How to tell compression ratios?

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   Any way or formula to figure compression ratios? I have a 301 diese that was built 25 years ago with decompressed pistons to hold a head gasket together,  wondering how much decompressed? 

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DougG Members Profile Send Private Message Find Members Posts Add to Buddy List Orange Level Joined: 20 Sep 2009 Location: Mo Points: 8091	Post Options Post Reply Quote DougG Report Post Thanks(0) Quote Reply Topic: How to tell compression ratios? Posted: 01 Sep 2013 at 4:44pm
	Any way or formula to figure compression ratios? I have a 301 diese that was built 25 years ago with decompressed pistons to hold a head gasket together, wondering how much decompressed?

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O.P.S. Heads Members Profile Send Private Message Find Members Posts Add to Buddy List Orange Level Access Joined: 02 Jan 2013 Location: Iowa Points: 574	Post Options Post Reply Quote O.P.S. Heads Report Post Thanks(0) Quote Reply Posted: 01 Sep 2013 at 10:02pm
	Static compression ratio is a comparison of how much volume is a BDC vs. how much is at TDC. Simple as that. With a 15:1 static compression engine, the cylinder at BDC with hold 15 times as much water (or air) as the same cylinder would hold at TDC. You will need to compare a stock piston to your machined pistons to see how much material has been removed either in the bowl, valve reliefs, off the diameter above the top ring, or faced off the top of the piston. This will include but is not limited to using water to measure the volume of the bowl and a caliper to measure the other dimensions of the piston. You also need to consider valve stand out and head gasket thickness if you are calculating the compression of what you have. This assumes the rods and crankshaft are of stock dimensions. A couple of cc's of TDC volume makes a difference on a small bore, short stroke engine like a 301. An easier method may be to remove what you have and install 15:1 pistons out of a turboed 301 or 14:1 pistons out of an early 6080 and call it good. One of those two ratios should fit your needs. Edited by O.P.S. Heads - 01 Sep 2013 at 10:08pm

Bill_MN Members Profile Send Private Message Find Members Posts Add to Buddy List Orange Level Joined: 12 Sep 2009 Location: Sioux Falls, SD Points: 1463	Post Options Post Reply Quote Bill_MN Report Post Thanks(0) Quote Reply Posted: 03 Sep 2013 at 11:07pm
	Service manual should have cranking compression, measure your compression and divide it by what the manual says, then multiply by the stock compression ratio which should also be in the manual, will give you a rough estimate. Of course it may be lower due to wear and any valve or cam variation
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