Ignition Timing Mechanical Advance

[Zed Head]

So the "real" question is why do they let the curve go flat...?  Not the actual numbers.

Here's a 72 curve that goes flat at 2000 RPM, 10 degrees advance.  On top of initial, 17 degrees, it might be only 27 degrees total.  All in 27 degrees at 2000 RPM.  Not sure really which advance goes with which initial.  They give distributor numbers in one chapter and car options in the Tuneup chapter.  I assume that high initial would go with low centrifugal advance.

[240260280]

The vacuum advance comes in at high rpm at low VE when cruising.

The advance IS related to the quantity of air and fuel in the chamber.

[AK260]

OK so just putting the question of why it goes flat to one side for a moment, here is a thing that I don’t fully get:

Conventional wisdom says best timing for these engines is around 34 degrees - yet none of the stock distributors go that far. In fact the majority top out at 27 degrees.



My tuner who spent a total of 4 hours on the Rolling Road was adamant that 28 degrees @ 3500 rpm was the ideal. Now I appreciate at 10.5:1 my engine isn’t stock and I also think he was playing it a little safe.


But the question is - how has the z world come to the conclusion that the ideal is 34 @ 2800ish and why?

[jonbill]

So the "real" question is why do they let the curve go flat...?  Not the actual numbers.
Here's a 72 curve that goes flat at 2000 RPM, 10 degrees advance.  On top of initial, 17 degrees, it might be only 27 degrees total.  All in 27 degrees at 2000 RPM.  Not sure really which advance goes with which initial.  They give distributor numbers in one chapter and car options in the Tuneup chapter.  I assume that high initial would go with low centrifugal advance.

I think the curve goes flat because that's the best you can do reliably with springs and weights and centrifugal force. Electronic systems may reduce advance after the initial peak as the engine gets up to peak torque and then increase again afterwards.

[jonbill]

34-38 ish is common to engines, not z specific. It was impirically determined back in 1429 by the Lindisfarne monks.
The variations are through different chamber designs having minor differences in combustion speed.

[240260280]

Chamber size, compression, location of plug and chemistry of the gas affect the maximum advance setting.  Higher compression/smaller chamber engines need less than lower compression/larger chambers.

[Captain Obvious]

18 hours ago, jonbill said:

First, although the combustion takes a reasonably constant time, that combustion doesn't proceed linearly. It starts slow with a small flame front and grows and gets faster.

At 1000rpm with 15 deg advance, the 1.3 ms of combustion takes 37 degrees of crank rotation. 15 BTDC and 22 ATDC (say). most of the burn and and nearly all the gas pressure happens when the piston has passed TDC (ATDC)

I know that the combustion doesn't proceed linearly and it grows. That's the whole reason the mechanical advance exists in the first place. The mechanical advance is supposed to account for the fact that the burn doesn't all happen at once. Takes time to build. (Note my point here is that it takes TIME. It doesn't take "degrees".)

And about the rest of your note above... I'm not sure I get the point of your examples, and I don't think the math is right.

At 1000 RPM, the engine is spinning at 0.167 ms per degree*. So your example of 15 degrees of advance at 1000 RPM would actually light off the spark at 2.5 ms BTDC.

And at 1000 RPM, 1.3 ms of combustion is only 7.8 degrees of rotation, not 37.

Here's some hard numbers using 1.3 ms as a fixed advance BTDC:

The question isn't so much about the down low range. In fact, those numbers are pretty much exactly like stock up until 3000 where the original mechanical advance tops out. It's back to the question about "why does it go flat?" above that and "would it be better to continue to advance?" if you had reliable means.

[240260280]

cube or square law of volumes?

Cube Root does not grow much:

 

Perhaps the advance change  is only for idle emissions and transition only?

[jonbill]

I know that the combustion doesn't proceed linearly and it grows. That's the whole reason the mechanical advance exists in the first place. The mechanical advance is supposed to account for the fact that the burn doesn't all happen at once. Takes time to build. (Note my point here is that it takes TIME. It doesn't take "degrees".)
And about the rest of your note above... I'm not sure I get the point of your examples, and I don't think the math is right.
At 1000 RPM, the engine is spinning at 0.167 ms per degree*. So your example of 15 degrees of advance at 1000 RPM would actually light off the spark at 2.5 ms BTDC.
And at 1000 RPM, 1.3 ms of combustion is only 7.8 degrees of rotation, not 37.
Here's some hard numbers using 1.3 ms as a fixed advance BTDC:

The question isn't so much about the down low range. In fact, those numbers are pretty much exactly like stock up until 3000 where the original mechanical advance tops out. It's back to the question about "why does it go flat?" above that and "would it be better to continue to advance?" if you had reliable means.

OK, I see your point now. If its OK to have first 0.5ms of combustion before TDC at 1000 rpm (where it is 15 degrees) , then why isn't it still OK at 6000 rpm.(where its 50 deg or whatever).
I don't know. Something to think about!

[Zed Head]

Piston speed should be considered.  Not crankshaft rotational speed.  Unless it's a Wankel.

Throw in the angle of the rod also.  At higher degrees advanced the leverage on the crankshaft increases dramatically.  Small pressure can overcome the inertia of the crankshaft and associated parts.  

Just tossing wrenches...

Edited March 12, 2020 by Zed Head

[Captain Obvious]

Many moons ago I calculated piston speed in depth when I was poking around with header designs. I've got a file around here somewhere the documents it very well. It was a couple of computers ago and exists on a CD ROM somewhere. I'll see if I can dig that up.

2 hours ago, jonbill said:

If its OK to have first 0.5ms of combustion before TDC at 1000 rpm (where it is 15 degrees) , then why isn't it still OK at 6000 rpm.(where its 50 deg or whatever).

Right. So is there a timing number that you think is "optimum". Pick an advance at a specific RPM that you think provides the perfect 22 ATDC pressure peak and I'll plot that out.

@AK260, you suggested two of them...

28 degrees @ 3500 rpm - your tuners numbers for your 10.5:1 motor. 
34 degrees @ 2800ish  - From "the Z world"

Also when I get a chance, I'll back calculate your mechanical advance degrees into time measurements and see how that looks on a curve.

[Captain Obvious]

15 hours ago, 240260280 said:

Chamber size, compression, location of plug and chemistry of the gas affect the maximum advance setting.  Higher compression/smaller chamber engines need less than lower compression/larger chambers.

@240260280  Thanks for your input too. All that stuff would certainly have an effect, as does VE changes with RPM.

With all your research... Have you come up with any curve that shows VE versus RPM for our motors? Any idea where it typically peaks?