Page 184 - Diamond Pistons
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Now the RATIO starts to become more obvious. This is what usually changes a piston advertised at 12.5-to-1 for a 454 cubic inch engine into an 11-to-1 piston upon assembly. Actual volumes are not veri ed (poured), or calculated at the time of assembly based on how the actual parts stack-up. The builder is unaware of the engine’s lower compression ratio until the engine is placed on the dynamometer or race track, usually with less than satisfactory results.
Accuracy is everything.
Using actual volumes rather than theoretical book dimensions provides more accurate ratio calculations. The key is to try and SIMULATE on paper how your engine will stack up at the time at the time of assembly. Remember, the bigger the swept volume (cubic inches) and the smaller the compressed volume, the higher the RATIO.
Bottom line:
Measure your parts and crunch the actual numbers— all the numbers—and know what your compression ratio will be for a particular piston before you buy it. If you use “theoretical” numbers out of a book you will end up having an engine with less ratio (and performance) than anticipated. Crankshafts, and connecting rods are usually very close to their advertised length, but engine block deck heights are rarely “spot on” the listed OEM specs.
Compression Ratios Explained
At Diamond, we make every e ort to accurately calculate and inform our customers how we arrive at the compression ratios we advertise. Integrity is everything to us. An engine’s compression ratio simply put, is the total swept volume (with the piston at BDC- bottom dead center) divided by the total compressed volume (with the piston at TDC-top dead center).
Actually, things are not that simple. Too often a piston with a tall dome is viewed as a 15-to-1 piston, but in reality, once the ratio calculations are performed correctly—with real numbers and ALL the volumes accounted for—things usually end up with an actual compression ratio far less than advertised. This is especially true for engines under 500 cubic inches. Interestingly enough, many piston manufacturers o er pistons for the same engine and application with volumes only one or two cc’s apart, but the advertised compression ratios vary as much a one full point of compression from brand to brand.
The key to  guring out an engine’s actual compression ratio requires understanding ALL the numbers necessary to compute its compression ratio correctly. The variance in advertised ratios is usually due to the following volumes being omitted from the calculations:
1. The compressed head gasket volume.
2. Deck volume (actual, based on piston position . .
at TDC, not assumed.)
3. Volume around the top ring (top land volume).
Usually 2.0 - 3.0cc’s.
These are CRITICAL volumes that must be accounted for when accurately computing an engine’s compression ratio. In some engines, these volumes can add up to more than 20cc’s, which DRAMATICALLY changes an engine’s actual compression ratio.
Prove it.
To prove this, take the number 1,000 and divide it by 100. Your calculated ratio in this case is 10-to-1. 1,000cc’s represents the swept volume of a cylinder with the piston at BDC. 100cc’s represents compressed volume with the piston at TDC. Simple right? Now, take the number 1,020cc’s and divide it by 120cc’s and see what you get. Your new ratio is now only 8.5-to-1. The di erence here is that extra 20cc’s now represents the gasket, the top ring (down) volume and deck volume added ON BOTH ENDS of the ratio calculation. That 20cc’s is crucial, because it’s there at both BDC and TDC.
Your compression ratio is calculated using volumes obtained from the drawing at right: For determining the various formulas, consult the tables on the following page.
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