[Christ]

There is a certain amount of carbon build up on the top of the pistons that creates better combustion efficiency, purportedly. I've never experienced this, because I've never cared to test it.

The layer will be very thin and crystalline, though. It's not enough to even notice. In fact, it's the layer of carbon that most people work so hard to clean off when rebuilding an engine, when it doesn't, in fact, need to be cleaned.

You can use something like SeaFoam - but if the carbon build up is heat-tempered, it won't be taken off by a short-acting solvent or even water. Carbon forms a deposit that is basically hardened and non-porous, so short of being chipped away, not much will take it off quickly. The point of the water mist is to instantly cool the heated carbon build up, causing it to crack and shrink. The heat buildup and shock cause it to loosen, and it's ejected through the exhaust valve. This can plug up your catalytic converter, at best.

If you really want to clean your engine of carbon regularly, give it a good spirited acceleration onto the highway, which will raise cylinder pressures and combustion temps, causing the non-crystalline carbon to be blown from the cylinder.