Not all pistons are created equally. Whether you choose to line your engine block with flat top, dished, or dome pistons all depends primarily on valve pocket requirements and compression requirements. We’ll go through the basics to help you understand the differences, so you can make an informed decision on what to stock up on with your next custom build or upgrade.
How A Piston Works
Inside every engine you find a cylinder. Within that cylinder are your pistons. The number of pistons you have, as well as their arrangement, is determined by the type of engine you have. The piston’s job in all of this is to transfer force from the exploding gas up to the crankshaft. Each piston inside the cylinder is connected by a rod which allows it to move up and down. Air and fuel are mixed together and pulled into the cylinder. The cylinder compresses the mixture, the spark ignites it, and you have power. The resulting expanding gases from this combustion drive the engine piston forward to move the same way that pushing down on the pedal of a bike causes the wheel to move.
Types of Pistons
There are three types of pistons, each named for its shape: flat top, dome, and dish.
As simple as it sounds, a flat-top piston has a flat top. Flat-top pistons have the smallest amount of surface space; this enables them to create the most force. This type of piston is ideal for creating efficient combustion. Flat-top pistons create the most even flame distribution. The difficulty that comes with this is that it can create too much compression for smaller combustion chambers.
Dish pistons present the least problems for engineers. That is more because of where they are used than any property they themselves hold. They are shaped just like a plate with the outer edges slightly curling up. Typically, dish pistons are used in boosted applications which do not require a high-lift camshaft or high compression ratio.
Opposite in concept to the dish pistons, these bubble in in the middle like the top of a stadium. This is done to increase the surface area available on the top of the piston. More surface area means less compression. While more compression does mean more force is generated, there is an upper limit of what each combustion chamber can handle. Reducing the compression rate in this way essentially prevents the engine from ripping itself apart. It’s just one tool in limiting the amount of force generated to what the engine is capable of handling safely.
If you are just beginning, this is only the start. You can’t understand the whole puzzle without putting the pieces in context with one another. So while this explains what pistons do and how differences in shape matter, it needs to be understood in the context of the whole engine to get the full picture. Keep studying and you’ll be on your way.