Accepting that there is probably more bullshit spoken about ignition curves than almost any other aspect of ignition....
In the 70s it was typical to use points ignition for racing, but with the advance retard unit welded solid, don't consider this to be using no advance, in reality it is 'no retard'. Ignition is retarded to cope with starting and at lower rpm running to make the machine accelerate more smoothly on a light throttle setting. The only timing figure of concern for racing is fully advanced, because you have little interest in engine performance lower than say 4000rpm!
Yes, it can be difficult to start an engine with a fully advanced spark....because it will kick back, but this is of no concern when the bike has no kick starter!
In '76 upgrading to an ex-Thruxton works race team Lucas Rita ignition seemed like a useful thing to do, and I was able to buy one! So I fitted it and started the motor and strobed it....only to find it had no advance at all and I had to set it fully advanced.....at least I did not then need to run it to 5000 for timing checks! The accuracy of the timing was better than points, but still limited by cam chain float (as always and see furher down).
When I opened the ignition box (not something you will do with a potted Boyer) I found that a jumper wire had been soldered in to link out the advance circuit....so...circa 73/74 the Norton race team ran their ignitions fully advanced, even though they did use electronic ignition, they did not use an advance curve....at least on one ignition, but I suspect it was typical of their set up.
Second fact is that If you run too much advance you will promote detonation. What is too much, well of course it is variable since the point at which detonation occurs is affected by fuel RON rating and load.
Running as much advance as you can to deliver as much power as can be achieved with good fuel and without causing engine damage typically means on a Commando, with say a PW3 or something similar, around 32 degrees of advance. (though I would like to hear from anyone using more advance what their setup is).
28 degrees was a good compromise for road use with a mild cam and variable fuel quality. With wilder camshafts than a PW3 you might benefit from running more advance...Manx Nortons for example, as far as I know, ran around 36 degrees with more radical valve timing.
Steve Maney will supply you with an analog Boyer ignition, it is the only one he uses, it is standard Boyer. He will also supply a mounting kit to use the ignition timing direct from the crank This will remove some of the timing fluctuations due to the cam chain flutter, though of course the valve timing is still moving around a bit! The Norton experimental shop ran tests on crank driven ignition timing in '75....I used to have the MK III crank that was used for these trials in my racer in the '70s, I got it from a guy made redundant by the closure in late '75.
Using crank driven ignition timing with a standard ignition you may realise means that the advance curve as shown in ludwigs diagrams above is now halved, since we are running the ignition at engine speed not half engine speed. Ignition is set at full advance anyway, and as we have said in this race engine application we have no need to retard it for light throttle openings, so fully advanced is all that matters to us, and as some of you may know, Mr Maneys bikes are quite quick on the Daytona banking!.
A side effect of this half advance curve seems to be that at the noise testing rpm or 3750 for racing....the recorded db figure is according to Steve lower than with a half engine speed Boyer fitted! Bizzare, but useful for racers struggling to get under the noise limit.
So, fiddle with the advance curve as much as you want.....some of us would just as soon live without one...but if my ignition does have an advance curve I will certainly make sure that the ignition is correctly fully advanced by 5000 (or less!), rather than set it at 3000 and have no idea what is actually happening at max advance and indeed where that occurs, since as has been noted, ignitions packed by suppliers in the same box actually vary.
