![]() Piston Diameter at Top Ring Land - usually runs 0.003" to 0.005" less than bore diameterĭistance from Top of Piston to Ring Land - may vary from manufacturer to manufacturer Gasket Thickness - varies from manufacturer to manufacturerĬompression Height - distance from CL of wrist pin to top of piston Gasket Bore Diameter - 4.100 is standard small block I only included the components for a flat-top piston with valve reliefs.if you're into domed or dished pistons you're on your own.Ĭylinder Bore Diameter - e.g. The catalog provides a great deal of information on the various Ford platforms, but if you're building a custom engine your particular dimensions will vary. I rigged up this compression ratio calculator using information from the Ford Racing Performance Parts catalog. Just to illustrate how concerned we are about the total quench volume, we literally avoid unnecessary valve jobs since a simple valve job can sink the valve 0.003" to 0.005" further into the head, thereby increasing the quench volume by that much! Properly done, a set of 60 or 65 cc heads can be milled down to 38 or 40 cc's. Its not uncommon to see a Street Warrior racer take a brand new set of heads and mill 0.150" or more off them right away, often cutting the heads right down to the valve seats. This includes raising the top ring land on the piston, minimizing the valve reliefs, avoiding valves with dished tips, and by aggressively milling the head. A Street Warrior motor goes to great lengths to reduce the effective quench by any means possible. by reducing the size of the combustion chamber. Since we don't have the domes to reduce the quench, we increase the compression by reducing the free volume above the piston, i.e. In contrast the Street Warrior class is held, by rule, to a true flat-top piston. The domes fill the combustion chambers on the compression stroke, minimizing the volume between piston and head (otherwise known as the quench). These are pistons with a raised combustion surface, often CNC cut from digitized molds of the their combustion chambers. The Street Bandit racers can get their high compression ratios through the use of domed pistons. This is a large increase compared to a stock engine with a compression ratio somewhere around 9.0:1. Its not uncommon to hear of compression ratios in excess of 13.0:1 in normally aspirated classes such as Street Warrior or Pure Street, and as high as 17.0:1 or more in classes like Street Bandit or Hot Street. Since we're limited to producing horsepower in the normally aspirated fashion, one of the best ways to increase the output is by increasing the compression in the motor. Increasing the compression ratio can generally be accomplished a number ways such as using domed pistons, shaving (milling) the heads, using a thinner head gasket or changing the compression height of the piston.Ĭompeting in a rules class like we have over the years, we are always left looking for ways to get every last bit of horsepower out of the engine. ![]() Compress the air/fuel mixture into a smaller volume (increasing compression ratio) and you've got a mixture that's easier to ignite, burns quicker and more completely, and hence delivers more power. In general, higher compression ratios mean more horsepower. Compression Ratio is an important variable in building a performance engine.
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