![]() Today, finding online calculators that quickly spit out the results is only Google click away. In the old days, calculating the CR meant getting out the slide rule (really long time ago) or working through a set of formulas on a hand-held calculator. To calculate dome volume: first, position the piston a measured distance into the cylinder, making sure the dome is below the deck. If you have a used block and don’t know the deck height, or you purchased a set of heads and don’t know the combustion chamber volume, then the potential for the types of problems mentioned by Stevenson is quite probable. When ordering pistons, for example, the company’s tech reps will need to know a number of factors to ensure the desired, or at least a safe, compression ratio can be provided. Much like measuring piston dome volume, the key is sealing off the chamber with a clear plate and measuring the amount of fluid it takes to fill up the chamber.įor non-racers it’s a good idea to know and understand the data necessary to calculate the CR-especially if building an engine from scratch. A burette and task-specific fixtures are needed to measure the combustion chamber volume. Knowing the CR will provide a strong foundation for the tuning strategy. ![]() Even if there are no rules for CR, the racer may be restricted to a specific of fuel. On the flip side, if the CR is lower than the allowed maximum, then the racer is giving up horsepower. If the CR is not calculated correctly, then the racer could be penalized for cheating if officials discover it’s too high. The piston manufacturer will supply the compression height for your calculations.Ī number of sanctioning bodies restrict the engine’s compression ratio, depending on the class or application. The piston with the shorter compression height on the right allows the use of longer rods, a longer stroke or a shorter deck height. Note how the wrist-pin bore is further away from the crown of the piston on the left. And, if the power isn’t quite there, another pound or two of boost easily makes up the difference.” The deck clearance volume will be affected by the deck height of the block, the crankshaft stroke, the rod length and the compression height of the pistons. The tuning window widens and provides more of a safety envelope in the event of a fuel pressure or delivery problem, or even a bad batch of gas. “In forced induction (FI) applications, erring on the low side is much safer than pushing your luck on the high side. On the high side carries greater risk in tuning and potential component failure if appropriately better fuel is not used,” says Alan Stevenson of JE Pistons. ![]() “Too little compression will usually result in unmet performance expectations. write the number on the piston as your checking for easy comparison. Move the indicator to the deck plane of the piston to read how far the piston is below or above the deck of the block. First, bring the piston to TDC, then zero the dial indicator on the deck surface of the cylinder block. The compression ratio is affected significantly by the deck clearance volume, The distance between the piston crown at TDC and the height of the deck surface. ![]()
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