hobot said:
Alan the Harley ignition is not a hi rpm related issue but for a lugging loaded down hi throttle low vacuum Anti-detonation feature that is not related to the reason comnoz and others use retard near top rpms, Which is to get more combustion pressure as volumetric efficiency increases [on cam/pipes] upping compression charge density when flame fronts can travel faster so need less lead time to give most pressure just after TDC.
Steve, if you read about the programming of the Harley ignition, the curves relate to a rev range up to about 6,000 RPM then the advance is held constant to 8000 rpm unless the pressure switch operates if the manifold vacuum is too low, and detonation might occur , - then the second less advanced curve comes into operation. To me those are the ranges the commando operates within. What it means is that the static advance can be increased without the danger of detonation because the timing will retard if you open the throttle wide and lose vacuum. I use methanol fuel which is said to have unlimited antiknock we also use it in both H1 and H2 Kawasaki triple two strokes which run extremely lean to get best power. My brother had a lead pop out of a coil, and just plugged it back in. This upset one of the boxes in the Lectrix ignition system , and there was a detonation which cracked a cylinder head.
In the end, there is a balance which must be maintained between comp. ratio, mixture, and ignition advance to maintain a certain condition in the combustion chamber. As the revs rise the timing should advance to allow time for the optimum combustion event to take place. If it has done that and you open the throttle wide and lose vacuum, the mixture becomes even leaner. Under normal circumstances, a fixed ignition timing means the amount of advance you can use is very limited. So more energy goes out the exhaust pipe (you don't capture the whole event even with the best designed cam ).
With petrol the situation is even worse, it actually is more prone to detonation and high combustion temperatures, and that is what limits the amount of advance you can use.
With the late 70s 350cc two strokes, power valves and power jets are used to try and stop detonation, particularly with some expansion chamber configurations. If a two stroke bike is not jetted lean enough, it will usually be very slow and sometimes not even run. I know that racing TZ350 Yamahas run much less static advance than RD350s and you always have to feed the throttle.
With my 850 commando, I add 4 degrees advance to the static timing over that used for petrol when using methanol, and I'm not that particular about the main jet size. The mid throttle jetting is extremely lean. I only use the bike on a short circuit with mainly twisty bits and only two fairly long straights. If I wanted to take it to Phillip Island, I'd have to make it go slower or it would do damage to itself. Your Daytona would be horrendous if you had the bike set up well for a short circuit. Gearing is another factor - if the bike is set up to pull a higher overall gearing due to the cams and exhaust you are using and the limitations of the crankshaft, the frequency of throttle wide open in high revs situations is increased. The six speed CR box should help.
