Let's begin at zero--the flattop...
Let's begin at zero--the flattop piston. Aside from the valve reliefs that do effect compression, we are starting with a crown that doesn't effect compression much one way or the other. Combustion chamber size, compression height, and deck height are the biggest factors in determining compression. We also need to factor in head gasket thickness.
Forged pistons are made from a variety of different aluminum alloys. the two most popular alloys are 4032 and 2618. Federal-Mogul's Speed Pro Division tell us they use an alloy called VMS-45, an alloy similar to 2618. Speed Pro forged pistons include 11-percent silicon for added strength. This additional silicon content also controls expansion properties, which has always been a challenge with forged aluminum pistons.
Probably one of the biggest advantages of forged pistons is durability. Not only do they hold up better than cast or hypereutectic pistons, they fail gradually under severe duty conditions. This buys time if you get into trouble. It prevents complete and total engine failure.
Installing forged pistons in your daily commuter doesn't make much sense unless you intend to throw nitrous or supercharging at them periodically. Using forged pistons in a mild-mannered engine is overkill and economic foolishness. You simply don't need them. And for all the hoopla over forged pistons, they're not always desirable in street engines because they're noisy when cold. They rattle up a storm in a cold engine because they have different expansion properties than cast or hypereutectic pistons. They are loose cold and snug hot.
We used to call these "pop-up"...
We used to call these "pop-up" pistons, with a crown that fit the combustion chamber perfectly. In the days of high-octane fuel, pop-ups and a 13.0:1 compression were the norm. The pop-up crown fills the chamber and raises compression dramatically. We don't see these much in street use today.
Piston selection is actually a simple process rooted in how you intend to use the engine. If you're building a daily driver or weekend cruiser, cast pistons will get the job done. If you're a tad nervous about it, consider this--cast pistons were used in 7,000-rpm screamers in SCCA competition in the '60s. Take comfort in knowing they will live nicely in your street-driven restomod.
If you intend to do a little weekend drag racing, hypereutectic pistons will get you through with smooth, quiet performance and reliability, as long as you stay away from nitrous and supercharging. Nitrous and supercharging call for the use of forged pistons without exception, due to the extremes of heat and shock presented by these power adders.
Piston Dynamics
With material issues out of the way, we're ready to look at piston design. There is much more to piston design than we have room to discuss here. Instead, we're going to touch on the basics. Piston design and shape greatly effect how an engine performs. When pistons are too heavy, we lose power. Design in too much skirt, and we lose power through excessive friction. Too little skirt, and the piston becomes unstable. Shoehorn in too much displacement, push the wrist pin into the ring grooves, and you have a formula for piston failure because this exerts too much heat on the pin and boss.
Checking valve-to-piston clearance...
Checking valve-to-piston clearance involves working modeling clay into the valve reliefs first, then bolting the cylinder head and valvetrain in place.
In the dreamy world of piston science, we dream of the perfect piston--the piston that creates very little friction (drag), weighs very little, carries just the right amount of oil up the cylinder walls, and provides a perfect cylinder seal. In the real world, it is nearly impossible to achieve all of these elements at once.
Piston development and research is a science that has been going on since the dawning of the four-cycle engine more than 100 years ago. The objective has always been to squeeze the mixture and spin the crank without losing power in the process. Older, modern-day piston engines had big, heavy slugs in their bores. Take the FE-series big-block, for example. Our old 352s, 390s, and 428s had heavy pistons with long skirts from the factory. These pistons were stable, but heavy.
 The opposite extreme of the...  The opposite extreme of the pop-up is the dished piston. When we dish the piston, we increase chamber volume and lower compression. Dished pistons show up in strokers and supercharged applications. We use them in strokers because we have increased compression height. With strokers, the piston travels further in the same bore and combustion chamber sizing. This calls for an increase in chamber size when the piston reaches the top of the bore. In supercharged applications, we reduce compression to make up for the boost. |  When we stroke more displacement...  When we stroke more displacement into an engine, we push the piston's design limits. This is a 351C stroker piston with a pop-up crown. Note the location of the oil ring grooves right up there with the wrist boss. This means a lot of heat in vulnerable areas. The things we do for power. |  This Probe stroker piston...  This Probe stroker piston tells us a lot about what is happening today in piston technology. Note the location of the ring lands and grooves, along with piston pin positioning. This allows us to stroke a lot of displacement into a small-block. However, it also loses reliability due to heat issues. With the pin up high in the ring grooves, it is exposed to a lot of heat at the crown. |