boating around

Ignition curves. 2 vs 4 stroke.

The most perplexing questionfor many people is why a 2 stroke has an ignition advance curve totally the reverse of a 4 stroke.
A typical 4 stroke will start with about 10 degrees of advance at idle and finish with around 32 @ peak torque rpm, extending to perhaps 37 @ peak hp rpm. However the advance curve shown below is what will be found in many 2 stroke engines.

The big difference is the 2 stroke suffers terribly from exhaust gas contamination of fuel/air mixture and limited cylinder filling @ lower engine speeds. Therefore the spark plug must fire a comparitively long time before tdc to give the diluted charge time to burn. Then as rpm rise the engine works more efficiently and combustion speeds considerably. The spark advance must be reduced to ensure that the pressure peak from this more rapid combustion accurs past tdc. If it were to peak before tdc or to soon after combustion pressure would rise to quickly and the unburned portion of mixture would explode. It is this uncontrolled burn. detonation, that destroys engines.

Dave