With an aluminum cylinder on a street engine, the tightest clearance recommended is about 0.030-inch, with 0.040-inch maximum. For iron cylinders, 0.040-inch is recommended. Race engines built to close tolerances and closely monitored street engines can be setup with slightly less squish clearance, but it is on a trial-and-error basis, and you have to pay attention to what you are doing and monitor the engine closely.
A leak-down tester is an excellent tool for checking cylinder sealing and more revealing t
Piston-to-bore clearance is another factor in determining squish clearance. All pistons tend to rock as they pass over TDC, and the rocking movement reduces the piston-to-head clearance. Large-bore pistons with loose piston-to-bore clearances rock more than small-diameter pistons with tight piston-to-bore clearances. Additionally, pistons with heavily notched skirts rock a bunch. The greater the piston rock in the bore, the greater the squish clearance must be.
For a given crankcase, stroke, rod, and piston, squish clearance can be juggled by changing cylinder length, head gasket thickness, or cylinder base gasket thickness. Squish clearance cannot be changed by milling the head if the squish band is located between the top of the piston and the head's head-gasket surface. However, combustion chamber design can change this. For instance, if the squish band is located between the piston dome and the combustion chamber (not the head gasket surface), such as with the Shovelhead and some modified heads, milling the head may reduce squish clearance.
The components of ring seal are: (1) cylinder, (2) ring, (3) piston ring groove, (4) piston stability in the cylinder, and (5) lubricating oil. It is impossible to tune an engine correctly unless it has excellent ring seal. Better cams, carbs, exhaust systems and all the hard-to-find stuff will not show power improvements if the engine's ring seal is not correct. Since maximum cylinder fill and cylinder pressure occur near peak-torque rpm, peak-torque power numbers will drop down first with a poor-sealing pump. Round, straight, and rigid cylinders, accurately machined piston ring grooves, piston stability in the bore (read: tall cylinders and long rods), and ring/cylinder wall finish compatibility are all crucial for achieving proper ring seal.
Compression ratio is a key factor in power production, efficiency, and detonation tolerance. More compression will improve power throughout the engine's rpm range, which makes for snappier acceleration and increases fuel mileage because the engine is more efficiently converting energy into power. However, once the limits of detonation are reached for a given fuel octane and combustion chamber design, so are the realistic limits of compression. Detonation is spontaneous, uncontrolled, and potentially disastrous combustion in the combustion chamber. Sometimes if an engine is on the edge of detonation, just insulating the fuel line with household copper water pipe insulation can eliminate the detonation.
Compression can be raised by replacing the pistons, milling the head, modifying the combustion chamber or increasing the displacement. The engine designer/builder has a delicate balancing act of optimizing compression for maximum power without encountering power robbing detonation or resorting to race gas on a street engine. Some of the major tools he has to work with are corrected compression ratio, combustion chamber efficiency, camshaft timing, carburetor/intake manifold efficiency, ignition timing, and gearing.
Final WordsAs you should realize by now, ending up with an optimized engine involves more than just selecting a bunch of parts. The next time you are planning on freshening up, rebuilding, or building a new engine, don't forget to go the extra mile and optimize its efficiency while your at it. Paying attention to details, such as optimizing engine components, can set you apart from the crowd and put you at the front of the pack.
SourcesThe Performance Pro SeriesCrystal Publications(800) 945-4890; (480) 654-9118