Piston design has long been a point of contention among hot-rodders, and for good reason. Piston heads can range from perfectly flat tops to dished pistons with a depression in the middle and their polar-opposite domed equivalents; each design has pros and cons that are heavily dependent upon other factors, like cylinder head design. Drawing a few basic conclusions based on what is known of flame travel and component weight can help to point you in the right direction.
Flame travel is one of the most important factors, so it's best to start here. Gasoline doesn't explode in your cylinders; it burns in an ever-expanding bubble called a flame front. Assuming the engine has some sort of swirl effect in the cylinder, as most do, the flame actually travels in something of a downward-moving vortex. A flat-top piston allows the flame front to disperse evenly along the piston head, whereas putting a dome atop the piston interrupts even flame front travel. So, all else being equal, the dome itself is almost guaranteed to cost you some power just because the mixture doesn't burn as evenly or as quickly as it could. In this case, the advantage in flame front travel goes to the flat-top piston.
The burning gases in your engine's combustion chamber move the piston by increasing cylinder pressure, as measured in pounds per square inch. Let's say you've got a flat-top piston with a diameter of 4 inches; a little math tells us it's got a surface area of 12.56 inches. If your combustion event gives you 200 lb. of pressure per square inch, you end up with 2,512 lb. of force pushing down on the piston. Now, put a triangular dome on top. The sides of the triangle have to travel a bit farther to span the piston's 4-inch diameter because they go up at an angle, resulting in more surface area. So, if you've got a dome that increases piston surface area by an extra 1.0 inches, the pressure on the piston is increased by extra 200 psi. For this reason, domed pistons have the advantage in power expression.
Weight and rpm
Weight is a major factor in piston design, primarily because it determines how high the engine can rev before snapping the piston's connecting rod. At high rpm, pistons experience acceleration forces measuring in the thousands of g's. At lower rpm, the piston may experience only about 400 g's of acceleration and deceleration. In other words, the piston exerts 400 times as much force on the piston as it weighs. At 10,000 rpm, the piston may see as high as 9,000 or 10,000 g's. At that kind of speed, an average flat-top piston weighing a moderate 500 g -- or 1.1 lb. -- would exert about 11,000 lb. of force on the rod. The addition of a 0.22-lb. aluminum dome effectively adds 2,200 lb. of load on the rod. The added weight can cause the connecting rod to snap and the engine to self-destruct at high rpm. Again, the advantage in weight and rpm goes to the flat-top piston.
Acceleration advantages between the piston types can be kind of a wash and depend largely on the engine. Extra mass means more weight to accelerate and decelerate, and as we've already seen, those accelerative forces can be pretty significant. Heavy pistons slow engine acceleration, which doesn't do much for performance, but more importantly they slow deceleration. An engine that decelerates slowly takes longer to drop in rpm between gearshifts, and that slows shifting. While this may sound somewhat nitpicky, piston weight can make the difference between slow, hard shifts and crisp, quick shifts. However, owing to the domed piston's increased surface area and superior power expression, you're not likely to lose any horsepower due to acceleration forces. In the end, the horsepower lost through acceleration is about the same between the two piston types, but the advantage in engine deceleration and shifting characteristics goes to the flat-top piston.
Consider domed pistons an item of last resort, to be used only if you've got a huge combustion chamber and desperately need the dome to increase compression to an acceptable level. While domed pistons do offer a few benefits in terms of power expression via increased surface area, dished pistons can do the same thing with far less weight penalty. About the only engines that really need dome pistons are those that use hemispherical combustion chambers; in this case, the chamber roof is so high and peaked that you almost can't maintain a decent compression ratio without a dome piston head. Otherwise, you'd do better to run a smaller combustion chamber and move the combustion event down into the piston with a dish. But this comes with a bit of a weight penalty versus a flat-top piston, so the dished piston is more suitable for large-displacement, low-rpm engines. All of this makes the flat-top piston preferable for most performance engines, particularly those that don't use hemispherical combustion chambers.
- Racing Engine Builder's Handbook; Tom Monroe
- Photo Credit Jupiterimages/liquidlibrary/Getty Images