FOUR-STROKE FUNCTION
From left to right we have the intake, compression, power, and exhaust strokes of a four-stroke or four-cycle engine. On the induction stroke, a fresh air/fuel charge is pulled past the open intake valve as the piston moves down the bore. At around Bottom Dead Center (BDC) the intake closes and the piston motion moving up the bore starts the compression stroke. At some point just before the piston reaches the top of the compression stroke at Top Dead Center (TDC), the spark plug will fire. At this point there is a small delay in the combustion before it really gets underway (hence the firing a little before TDC). As the piston goes over TDC so the combustion event burns the charge and the heat generated causes the contents of the cylinder to rapidly rise in pressure. This pressure pushes the piston down the bore on the power stroke. As the piston approaches the end of the power stroke, the exhaust valve starts to open. Initially the gases, still at relatively high pressure, vent themselves out through the progressively opening exhaust valve. By the time the piston starts to move up the bore the exhaust valve is already well off its seat. After this initial cylinder "blow down," the piston's motion up the bore pushes out the remaining spent charge through the exhaust valve. At the top of the exhaust stroke the intake begins to open and the whole sequence of events starts over again.

Compression & Cylinder Pressure
Raising the CR increases torque, and consequently power, throughout the rpm range. Because raising the CR increases thermal efficiency, it brings about an increase in fuel economy. If a longer duration cam is installed, raising the CR at the same time can be worth considerably more than these two moves considered separately. When the CR is raised, peak combustion pressures are increased. And since cylinder pressure equals torque, that's a good thing. A rule of thumb for typical production engines is that combustion pressure is equal to the CR times 100. This tells us that, from a 10:1 engine, we would expect to see about 1,000 psi of peak combustion pressure. For a well-developed high-performance engine, combustion pressures can be as much as the CR times 120.

SOURCE
Calico Coatings
Denver
NC
704-483-2202
www.calicocoatings.com
Lunati (Div. of Holley)
COMP Cams Ross Racing Pistons
625 S. Douglas Ave.
El Segundo
CA  90245
310-536-0100
Joe Gibbs Racing oil CV Products  800-448-12 Scat Enterprises Inc.
1400 Kingsdale Avenue,
Dept. MMFF
Redondo Beach
CA  90278
JE Pistons Total Seal Inc.
22642 N. 15th Ave.
Phoenix
AZ  85027
800-874-2753
United Engine & Machine (KB Pistons)