I think Kevin Cameron addressed this topic a couple issues ago in CW. Once again, my memory is not what it should be, so I can't cite his findings, but it had to do with how timing adjustments effect pulling the straight and top speeds. I think...
(I know it's not "scientific" but it's all I got.)
BTW, how much of a PITA was it to index the timing on the SR motor? I hadn't considered it when I built my XT, but I may try it next time. I wonder if I could do it without pulling the engine from the bike?
It was pretty easy, dial gauge, index wheel, etc, but you have to have to be able to take the cam gear on and off (assuming you have bought the indexable type cam gear) to change the position, which means you have to take the head cover off, which means out of the frame. The stock assembly is not indexable.
I found it 8 degrees off (Intake valve opening too late--which makes sense, with bigger lobes and increased lift times). The interesting part was that the megacycle cam would not match their exhaust specs. Once I had the intake dialed in, (I was informed by many sources that intake dialing in was more important than exhaust) I could not get the exhaust to match the specs. Gotta find out more from MC
I did the procedure 5 times to make sure I was getting it right . . .
As far as advance / retard, I am just talking about at idle.
Changing ignition timing at idle or thereabouts changes the efficiency at which the engine is running. At idle conditions...very little air and fuel, no load, these small changes in efficiency are reflected in engine rpm.
You might ask yourself...well why not time the engine for max rpm at a given throttle opening and achieve max efficiency....problem is it's only valid at that throttle opening and load. An engine will will typically increase idle rpm up to about 3-4000 by adjusting the timing up to about 40 degree or so at idle. Problem is under load, with large throttle openings at these low rpms the engine will no longer be efficient with that timing....or won't have the torque charateristics necessary for ridability.
Ignition advance basically gives the combustion a "head start" over piston position. At a certain point an ideal compromise is reached between this "head start", flame travel speed in the combustion chamber and piston position. But since flame travel speed varies with throttle opening, fuel mixture, compression and load, this ideal point changes when the bike is being ridden. Even dynamic compression changes with rpm and throttle position.
Camshaft profile, exhaust and intake design also effect the ideal timing....as these things effect volumetric efficiency...which affect torque and hp. That's why I want a programmable ignition....I run 47 degrees of straight advance...which is good for high rpm operation...but down around 6-7,000 rpm where the engine has a good volumetric efficiency I'm handicapping the engine by running an advance that's about ten degrees off the ideal.
It's more complex than this of course...and I don't know all the science involved...but as long as you know enough to estimate how changes will effect the behavior of the bike you have a good start.
Actually...I did a little cost benefit analysis and determined that I would probably be better off sourcing the system from someone like PVL. I've got the part numbers I need now...I've got Aaron pricing them for me.
To build it myself I'd have to go with a higherspeed MicroController than what I'm working with now....it's just too slow to make the calculations needed at higher rpms. The faster controllers are not particularly user friendly....I'd be looking at learning a C variant, buying the compiler software (at least $500) plus the hardware to burn the chips...plus most of them are "Burn once" chips, so the only way to make them programmable is to burn an interperter into the eprom and then build a flash memory setup to hold the actual program.
I really wanted to do this myself...but in the final analysis the payoff just does not justify the money and the hundreds of hours I'd spend on perfecting a design.
I'm going to keep playing with the BasicStamp MC anyway...see what can be done with it. On the internet I have seen some basic programmable systems made with inexpensive PIC's, most of them in some small way don't do what I want...and I don't have the knowledge at this point to modify them to my needs.
Nothing pisses me off as much as not learning something I want to learn....but I'd just have to dedicate too much time to this, to the exclusion of other things that need to be done.
I am playing with ideas like a variable length intake track driven by rpm and throttle position, using a PIC driving a servo motor that moves some velocity stacks. Kind of a reverse EXUP valve....electronics are a cinch....the mechanism would be a bitch.
I could slip it by tech as part of the "electronic ignition"