I was thinking of ways to make PCM tuning easier and had an idea.

Is there a mathematical formula to determine what ignition timing is needed or "optimal" at certain RPMs and MAP values, if one knows the cam specs and compression?

I am thinking that with such information, one would know where the piston would be in each bore, and also what valves would be open and how far... and thus would know what would be optimal timing for best combustion.

If my thinking is correct, or even close, wouldn't it be possible to get a fairly close timing map by using the formula for all RPM/MAP intersections? And then, perhaps, even write a small computer program to do it?

Someone shoot some holes in this or tell me I'm on to something.

 

Sure, I’m surprised that no one else has shot holes in it.

But without spending too much time on it, ignition advance is all about compensating for the delay between when the spark fires and the charge ignites, and between the delay from ignition to the pressure peak, to get the pressure peak at a point of the piston’s power stroke where it could use it most efficiently. If the spark fired and you got instant, complete combustion you could just set the timing and that would be it, no advance necessary (and you’d need a much stronger bottom end).

The 2 delays depend on tons of things besides piston travel speed, pressure, charge temp, primarily ignition speed, flame front speed, combustion chamber shape, how that shape changes and how the gasses in the chamber are moving. RPM then just adds the variable that you’ve got to take care of all of that before the piston moves down too far in the bore to take advantage of the power produced.

If you could model all that (and more) with a relatively simple equation… well, go for it.

 

Oh, BTW, if you can come up with a relatively simple equation to do it, then take a look at a pressure graph for combustion in a cylinder and you’ll find that it’s not a simple curve, but more of an island because in most chamber designs, even with the same chamber with seemingly identical conditions you still get a wide range of results.

 

AFAIK even the OEM's do this by trial and error. There is absolutely no way any simple formula could work. Variables include compression ratio, fuel used, combustion chamber design, materials used (aluminum v. iron heads, etc.), valve timing (cam related), operating temps, etc. If you had a very sophisticated model of your engine, you could probably derive some sort of a formula. But it would take a very long time to derive and would only apply to that particular combo.

It's not hard to get to achieve close to optimum timing using trial and error anyway, at least when tuning on some sort of a dyno. Start somewhere "reasonable" for your combo, make runs a couple of degrees up and a couple down and see which trends in the right direction. Repeat until max hp is achieved. Usually takes 30-45 min on a chassis dyno with time between runs to cool off. Of course, this leaves out of consideration subtle differences between running under varying loads on the road, etc. But it works well.

Beware of simple formulas such as "retard timing x degrees for y amount of boost", and the like. They are seldom accurate. My friends 5.0 Mustang with a Roots type blower likes ~20 degrees of timing using C-17, my centrifugally SC LT1 likes ~30 degrees running 100 octane unleaded. Awfully hard to think of a simple model that would predict that kind of discrepancy.

Rich Krause

 

OK, thanks a lot for the input.

But, can you guys clarify one thing--is this basically a matter of, "yes it's possible, but I wouldn't want to actually do all the math," or that no, there are some formulas needed that don't exist?

If it's the former, why not simply take the time to look at all the equations necessary, and come up with a new formula that includes everything? Is this possible or not? If so, why has no one done it?

Thanks again, this is very interesting.

Solomon

 

From the little bit of knowledge that I’ve got about the topic, it’s not possible because there’s some randomness (a possible range of outcomes) to the cylinder pressure spike that’s nearly impossible to predict from cycle to cycle, much less from cylinder to cylinder. I actually did run across an effort to predict the seemingly random part of it that has been referred to as the conservation-of-delay theory which supposedly comes within a fraction of a millisecond in predicting the onset of detonation, but this would only be applicable in situations where max power/timing is limited by detonation.

In a nutshell, it’s much harder to do mathematically then it is empirically.

 

Solomon, it must be possible on some level, Engine Analyzer shows timing stuff, and even warns that it would expect there to be knock at x timing with x gas and so on.

I think we talked about some stuff like this in the past, to come up with some software to make tuning easier, at least as a baseline - I forget if its MoTeC or who, that has software that calculates base timing and VE maps, so someone is doing it.