The ideal ignition advance curve

On reading about programmable ignition systems, there has been a comment which interests me. Some claim it is impossible to calculate the most desirable curve. I think it might be difficult but not impossible.
If you fit longer con-rods into a motor, the 'rock-over time' gets longer. Rod length to stroke ratio in a motor affects the leverage the piston has on the crank. I suspect the Commando rod length to stroke ratio is ideal for a motor which normally runs below 7000 RPM. If I was going to programme the ignition system, I would want to create maximum power at all revs throughout the usable rev range. Because the combustion is a chemical reaction, it probably takes a fairly constant time to run from start to finish. But if it starts at 28 degrees at 1000 RPM and the rock-ever time shortens as the revs rise, then ideally the spark should be advance to compensate for the shorter rock-over time. So the ideal advance curve is the rate of change of rock-over time. It changes for motors which differ in rod length to stroke ratio.

It might be interesting to run a motor with fixed advance and compare it with the same motor with the ideal advance curve with the same 28 degree static advance

The advance of the ingition from setting it on static to full is so short, it takes only about one - two thousand rev to go from static setting to full advance it wouldn't be worth worrying your bonnie little head about it!
But. . . . if you have a bee in your bonnet. . . . . .

The OEM curve is excellent for an essentially stock engine. That's the only curve I would use unless the engine is extensively modified. In that case I would put it on a dyno and determine the best advance setting with the type gasoline you will use. In that situation I suspect that the best advance curve would parallel the original "curve" but with a change to the maximum advance number.

I think that if you buy an ignition system which has a programmed curve and it works well, the curve was probably a good guess. - Or perhaps it does not matter ?

MexicoMike said:

The OEM curve is excellent for an essentially stock engine. That's the only curve I would use unless the engine is extensively modified. In that case I would put it on a dyno and determine the best advance setting with the type gasoline you will use. In that situation I suspect that the best advance curve would parallel the original "curve" but with a change to the maximum advance number.

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When you change the static advance, you simply shift the whole curve forwards or backwards. The best shape of the curve depends on the rod to stroke ration which differs in various motors. The other thing is, as you wind the throttle on and the revs rise, the taper on the needles comes into play and the balance between mixture and advance has an effect.

Bernhard said:

The advance of the ingition from setting it on static to full is so short, it takes only about one - two thousand rev to go from static setting to full advance it wouldn't be worth worrying your bonnie little head about it!
But. . . . if you have a bee in your bonnet. . . . . .

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If I thought a programmable ignition system might provide a major performance benefit, I would install one on my bike. I think a lot depends on what fuel you use. With petrol, getting the tuning right is more important than it is if you use methanol.

My Pazon Surefire works great with 110LL. No kickback, just starts.

acotrel said:

When you change the static advance, you simply shift the whole curve forwards or backwards. The best shape of the curve depends on the rod to stroke ration which differs in various motors. The other thing is, as you wind the throttle on and the revs rise, the taper on the needles comes into play and the balance between mixture and advance has an effect.

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The best performance for a typical 4 stroke motor has always been to have all the advance in by around 3k RPM. The "shape" of the curve is really not too important since we are only talking about what happens between idle and 3k. There are some high max RPM motors that benefit from a reduction in advance at the very top of the range but that is way beyond the RPM range of a Norton motor. It also requires a computer-controlled engine where all aspects of the fuel/air mixture/load can be considered/adjusted.

Ideally, for optimum low speed operation/efficiency, these engines would benefit from a vacuum advance as cars used to have...

ignition advance is arcane. like acotrel says, there are purely mechanical considers that have to do with stroke radius and rod length. but the way the mixture burns is wildly variable, because so many other things are going on in the cylinder-- compression ratio, mixture velocity, vacuum, swirl, tumble, squish, reversion, extraction, lift, duration, temperature, density, mixture strength, and so on. and while the chemical reactions might run at a constant rate at any fixed rpm/temperature/load/etc, the actual burning of a charge changes with turbulence, which is why additional advance isn't necessary above some intermediate rpm, even as engine speed increases. and you can theoretically say that the best spark curve is the one that produces the highest BMEP at each rpm, but then you have to decide whether biasing the curve to take away pressure at one rpm and deliver more at another rpm is a "better" or "worse" curve.

the way to do it is to test and map it. even this one with only one variable--load-- is way more complex than a straight line or a simple curve. look at the hole at light load at 5000 rpm, where the ignition advance needs to go down, rather than up



but you have to have a computer to do it, and EFI to apply it, and none of my beaters will ever have that. so we're stuck with a pretty rudimentary view of what's going on. i still think testing is the way to go, because it skips all the science and goes directly to where you want to look. but we'll never be as sharp as an ECM could be, in a different world.

just my two cents. i ain't no expert

dorset said:

ignition advance is arcane. like acotrel says, there are purely mechanical considers that have to do with stroke radius and rod length. but the way the mixture burns is wildly variable, because so many other things are going on in the cylinder-- compression ratio, mixture velocity, vacuum, swirl, tumble, squish, reversion, extraction, lift, duration, temperature, density, mixture strength, and so on. and while the chemical reactions might run at a constant rate at any fixed rpm/temperature/load/etc, the actual burning of a charge changes with turbulence, which is why additional advance isn't necessary above some intermediate rpm, even as engine speed increases. and you can theoretically say that the best spark curve is the one that produces the highest BMEP at each rpm, but then you have to decide whether biasing the curve to take away pressure at low rpm and deliver more at higher rpm is a "better" or "worse" curve.

the way to do it is to test and map it. even this one with only one variable--load-- is way more complex than a straight line or a simple curve

but you have to have a computer to do it, and EFI to apply it, and none of my beaters will ever have that. so we're stuck with a pretty rudimentary view of what's going on. i still think testing is the way to go, because it skips all the science and goes directly to where you want to look. but we'll never be as sharp as an ECM could be, in a different world.

just my two cents. i ain't no expert

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The data is multivariate, but if you don't compare various motors and only concentrate on your own, many of those variables remain constant. It is difficult to visualise the interaction of more than four variables. But most of us can think in terms of 3 dimensions and a time variable. The pattern becomes a landscape. In games theory, you optimise by winding your way along the bottom of the valleys where the energy is minimised.. With our motors the comp, ratio, fuel type and exhaust system usually stay pretty constant. What we play with is throttle position, jetting and ignition advance. As you wind the throttle on you raise the needles in the carbs and the taper compensates for loss of vacuum. If the taper is too quick, you richen the mixture at a rate which might be detrimental - you get less power. But if the igniition advance increases, it has the same effect as leaning off, so you ,might get restoration.

With my motor, I use the slowest taper Mikuni needles and fixed ignition advance. It means I need to feed the throttle on in a controlled fashion. If I used a programmed advance curve, I might be able to opemn the throttle much quicker. But because I use methanol fuel which is much more flexible than petrol, for me a programmed curve would probably not deliver an advantage.. But with all these things, you never really know until you try it.

what compression ratio and advance do you use with methanol? i have read in the Old Books that the normal retarding you can expect to use with higher compression is cancelled by the slower burn of the alcohol, so that the end advance is mostly the same. but then i see in their charts that their final timing is still about 4 degrees retarded compared to non-alcohol fuel.

MexicoMike said:

The best performance for a typical 4 stroke motor has always been to have all the advance in by around 3k RPM. The "shape" of the curve is really not too important since we are only talking about what happens between idle and 3k. There are some high max RPM motors that benefit from a reduction in advance at the very top of the range but that is way beyond the RPM range of a Norton motor. It also requires a computer-controlled engine where all aspects of the fuel/air mixture/load can be considered/adjusted.

Ideally, for optimum low speed operation/efficiency, these engines would benefit from a vacuum advance as cars used to have...

Click to expand...

At the other end of the spectrum, there are some race camshafts that allow higher revs, but require far more advanced ignition than normal.

dorset said:

what compression ratio and advance do you use with methanol? i have read in the Old Books that the normal retarding you can expect to use with higher compression is cancelled by the slower burn of the alcohol, so that the end advance

dorset said:

what compression ratio and advance do you use with methanol? i have read in the Old Books that the normal retarding you can expect to use with higher compression is cancelled by the slower burn of the alcohol, so that the end advance is mostly the same. but then i see in their charts that their final timing is still about 4 degrees retarded compared to non-alcohol fuel.

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is mostly the same. but then i see in their charts that their final timing is still about 4 degrees retarded compared to non-alcohol fuel.

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With my 850, I have not increased the comp. ratio. I run 28 degrees +4 = 32 degrees advance. But I run it jetted very lean. Methanol works well at all comp. ratios. If you raise the comp. ration, you usually need to use bigger jets. For 9 to 1 comp. I use 0,116 inch needlle jets. If I was using 12 ro 1 comp. I would use 0.120 inch needle jets, and 28 degrees advance. At 14 to 1 comp. it is normal to back the advance off about 4 degrees.
The beauty od methanol is it's flexibility. Even if you get it too rich, you still get an increase in power compared with when you run petrol. The main reason is methanol hides-up the tuning errors.
That is not to say that if you tune with it with the same fastidious care you need with petrol, you would not get much more power. Methanol has about 0.8 times the calorific value of petrol,, however even when jetted lean, you use about twice as much of it, due to it's much higher latent heat of vaporisation.
It is much easier to get a motor going fast if you use methanol, than it is when you use petrol. Your margin for tuning errors is much greater. With petrol, getting it tuned right is much more critical.
Ignition advance, comp ratio and jetting all interact in balance. If you do not change your fuel, when you change one of those factors, you usually change one of the other two. If you have a programmed advance curve, you can probably tune closer to the optimum at all revs.

I wonder how many guys experiment by fitting different taper needles ? I use the leanest and feed the throttle on. - You would not do that with a road bike.

Bernhard said:

At the other end of the spectrum, there are some race camshafts that allow higher revs, but require far more advanced ignition than normal.

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I don't think I've ever seen a modified 4 stroke engine that wanted more than a couple of degrees difference in max advance over the stock engine to make max power. High profile cams do require a lot more initial advance or they won't idle at all but the max advance for them is typically within 2-3degrees of the optimum max advance for the stock setup. Re that, optimum for stock is typically NOT per factory specs - needs to be evaluated/adjusted on a dyno.

All engines are different - even identical builds can turn out to need very different initial advance for good low/mid operation. I've seen the need from as low as 6 degrees up to 18 degrees of initial advance on two "identical" engines.
Of course, that requires tailoring of the curve with different weights/springs/inserts on a standard distributer because in the example, the final max advance figure for most power - 7500 RPM all advance in at 3200 RPM) was 36 degrees (same as stock) on one engine, 37 on the other.

FYI, the 500 Domiracer purchased from the factory by Paul Dunstall was one, but this was full race tune. He mentioned it required a lot of advance.
The Yoshimura cam I that i purchased for a Jap bike also required the same......or it wouldn't rev to 13,000!

MexicoMike said:

The best performance for a typical 4 stroke motor has always been to have all the advance in by around 3k RPM...
Ideally, for optimum low speed operation/efficiency, these engines would benefit from a vacuum advance as cars used to have...

Click to expand...

The engines I routinely ( used to ) work on, Mercedes M130 with mechanical fuel injection, have vacuum retard, ignition at starting rpm
5° ATDC.
MM : advance figures are given at sea level.
I believe you are in Mexico City.
Do you compensate for the altitude ?

Yes, I set the timing 5 degrees advanced over the stock setting.

DogT said:

My Pazon Surefire works great with 110LL. No kickback, just starts.

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Dog, I've got to ask, what color is 110LL? I think you made a typo there. 100LL is blue.