Ignition timing and how it affects engine temps question.

[patthesoundguy]

How does ignition timing affect operating temps? Im tuning my express and I'm curious about timing, as a dude I chat with mentioned that the timing could effect the temps.

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[deuce217]

Not positive on this, but increased temp would make sense due to longer duration of combustion in the cylinder.

[Wheelman-111]

Greetings:

Timed too early, the combustion starts well before the piston reaches TDC. The combination of rapidly expanding gases and the piston still on its way up can radically increase the average temperature and burn a hole in your piston shockingly fast.

PistonHoles.jpg (134.51 KiB) Viewed 7179 times

The timing/temp curve is exponential - nanoseconds sooner result in tens, then hundreds of degrees hotter.

Timed too late, the performance suffers because the expanding gases "chase" the piston down the barrel. It should run much cooler that way, but it'll be slow.

[patthesoundguy]

Thanks wheelman, that is super awesome info. I swapped in main jet from 95 to105 just to see if the temps would be lower and diddnt help too much it diddnt rip as hard was on the rich side. I had a 86 spree head on it yesterday and it seemed to have more even temps over the stock express head I put on today. The temps like to go up to 380 but drop fairly quick if I let off the throttle and let it drag on the pilot. So to time it to drop the temps a bit do I need to make the points close a hair later? The thing rips with the pipe on it but I can't let it rip too long because I'm sure its gonna spike and burn it up. I set the timing up according to the manual but it doesnt seem that accurate. It will bury the needle on the speedo with a .25 over stock cylinder with 19mm delorto carb, MLM pipe and some porting. I love the express almost as much as my spree.

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[Wheelman-111]

Greetings:

Jetting big lowers temps but really hurts performance, plus the hassle of frequent head decarbonizing and risks clogging up that Valuable MLM Pipe. Why the big jump from 95 to 105. Consider stopping halfway at ~100? Backing off the timing may help, but also results in lackluster performance. What to do?
Consider adding a layer of head gasket to decrease compression, at a lesser cost in performance than the first two options. If there's an airport near you, half-and-half mix with Premium and 100 Low-Lead AvGas would give additional insurance against detonation if it does get hot. That's all Flash6 drinks (165 Compression...), and well, so far so good.

[patthesoundguy]

Thanks Wheelman I really appreciate the help. The big jump was the only larger jet I had for the dellorto right now the temps aren't too too bad, just did a run with it, but you are right it just doesn't rip like it should that rich. I've got head gaskets so I'll try a second one with the current jet and see how it goes then I'll drop down to the 95. I'm running 92 octane which is the highest the stations have here. I'm running 32:1 mix just because I'm breaking in my spree big bore kit and that's what I have mixed up. And after the big bore kit went into oil free mode I figure as long as its jetted to accept it I'm good. I may check the timing again just to make sure its right, I may check it with a multimeter instead of the light bulb they say to use in the manual just to be sure its right on. I like the head gasket idea, thanks again for that B-) I'll keep you posted

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[patthesoundguy]

I have lots of small jets maybe I'll drill one in between 95 and 105 after I try some of the other things first

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[deuce217]

What plug do you run? I run #8 in all of mine. You could also try a colder one. I wonder if you may have a slight air leak causing it to lean out, since it was still hot after the 105 main jet, I would maybe take another look at the usual intake gasket areas prone to leaking.

[patthesoundguy]

I've got a 6 in for a plug, it calls for a 4 stock. I have an 8, gonna try it. I was just out data collecting on the express, it likes to spike going uphill the most. Does that indicate anything? That's with the second exhaust gasket too.

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[Wheelman-111]

Greetings:

The added work of pushing uphill will raise CHT readings. I found oddball plugs like 7s and 9s by Bing-ing B7HS, for example. An 8 with that high compression isn't a bad idea.

[patthesoundguy]

I'm going to take it for another spin later tonight and gather some more data and try the 8.

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