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Old 08-14-2012, 07:02 PM
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compression ratio help.

ok i have a sbc 350 40 over with Speed Pro 125 Dome Pistons with 76cc heads i was told if i use a Fel-Pro steel-shim head gasket 0.015-inch compressed thickness my cr should be close to 10.1 ..any one know what my compression should be?

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Old 08-14-2012, 09:16 PM
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What do you mean?
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Old 08-14-2012, 09:27 PM
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What do you mean?
Im trying to find out how much compression my engine has.
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Old 08-14-2012, 09:42 PM
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There's a lot of calculators on line that will give you and exact number.
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Old 08-15-2012, 04:47 AM
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Quote:
Originally Posted by the82elco View Post
ok i have a sbc 350 40 over with Speed Pro 125 Dome Pistons with 76cc heads i was told if i use a Fel-Pro steel-shim head gasket 0.015-inch compressed thickness my cr should be close to 10.1 ..any one know what my compression should be?
A piston such as the Speed Pro H618CP (0.125" dome) has a dome volume of 3.5cc and a compression height of 1.56". These numbers will be needed when it comes time to use a compression calculator.

You have to be careful when entering the dome or dish volume into the online calculators- some will want you to use a negative number for a dome, others will want a positive number for a dome. The one I linked to above is clearly marked to use a "-" for dished pistons, so in your case enter the dome volume of 3.5cc as a positive number (use no sign in other words).

Most head gaskets for a 4" bore SBC will have a gasket bore diameter of 4.09 to 4.100"

You will also need to know how far "in the hole" the piston is. A stock Chevy 350 block will be close to 0.025" in the hole. But when a block is prepped for a rebuilt engine, often metal is removed from the deck to clean and square up the surfaces. This decreases the piston deck clearance. Also there are "rebuilder" pistons that have a shorter compression height to compensate for milling the castings. Usually the rebuilder pistons measure 1.54", or 0.020" shorter than usual.

The cylinder head will also need to be measured to see exactly what the volume is. For the same reasons as the block is milled, the heads are often milled and also milled to gain compression in some cases. So you need to know the actual volume to be accurate.

All that said, if the heads are actually 76cc and the block is unmilled, pistons have a 1.56" compression height and using a 0.015" head gasket, the static compression ratio is going to be right at 10:1.

To see what the dynamic compression ratio is you need to know the rod length (stock is 5.7") and the intake closing point of the camshaft. This lets you estimate whether a combination of parts will be able to use pump gas w/o detonation, for example.

A couple calculators for dynamic compression ratio:
Dynamic CR

Another dynamic compression ratio calculator

Here is a short page on the compatibility of different cam durations to the compression ratio.

One last thing- do yourself a favor and read up on "quench". It's an important part of the engine build and can make the difference in having an octane hungry pinging engine or a smooth running engine w/o detonation.

BTW, no need to click "thanks".
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