Captain Obvious

Ignition Timing Mechanical Advance

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    So the recent chatter about timing and distributers has me thinking.

    The stock distributors include a mechanical (centripetal) advance function and the spec varied some over the years, but as an example here's the spec for 77:

         0 degrees at 600 distributor RPM (1200 RPM at the crank)
         8.5 degrees at 1250 distributor RPM (2500 crank)

    There should be no mechanical advance below 1200 RPM, then start ramping up to a max of 8.5 degrees by the time you reach 2500 RPM. Once above 2500 RPM the mechanical advance is topped out and holds at 8.5 degrees all the way up to redline.

    Specs like this are given in degrees because that's what can be measured and that's what you can see with a timing light. But if you convert the timing specs from the angular domain (degrees) over to the time domain (seconds), it looks like this:

         At 1200 RPM, the engine is spinning at 20 rotations per second.
         At 20 rotations per second, each rotation takes 50 milliseconds.
         If each rotation takes 50 milliseconds, each degree of rotation (out of 360) takes 139 microseconds.
         So at 1200 RPM, the 10 degrees of base advance provide about 1.4 ms of advance.

    Doing similar math, if the engine is spinning at 2500 RPM, then each degree of rotation takes 67 microseconds. The 18.5 degrees of advance (10 base + 8.5 centripetal) provides about 1.23 ms.

    If you go through all the math and plot the results, you come up with something like the following. Notice that the advance value where the centripetal advance is active is relatively flat, but then the advance falls off above that once the mechanical advance has topped out:
    timing1.jpg

    Studying the numbers, I come up with a relatively constant average of about 1.2- 1.4 milliseconds of advance in the centripetal advance region.

    So my question is.... Why didn't they continue to advance the centripetal timing above 2500 RPM? Is that just a limitation of the mechanical functioning of the distributor? They just couldn't get the springs, weights, slots, etc to swing a wider range all the way up to redline? Isn't necessary for some reason? Or wished they could have, but just couldn't come up with a cheap enough way to do it?

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    Maybe the benefit of more advance tails off itself.  No need for more.  Or...black hole.

    You could compare to what the pros run using electronic engine control.  Seems like there must be some dynamometer studies out there somewhere.

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    The purpose of firing the spark before TDC is to get max cylinder pressure at the optimum point - I believe I read that the maths works out at about 22 deg ATDC is the optimum point. If combustion was instant, then 22 ATDC would be the spark timing, but combustion takes time and so the ignition is started at the appropriate time before hand.

    Since the speed of combustion is reasonably constant, as engine speed goes up, you have to start ignition earlier. I.e as the engine goes faster, you need more degrees of rotation for the time it take for combustion to reach its peak at 22 ATDC.
    So that's why the advance curve is set to give about 15-20 advance at idle and 30-35 at 3000 rpm.

    The question then is, why not keep advancing the ignition timing after that? The speed of combustion is still pretty constant and the engine is getting faster....
    And I think the answer is something to do with by the time you're igniting at much more than 35 BTDC, the amount of cylinder pressure you've developed while the piston is still rising becomes significant and a problem (detonation) , so you have to compromise and accept that peak pressure must come increasingly after 22 ATDC as the engine speeds go up beyond that.

    Edited by jonbill
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    I think JB said it perfectly.

     

    The only other thing I’be read in addition to what has been said already is that above 3kish rpm the VE increases which helps with better air / fuel mixing and not as much time required for a full burn. I could be wrong so happy to be corrected. I have to dig out the articles again.

     

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    6 hours ago, jonbill said:

    The question then is, why not keep advancing the ignition timing after that?
    The speed of combustion is still pretty constant and the engine is getting faster....

    Yes. That's exactly the question. And thanks for the input into potential answers.

    I get the need for the advance simply because the engine is spinning faster. That's the easy "first order" part. The more difficult part is "Why is it all in at 2500? Would it be better if it went further?" I guess I'm looking for the second order kind of effect, right?

    5 hours ago, AK260 said:

    The only other thing I’be read in addition to what has been said already is that above 3kish rpm the VE increases which helps with better air / fuel mixing and not as much time required for a full burn. I could be wrong so happy to be corrected. I have to dig out the articles again.

    http://www.s262612653.websitehome.co.uk/DVAndrews/

    And thanks for that input as well. That website is a good read, but unfortunately, not much info to answer the question about "why did they stop". About the only part that pertains to that question is the table of "Establishing maximum advance requirement" - "Notwithstanding the compression ratio and other factors, the characteristic that determines the maximum advance setting is the shape of the combustion chamber and the position of the spark plug."

    So it seems that the VE could be a factor. That's one of those "second order effects" I was talking about.

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    I think it's just colliding asymptotes.  You're asking why did "they" stop, or why is the effect there?  


    I wonder if it changes with a turbocharger.  Get away from the 14.7 psi limit.

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    Using mostly jonbills words (since they were so beauteous)...

    Since the speed of combustion is pretty much constant, why would there be an upper limit to the mechanical advance?

    In other words,... Why did Nissan stop advancing the mechanical timing at 2500 RPM? Why not keep advancing the timing mechanically all the way to redline?

    At first blush, there's no (mathematical) reason to stop. The simple math is the "first order" effect. But things like volumetric efficiency isn't simply math.

    Edited by Captain Obvious
    its not simply math

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    And just because everyone likes pictures... Here's what the timing would look like if you just hard peg the advance at 1.28 milliseconds BTDC. I tried to model the extended curve using the stock slope as a guideline and that's what I came up with. Mechanical only (no vacuum):
    timing2.jpg

     

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    The combustion is happening both sides of TDC. With low engine rpm, most of the combustion and gas expansion happens on the downside after TDC. As engine speed goes up, in order to keep peak pressure at 22 ATDC, more and more of the gas expansion has to happen on the upside, before TDC. Somewhere around 2500 to 3000 rpm the benefits of increasing advance and the downside of increasing combustion before TDC cross over and so Nissan (and everyone else) decided to limit the mechanical advance at that engine speed.

    Imagine the ignition has been advanced to 80 deg BTDC at 6000 rpm. (I'm just guessing at numbers!) combustion had to complete between 80 BTDC and 22 ATDC. That means approx 80% of combustion has happened before TDC, and that's a lot of gas pressure trying to stop the piston from coming up, and the gas is under very high pressure and any remaining fuel mixture is going to detonate. So you get serious knock and lots of heat but no more power.
    Something like that anyway :)

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    Thanks again for the input. I appreciate the discussion (which is clearly academic in nature).

    You said above "the speed of combustion is reasonably constant." If that's the case, then the same amount of time after a spark would develop the same pressure at the same rate regardless of the rotational speed of the engine. That's the whole point we both made above (and it's the theory behind adding centrip advance at all in the first place).

    I'm saying don't think degrees. Think TIME. The problem is that "degrees" changes with rotational speed. Time does not.

    If the burn takes 1.3 milliseconds to develop optimum pressure at 1000 RPM, wouldn't it take 1.3 milliseconds at 2000 RPM as well? And 3000 RPM? And 6000 RPM? Why would the burn occur faster at 6000 RPM that would necessitate backing off the advance* at that speed?

    If the burn rate is constant (in TIME), then shouldn't it be the same regardless of RPM?

    In other words, the fuel mixture doesn't know or care what the engine RPM is. All it knows is at what TIME it was lit off. If it burns at a constant rate you should adjust the light off to the same TIME regardless of how fast you're approaching TDC.

     

    * I say "backing off" because the speed goes up, but the amount of advance does not. It's like volunteering because everyone else in the line steps back. Just a different point of reference.   LOL

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    @AK260, you had posted a pic earlier showing the timing you're using with your 123 device...

    The button at the bottom right of the screen "Edit centrifugal curve" indicates that this is what you're running for your mechanical advance. Is that correct? That pic looks much closer to what I posted above than the original stock configuration.

    Is this just mechanical, or is this mechanical plus vacuum? :
    c94d1edfbd784d14af395edbc9b9a7ef.png&key

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    So Captain, this is purely my mechanical advance. I have zero base advance, which does mean I have to crank the car a bit when cold but at least it’s WISIWYG and I won’t do something dumb + cranking the extra 5 - 10 secs helps to oil get round before the action starts.

     

    The vac curve is a work in progress at the moment.

     

    Here is the stock 240z vac curve in 123 speak ...

     

    cffcd44038f1185ed184b207b4505db2.png&key=d81da94e5054dfc1a54569da18bc60e0980ab09473af09d3a940557d3017573e

     

    It is basically a recreation of the upper vac curve of this ...

     

    ef62d6269f81501d9085b98d45105ad9.jpg&key=d10e366c9ab424f104906406cc9792359aab96380b858470e72a8f1e24579e8c

     

    Does that help?

     

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    OK - here is another rookie thought for discussion - doesn’t the dynamic compression increase with revs? Wouldn’t that heat the charge more and so easier for flame front to propagate?

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    Volumetric Efficiency must come into play.... that is the amount of air (and fuel) filling the chamber.

    As the throttle opens wider,  more air  and fuel enter the chamber. Somehow this must be part of the reason why advance is limited at higher RPM WOT.

     

     

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    Piston speed, and combustion chamber volume, varies dramatically with crankshaft position.  If you created a plot of chamber volume with time, there would be a long stretch where it was close to its minimum, almost unchanging.

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    Thanks again for the input. I appreciate the discussion (which is clearly academic in nature).

    You said above "the speed of combustion is reasonably constant." If that's the case, then the same amount of time after a spark would develop the same pressure at the same rate regardless of the rotational speed of the engine. That's the whole point we both made above (and it's the theory behind adding centrip advance at all in the first place).

    I'm saying don't think degrees. Think TIME. The problem is that "degrees" changes with rotational speed. Time does not.

    If the burn takes 1.3 milliseconds to develop optimum pressure at 1000 RPM, wouldn't it take 1.3 milliseconds at 2000 RPM as well? And 3000 RPM? And 6000 RPM? Why would the burn occur faster at 6000 RPM that would necessitate backing off the advance* at that speed?

    If the burn rate is constant (in TIME), then shouldn't it be the same regardless of RPM?

    In other words, the fuel mixture doesn't know or care what the engine RPM is. All it knows is at what TIME it was lit off. If it burns at a constant rate you should adjust the light off to the same TIME regardless of how fast you're approaching TDC.

     

    * I say "backing off" because the speed goes up, but the amount of advance does not. It's like volunteering because everyone else in the line steps back. Just a different point of reference.   default_laugh.png

     

    Yep, if you only pay attention to the interest of having peak gas pressure at 22 ATDC, you would keep advancing the timing forever. But there are competing interests.

    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.

    Second, keep this in mind: gas pressure when the piston is on the way down is good. gas pressure when the piston is on the way up is bad.

    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).

     

    At say 5000 rpm, the engine is going 5x faster but the combustion still takes 1.3ms. So you need 5x more degrees of rotation to give time for that 1.3ms burn.

    So we need 5x37=185 degrees for that 1.3ms. If it is to complete at 22 ATDC then it has to start at 163 degrees before TDC. So now nearly 90% of the combustion and a lot of the gas pressure happens before TDC, while the piston is still on the way up. So although peak pressure will still come at 22 ATDC, very significant pressure has happened while the piston is rising, which slows it and will cause some of the fuel mixture to explode. (detonation) so the negative impact of so much gas pressure before TDC outweighs the value of having peak pressure at 22 ATDC.

    Caveat: I am guessing at the causes and science here (although the article zedhead posted seems to agree with me) . But.. I am sure it is impirical fact that there is no more power to find by having more advance than that approx 35 number, and actually you get a broken engine instead through detonation.

     

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    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.

    image.png

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