Guffin Posted October 7, 2003 Share Posted October 7, 2003 Can anybody explain this: My Buick 32-87 has, acording to the manual, a compression ratio of 4.5:1. As far as I know the compression is calculated as the total volume of the cylinder (piston area times stroke)plus the volume in the head divided by the volume in the head (minimum volume piston at top). If I calculate this from measurements on the engine I get a compression ratio of 5.23:1. How come?Jan <img src="http://www.aaca.org/ubbthreads/images/graemlins/confused.gif" alt="" /> Link to comment Share on other sites More sharing options...
Guest Hal Davis (MODEL A HAL) Posted October 7, 2003 Share Posted October 7, 2003 Does your piston come all the way to the top of the block? Does part of it come above the block and into the head? Is the piston flat on the top? Although it is probably negligible, did you include the thickness of the head gasket in your volumetric calculations? Link to comment Share on other sites More sharing options...
Guffin Posted October 7, 2003 Author Share Posted October 7, 2003 The piston comes all the way to the top of the block. The pistons have a flat top. The gasket is thin, only 1.2 mm. There was also a Low Compression Gasket, ca 2.5 mm thick. This is not enough to give 4.5 in compression.I think 4.5:1 in compression is rather low for a car from 1932. My Hupmobile 1928 has a compression oc 5.5:1. I have seen the value given for the Buick not only in the chop manual but also on the web: www.carnut.com/specs/gen/buick30.htmlJan Link to comment Share on other sites More sharing options...
Guest BillP Posted October 7, 2003 Share Posted October 7, 2003 The compression ratio is the ratio between the volume above the piston at bottom dead center and the volume above it at top dead center. The only (best) way I know of to determine these volumes is with a burette. This method eliminates any guessing of where the piston is at either TDC or BDC. Then it's a matter of using a calculator to determine the number. Link to comment Share on other sites More sharing options...
Guest Hal Davis (MODEL A HAL) Posted October 8, 2003 Share Posted October 8, 2003 Bill,I'm not familiar with a burrette. Could you explain how one works? I will have to agree that measuring volume of cylinders is not an easy task, especially if there is any complicated geometry. Sounds like you have the right idea.As for compression ratios of 1932, I'm sure it varied from manufacturer to manufacturer, but Ford had a 4.22:1 as late as 1931, so 4.5 doesn't sound like it is out of the ball park. Link to comment Share on other sites More sharing options...
Guest BillP Posted October 8, 2003 Share Posted October 8, 2003 A burette is a glass tube, open on one end and necked-down with a drain cock on the other. It is precisely calibrated, usually in cc's. An example of its use follows. To check the volume of the combustion chamber in an overhead valve cylinder head, insert both valves with a little white grease on the seats to seal them, insert a spark plug. Place a plexiglass plate(also lightly sealed w/white grease) firmly over the chamber. It has a 1/4 inch chamfered hole in it. Drizzle liquid into the chamber until full. Observe how many cc's required to fill the chamber. Do this seven more times (V8) and make them all the same by slight removal of metal of the small ones. This is one step in blueprinting an engine. The geometry is straightforward as it is mainly cylindrical shapes unless using domed or dished or flycut pistons, etc., then need burette. Link to comment Share on other sites More sharing options...
Guest Hal Davis (MODEL A HAL) Posted October 8, 2003 Share Posted October 8, 2003 Thanks, Bill. Makes sense to me. Link to comment Share on other sites More sharing options...
Guffin Posted October 8, 2003 Author Share Posted October 8, 2003 Perhaps my cylinder head comes from a 1934 car. For that year the compressin ratio was rised to 4.95:1. If this is with the Low Compression Gasket it is not unlikely it will be 5.23 with a thin gasket.Jan Link to comment Share on other sites More sharing options...
novaman Posted October 13, 2003 Share Posted October 13, 2003 The way BillP described is the way used to measure cylinder and combustion camber volume in racing. Very accurate. Link to comment Share on other sites More sharing options...
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