Combustion Chamber Shape vs. Ignition Advance
I am of the opinion that initial timing should only be advanced when the combustion chamber shape has been compromised. I've said it before and I'll say it again - It is always best to run the least amount of advance as possible. The combustion chamber is designed properly when it needs very little help in the form of advancing the timing to promote a better or more complete burn. When advancing the timing produces dramatic results - especially throughout the entire power band, it's probably best to "shelf" the design and start over with a new combustion chamber design.
Many new engines have compromised shapes to allow their owners a fudge factor of several octane points. The factories don't want warranty claims due to inadequate octane and they know they can't count on the public to always feed their machine the proper fuel - enter the compromised chamber shape. This is just one example of why a chamber shape is compromised. There are many though many have to do with the intended purpose of the engine; constant RPM sled engines, almost always on the pipe watercraft or "jet it and forget it" mopeds. Most of the time these engines can have their drive ability improved, throttle response enhanced, fuel efficiency increased and overall power strengthened through the proper design of combustion chamber shapes.
Secondary to shape, though at least as important is the ability of the head to rid the engine of power robbing heat. Together when working right, an engine will make more usable power over a broader range for a greater length of time. This is a common target engine builders shoot for, and it all stems from the commitment to a specific minimum fuel octane rating.
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