a domed piston for a 355 for 64cc heads and pump gas.

Get a flat top piston with a 1.56" compression height and select the head gasket thickness accordingly.

Any domed piston on a pump gas 355 with 64cc heads is a mistake on this motor.
It will result in a excessive high compression ratio

not that hard.
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Old 02-18-2013, 03:32 PM
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Regular production domed pistons such as Speed Pro, etc., are about 11:1 Static Compression Ratio. Flat tip pistons are about 10.0:1 SCR.

Generally, the octane requirement with 11:1 to 12:1 SCR is 112 octane and 10:1 to 11:1 SCR is 100 octane if the initial and total timing advance is set where the engine perform best.
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Old 02-18-2013, 06:56 PM
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Thanks everyone.
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