2-stroke engine tuners know that observing the amount of piston wash on the piston dome is a very accurate indicator as to the best fuel-air mixture for maximum power production in competitive situations. These small â€œclean areasâ€ on top of the pistons, located where the fuel enters the combustion chamber at the transfer ports, are indicative of the amount of excess fuel entering the engine. The smaller the â€œwashâ€, the leaner the fuel-air mixture.
How much is the perfect piston wash? The engine builders at Black Magic Racing tell us we should keep two important items in mind when we are looking at the wash on top of the pistons. Based on their experience (with Suzuki engines) the engines with flat top pistons can run less wash and not lose performance or detonate. The domed pistons need to run more wash than a flat top piston in order for the engine to run at peak performance.
Flat top piston wash, such as in a ZR 600 or ZR 500, at peak performance, will be the size of the nail on your little finger. You can get slightly smaller than this, but then the performance will begin to go down hill and the detonation point gets closer.
Domed style pistons need to have a wash the size of the nail on your thumb or slightly less. Running a ZR 800 or ZR 900 with a wash much less than this will lose performance. The 8â€™s and 9â€™s, along with the F6 and 7â€™s, will begin to lose power if the wash gets much smaller or becomes non-existent. The reason these engine just lose power and sometimes not even detonate, is due to the timing curve which is much milder than the 5â€™s and 6â€™s were back in â€˜98-99.
Remember, the amount of piston wash explained here is based on regular octane fuel. Running race fuel, if you have the compression to burn it, will allow the size of the wash to become less through jetting and not lose as much HP. If you are into getting the most out of your engines, use this rule of thumb to keep enough fuel thrown at it and avoid the burndowns.