Timing adv/retard affect on EGTs & knock

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    Premium Member Array black2003cobra's Avatar
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    Default Timing adv/retard affect on EGTs & knock

    I came across a comment made in another thread regarding the impact of spark advance on EGTs. Specifically, the comment made was that EGTs go up with less advance. Because high EGTs are generally looked upon as being bad, I believe the comment was meant to suggest that retarding timing wasn’t necessarily “safe.” I was a little surprised that this wasn’t challenged, since we all know that retarding timing is a typical way of reducing the octane requirement of the motor.

    Although I don’t have any actual data, it is true in theory that EGTs would go up with less spark advance. So that does seem to beg the question, that if EGTs go up, then how is it that pulling spark is “safe”? (Yes, it is.) Therefore, I thought this would be a great topic for discussion in a separate thread. (Sorry…no prizes for getting the right answer.)

    And if anyone has any EGT vs timing data, that would be great. Keep in mind the comparison needs to be made with everything else the same except timing, (viz., at the same IAT1 & IAT2s).

    On your marks, get set, discuss!

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    Premium Member Array Eric Brooks's Avatar
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    Default Re: Timing advance/retard affect on EGTs

    This is correct. Less timing causes higher EGTs. The reason is some of the mixture can still be burning as it exits the exhaust valve.

    On cammed cars, lowering spark at idle helps to stabilize idling. Otherwise there is no real reason to have really low spark.

    E

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    Senior Member Array Joe Gorm's Avatar
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    Default Re: Timing advance/retard affect on EGTs

    I've seen EGTs go up with reduced timing. With low timing and rich mixtures the EGTs are VERY high (like 1800F) on my low boosted 2V measured at the start of the midpipe. This is my understanding on EGTs.

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    Premium Member Array black2003cobra's Avatar
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    Default Re: Timing advance/retard affect on EGTs

    Good to have experimental confirmation! I will add that even if all the fuel is burned prior to the exhaust-valve opening event, EGTs will still be higher with less spark. So again, the question is: if EGTs are higher, then what is the mechanism for why the octane requirement is reduced with less spark?

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    Default Re: Timing advance/retard affect on EGTs

    that's why EGT's are not the best tool being by themselves...you may have made your tune a bit safer by retarding a bit, but because of the unburned fuel in the exhaust, EGT might show as high.

    But consequently if you start advancing timing so it starts to detonate, you should see high EGT's too.

    EGT's are awesome, but I'd definately pair them w/ a wideband...it's good to know what EGT's do when tuning parameter are altered...

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    Boost-aholic Array JrSVT's Avatar
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    Default Re: Timing advance/retard affect on EGTs

    I'm by no means an expert in technical data, but is there a difference in temperature that the higher octane combusts as compared to lower octane? I guess what I'm trying to convey is, does lower octane have a higher combustion temp compared to higher octane?

    skool me!

    Jr.

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    Default Re: Timing advance/retard affect on EGTs

    Quote Originally Posted by black2003cobra View Post
    Good to have experimental confirmation! I will add that even if all the fuel is burned prior to the exhaust-valve opening event, EGTs will still be higher with less spark. So again, the question is: if EGTs are higher, then what is the mechanism for why the octane requirement is reduced with less spark?
    I'm gonna take a stab by saying if you're retarding timing so much do avoid detonation, you've definately got an octane and or fueling deficiency...and being that you're EGT's would climb because of the timing retard you're probably trying for too much horsepower for the given fuel condition and you're set-up would be deamed inefficient...

    What is the best way to avoid too much retard (2 ways)...use more octane to keep up w/ the spark...and richening the tune is the other way to keep up w/ the spark, but its raising your EGT's....so go w/ what's more efficient by raising the octane...

    Not sure if I answered your Q or if that made any sense at all...LOL

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    Default Re: Timing advance/retard affect on EGTs

    Quote Originally Posted by JrSVT View Post
    I'm by no means an expert in technical data, but is there a difference in temperature that the higher octane combusts as compared to lower octane? I guess what I'm trying to convey is, does lower octane have a higher combustion temp compared to higher octane?

    skool me!

    Jr.

    I would say that it takes less heat to ignite 87 octane than it does 91 octane...the higher octanes ward off pre ignition by requiring a higher temperature to ignite...

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    Default Re: Timing advance/retard affect on EGTs

    Quote Originally Posted by black2003cobra View Post
    Because high EGTs are generally looked upon as being bad, I believe the comment was meant to suggest that retarding timing wasn’t necessarily “safe.” I was a little surprised that this wasn’t challenged, since we all know that retarding timing is a typical way of reducing the octane requirement of the motor.
    I would say that the higher EGT's due to retard is bad because it would start to melt the material w/ in the cats..I believe it causes backfires too..would be interesting to know what temp is bad for cats...

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    Senior Member Array oldbones's Avatar
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    Default Re: Timing advance/retard affect on EGTs

    Retarding too much will burn exhaust valves.

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    Senior Member Array PhillyCobra's Avatar
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    Default Re: Timing advance/retard affect on EGTs

    I think you need to differentiate between high EGT and detonation. Retarding timing can raise EGT, as discussed, but will not cause detonation. The link between high EGT and detonation is primarily when you get into a lean situation, which raises EGT and can cause detonation. In the opposite direction, I would guess that if you get detonation from advancing timing too much that you would probably see decreased EGT, as not all of the fuel would be fully combusted. EGT is used more to monitor the effect of A/F than timing.

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    Senior Member Array Joe Gorm's Avatar
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    Default Re: Timing advance/retard affect on EGTs

    Quote Originally Posted by black2003cobra View Post
    So again, the question is: if EGTs are higher, then what is the mechanism for why the octane requirement is reduced with less spark?
    Reduced cylinder pressures
    Quote Originally Posted by doubleblack View Post
    But consequently if you start advancing timing so it starts to detonate, you should see high EGT's too.
    Usually detonation will cause a reduction in EGTs because the extra heat is more concentrated in the combustion chamber, but EGTs should rise as you increase timing but before the knock limit.
    Quote Originally Posted by doubleblack View Post
    I would say that the higher EGT's due to retard is bad because it would start to melt the material w/ in the cats..I believe it causes backfires too..would be interesting to know what temp is bad for cats...
    Exhaust valves will become damaged with high EGTs because the metal starts getting softer. Seems to be more common in the import world. Your O2s will also be damaged. Cats are probably the least of your concern because they are made for high temps. Rich mixtures followed by excessively lean mixtures (coasting fuel shutoff) will cause the catalyst to get VERY hot, probably much more so than from high EGTs.

    As far as the octane.. Its my understanding that its more of resistance to high pressures that come from higher octane than resistance to higher heat, but they sort of go hand in hand. I believe higher octane fuels actually burn cooler, but am not sure on that one. Adding octane to increase the timing beyond the lower octane knock limit will reduce EGTs as well. Water/meth also has a HUGE effect on reducing EGTs by hundreds of degrees.

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    Premium Member Array black2003cobra's Avatar
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    Default Re: Timing advance/retard affect on EGTs

    Quote Originally Posted by Trick Tuners View Post
    Reduced cylinder pressures
    Winner! Reduction in peak cylinder pressure and temperature. See modeling results below. Engine 1 has less spark advance.

    As one can see, at the end of the power stroke, indeed the gas temp is greater on the engine with less timing advance, which is simply because ignition was delayed.
    Attached Images Attached Images   

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    Senior Member Array Joe Gorm's Avatar
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    Default Re: Timing advance/retard affect on EGTs

    Quote Originally Posted by black2003cobra View Post
    Winner!
    Ah ha. I figured a smart guy like you would already know the answer to a simple question.

    I like this one!

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    Default Re: Timing advance/retard affect on EGTs

    Quote Originally Posted by Trick Tuners View Post
    Usually detonation will cause a reduction in EGTs because the extra heat is more concentrated in the combustion chamber, but EGTs should rise as you increase timing but before the knock limit.
    so after the combustion chamber has detonated, it quickly begins to cool...thus having lower EGT's by the time it makes it's way into the exhaust manifolds, is how I'm understanding this...

    that's valuable info because lower EGT's is seen as a fairly good thing, in this case, the lower EGT is the result of an aleady detonated combustion chamber...

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