How to tell compression ratios?
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Topic: How to tell compression ratios?
Posted By: DougG
Subject: How to tell compression ratios?
Date Posted: 01 Sep 2013 at 4:44pm
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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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Replies:
Posted By: O.P.S. Heads
Date 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.
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Posted By: Bill_MN
Date 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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1951 WD #78283, 1918 Case 28x50 Thresher #76738, Case Centennial B 2x16 Plow
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