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You could move the piston to BDC, fill the cylinder completely with a measured amount of fluid, move the cylinder to TDC and measure how much comes out. The ratio of the two volumes of fluid will be the compression ratio. It may take a little ingenuity to fill it completely, but it could be done.

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67 Nova 385" daily driver - 12.1 @ 112
64 Chevelle Road Racer in the works
67 Nova Wagon under construction
 

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Without cc'ing the heads and pistons, and measuring the deck hgt, the best you can hope for is an estimation. By using published data, you can get pretty close. For flat top pistons, many brand's valve reliefs are around 6 cc. Most composite gaskets are between .039" and .041", however there are other thicknesses (.032", .051"), the bore on a sbc head composite gasket is 4.166". Stock deck hgt should around .025". You'll have to find out what heads you have to determine the cc's for them.

Click on the link in my sig and you will find a compression calculator.

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Pat Kelley
66 El Camino, daily driver
67 El Camino, STRIP/street
Pat's Page (CR Calculator, Utilites, car specs)

[This message has been edited by Pat Kelley (edited 03-31-2001).]
 

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isn't there an instrument that screws in the sparkplug hole that measures youre 'real' comession ratio? (it doesn't account for leaks and extended intake duration)
 

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dc & 80Classic,

There apparently is a tool for this. I was just up in Detroit yesterday morning having my buddy's 68SS Chasis Dyno'd and we were talking to the owner about different stuff and he told us about a guy that brought in a BB Chevelle with a motor that the Chevelle owner bought from a friend that was suppose to be 11:1 motor and when they checked with his equipment it was 8.7:1 The owner of the speed shop made a point to tell us he has the capapbility to check compresion ratio not just compression. I wasn't aware of it but based on this feedback there must be some kind of equipment to do that an you might check your local speed shops.

Mytmouse

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67 SS396 350HP 4 sp 3:55 Posi Butternut Yellow w/Black Vinyl
68 RS Z/28 302 350+Hp? 4sp 4:10 Posi, Euro Red on Black
70 SS396 350HP 4 sp 3:55 Posi, Black on Black

Mytmouse A.K.A. Robert Stacho
ACES Member # 04359

[This message has been edited by Mytmouse (edited 04-01-2001).]
 

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Oneovakind is closest on this. The compression ratio of an engine is simply a comparison of a cylinder's displacement with the piston at bottom dead center versus the displacement of the cylinder with the piston at top dead center. A 10:1 compression ratio means that the cylinder has a displacement of 10 cubic inches when the piston is at bottom dead center and a displacement of 1 cubic inch when the piston is at top dead center.
 
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