Seek comments from engine studs

20050_3's ignition is retarded 3 degrees in all cells from 2,000 RPMs to 7,000 RPMs in the 50% to 100% throttle position while 20145_1's ignition is retarded 3 degrees in the same fashion except the retarding starts at 2,200 RPMs instead of 2,000 RPMs.

If you put Jardines and triples (foam or K&Ns) onto your bike, you are bringing more air into the engine; and with a remap you'll be bringing in more fuel, so I'm assuming the the A/F mixture will be denser...correct? And is it true that pressure increases with density, so adding more air and more fuel will increase your pressure...or do you need higher compression pistons to increase the pressure?

Does a denser A/F mixture in the combustion chamber mean it doesn't need as much time to ignite, hence the reason for retarding the ignition?

Or does a denser A/F mixture takes up more room in the combustion chamber and therefore need more time to explode, hence advancing the engine.

For you engine studs who know the answer, please help me out here. I tried like hell to find my answer surfing various websites but couldn't do it.

Intake and exhaust mods increase the volumetric efficiency of the engine. So more air can enter and exit the engine quicker, which means more fuel is needed. When ignition happens with more air and fuel there is a bigger bang which equals more hp. Complete combustion is achieved at an A/F ratio of 14.5 to 1, otherwise known at stoich. However, more power can be had (bigger bang) at lower A/F ratios, however it is not a "complete" combustion. The more a/f mix entering the engine the denser the mix (meaning 13/1 vs 14.5/1) and the higher the compression. However a compression ratio is static and based on bore diameter, stroke and cylinder volume. So to answer your question, yes higher compression can be achieved by increased volumetric efficiency. Higher compression pistons and different head gasket sizes will allow the mix to be compressed even more, making the compression ratio higher.

Does a denser A/F mixture in the combustion chamber mean it doesn't need as much time to ignite, hence the reason for retarding the ignition?

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Not necessarily, it means air and fuel are entering the engine quicker. It also means that at higher RPMs the engine can develop more power because it can "breathe" better at higher RPMs.

Or does a denser A/F mixture takes up more room in the combustion chamber and therefore need more time to explode, hence advancing the engine.

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I think the reason why Wayne achieved more power in the those RPM ranges by retarding the ignition is two fold. First is because total combustion time was decreased by the addition of the intake and exhaust mods. It takes less time to fill and evacuate the combustion chamber thus the spark could be slowed to allow ignition happen as close to the power stoke of the piston as possible. And secondly the mix is more volatile since it has a lower A/F ratio and burns quicker.

But whatever you do, don't use the timing changes that Wayne did as a standard for tuning all engines. Remember that Wayne started out with Triumph's stock tune and retarded the ignition from that. The stock tune was developed by Triumph to not necessarily make the most power but to meet emission and noise standards and to allow the cat box to operate efficiently. From what I remember he was equally surprised to of gained hp by retarding the ignition in those ranges.

Thanks Pig9r! The part about the combustion chamber filling up quicker was something that puts retarding the spark in a much better context. :bch: