Basic Question About Timing

You answered your own question! There is a delay involved!

The delay is the amount of time that it takes for the air/fuel mixture to ignite, burn, start to expand, and therefore cause a resultant increase in pressure in the cylinder. That's what pushes the piston back down in the power stroke of a four-cycle engine.

Remember your four stroke cycle? Suck - Squoosh - Bang - Whoosh ! (Or Intake - Compression - Power - Exhaust, for you purists!)

In order to get the most power, you have to ignite the air/fuel mixture BTDC (Before Top Dead Center) in order for the mixture to start burning and expand by the time the piston actually reaches TDC.

If you fired the plug exactly at TDC the piston would already be on its way down in the cylinder before the expanding A/F charge even had a chance to push it down, resulting in lost power.

The faster the engine turns (higher RPMs) the farther in advance you must ignite the mixture to get it to burn and expand as the piston reaches TDC. Spark advance is typically around 10 to 14 degrees BTDC at idle up to 30 to 35 degrees BTDC at top engine RPMs.

Keep in mind that an engine turning at 6,000 RPM is really hauling butt. (That translates to 100 revolutions per SECOND!) That's really fast for a rotating assembly the size and weight of a crankshaft, connecting rods and pistons!

Hope that helps.

Thanks for that explanation, it helped me to better understand a few things.