Compression Ratio Calculator
Compression Ratio is an important variable in building a performance engine. In general, higher compression ratios mean more horsepower. 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. 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.
Competing 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. 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. 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. This is a large increase compared to a stock engine with a compression ratio somewhere around 9.0:1.
The Street Bandit racers can get their high compression ratios through the use of domed pistons. These are pistons with a raised combustion surface, often CNC cut from digitized molds of the their combustion chambers. The domes fill the combustion chambers on the compression stroke, minimizing the volume between piston and head (otherwise known as the quench). In contrast the Street Warrior class is held, by rule, to a true flat-top piston. 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. by reducing the size of the combustion chamber. A Street Warrior motor goes to great lengths to reduce the effective quench by any means possible. 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. 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. Properly done, a set of 60 or 65 cc heads can be milled down to 38 or 40 cc's. 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!
I rigged up this compression ratio calculator using information from the Ford Racing Performance Parts catalog. 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 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.
There are a few rules of thumb to be observed with your typical OEM Ford, open combustion chamber style heads:
Flat milling of a head will reduce the combustion chamber 1.0 cc for approximately every 0.007 of an inch removed.
Every point of compression (e.g. from 10.0:1 to 11.0:1) will net 3% to 5% horsepower improvement.
The better the volumetric efficiency of your engine combination, the better the return on the increase in compression.
When checking piston-to-valve (P-V) clearances, a conservative builder will recommend maintaining 0.100" on the intake valve and 0.150" on the exhaust valve. Greater clearance is warranted on the exhaust valve since the piston is always 'chasing' the exhaust valve motion. Always check the clearance of the valve to the radius of the relief eyebrow in addition to the depth of the eyebrow. Often times the valve will contact at the edge of the eyebrow before it bottoms out in it.
When aggressively milling heads, be sure to check the spark plug to piston clearance. On heavily milled heads, indexing washers or installing the plugs with the piston below the TDC position may be required to avoid the piston contacting the electrode.
Milling the heads lowers the position of the intake port relative to the block deck so the intake will need to be milled for the ports to align properly. A good starting point is to mill the sides of the intake the same amount as the heads have been milled and mill the valley pan rails of the intake 1.5 times what the heads were milled. The intake bolt holes will also need to be elongated towards the intake centerline for proper fit.