Deck Height vs. Compression

[WE2Regal]

Most of the "off the shelf" NA 231 replacement pistons are listed as about 8:1 compression and are supposed to be stock replacements for 79-87 engines. There was a change in deck height in 1986 with the 109 block. Wouldn't there be a difference in compression because of different deck heights using the same piston? Ruggles said the flat top 3.0 piston made "12:1 compression in an otherwise stock 3.8." At the time that book was written, I'm guessing he was referring to using a 79-85 engine block. Would flat top 3.0 pistons make more compression in a 109 block? Is the compression ratio affected at all by the year of the engine block when using off the shelf pistons?

 

[ek02]

The deck height on a 1985 and earlier NA block is 9.560. The 109 is supposed to be 9.522 from what i have read on this site, so a 109 block should have more compression with flat top pistons. The deck on the 109 was reduced so thicker composite head gaskets could be used instead of the steel shim gaskets used on NA engines. The pistons seem to be the same between engines as far as compression goes. I think Turbo pistons were made a bit stronger than the NA pistons.

 

[WE2Regal]

Thank you. All manufacturers used to report their engine specs to NHRA for Stock Eliminator use. I like to use that guide to compare different year engine specs. The 1984 NA 231 is listed as 8.39 compression with a compressed head gasket thickness of .021 and deck clearance of .061. The 1987 NA 231 is listed as 9.5 compression with a compressed head gasket thickness of .017 and deck clearance of .020. Both engines use the same 8445 head.

 

[bison]

Thank you. All manufacturers used to report their engine specs to NHRA for Stock Eliminator use. I like to use that guide to compare different year engine specs. The 1984 NA 231 is listed as 8.39 compression with a compressed head gasket thickness of .021 and deck clearance of .061. The 1987 NA 231 is listed as 9.5 compression with a compressed head gasket thickness of .017 and deck clearance of .020. Both engines use the same 8445 head.

These numbers don’t apply to assembly line engines. The deck clearances and gasket thicknesses are incorrect for the years stated


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[WE2Regal]

I would think those are blueprinted engine specs? Who knows, Buick Motorsports may have fudged on things a bit when those dimensions were reported to NHRA. Wouldn't be the first time that happened in Stock. In that game, the men with the pen can make your car competitive or not.