04-11-2020, 08:36 PM
Hi Doug,
Yes "free-porting" means that the bottom of the piston skirt opens the exhaust ports into the crankcase instead of being long enough to keep the exhaust ports closed as the piston reaches the top of the cylinder. In chain saws and at the RPM"s they are running and with open headers or mufflers, it will be acceptable, though NOT the best, performance wise.
In a "high reving" modified Kart engine, free-porting causes a breaking of the standing waves through thru the carb/s into the crankcase that happen above certain RPMs. When this is happening, the reeds NEVER close, but stand open ALL the time. The engine will run with a reed GONE and there is NO sign that a reed has broken and is gone. The engine starts to act up only when the RPMs drop below the point that the standing waves form. If one can keep the engine running, and slowly get it back up in the RPM range it will start running perfectly again. Steve Ohara learned this when he lost a reed while racing, and has talked about this in posts. He re-learned something that had become lost knowledge that "old timers" knew about, but they had stopped racing so it had become lost knowledge
AND NO, NO, NO free-porting a "piped" engine is sure DEATH for the engine!! For the pipe to work right, the engine MUST NOT free-port. Pipes use BOTH sound waves and pressure waves to work their magic. Free-porting would destroy how they work.
Best Regards TORQUE
Yes "free-porting" means that the bottom of the piston skirt opens the exhaust ports into the crankcase instead of being long enough to keep the exhaust ports closed as the piston reaches the top of the cylinder. In chain saws and at the RPM"s they are running and with open headers or mufflers, it will be acceptable, though NOT the best, performance wise.
In a "high reving" modified Kart engine, free-porting causes a breaking of the standing waves through thru the carb/s into the crankcase that happen above certain RPMs. When this is happening, the reeds NEVER close, but stand open ALL the time. The engine will run with a reed GONE and there is NO sign that a reed has broken and is gone. The engine starts to act up only when the RPMs drop below the point that the standing waves form. If one can keep the engine running, and slowly get it back up in the RPM range it will start running perfectly again. Steve Ohara learned this when he lost a reed while racing, and has talked about this in posts. He re-learned something that had become lost knowledge that "old timers" knew about, but they had stopped racing so it had become lost knowledge
AND NO, NO, NO free-porting a "piped" engine is sure DEATH for the engine!! For the pipe to work right, the engine MUST NOT free-port. Pipes use BOTH sound waves and pressure waves to work their magic. Free-porting would destroy how they work.
Best Regards TORQUE