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Detonation?!?

Bolisk

Well-known member
Joined
Nov 2, 2007
Messages
441
Location
Crystal Lake IL
Corvette
1972 LS5 Convertible PS, PB, A/C
Two wrongs don't make a right. Your advance curve is still 'too much too soon'. By having all the advance come in so early, the initial advance has to be retarded from optimum setting to compensate and you end up with less than ideal 'all in' mechanical. Being that the stock curve was far lazier than what you have now and you've got the strongest springs available, the weights must be some weird aftermarket set up.


Makes sense. However, please check my logic. The stock curve was "all in" much later in the RPM range. I'm assuming that was more because of the strength of the springs rather than the weights. If we want to have the "all-in" moved earlier in the RPM range (near 3000) then the stock curve numbers are all crap. . .because everything is now condensed into a smaller RPM range. Right? So I have to throw the stock curve numbers the window.

So I'm guessing that the two most important factors are:
1. Centrifugal advance is 0 at idle
2. Centrifugal advance is all-in as close to 3000 rpm as possible.

I'm guessing that whatever happens between those two extremes is kinda "let then numbers fall where they may" type of scenario.

One thing that I have noticed, is that I cannot get 0 centrifugal advance at 750 RPM. Note: when you disconnect the vacuum advance the idle drops to about 450 to 500 and at that speed I have 0 centrifugal advance. But shortly after 500 rpm (closer to 600) the centrifugal advance starts to kick in ever so slightly. With all-in near 2500.

Granted, I'm measuring all this with my wife in the drives seat calling out rpm ranges. So there is a error factor in communications, and a potentially inaccurate tachometer.

I will borrow my friends inductive digital tachometer. . .and try to get a more "real" map of what is actually happening. I will post that here as soon as I can get the numbers.

Again, thanks for everyone's continued technical support.


UPDATE:

After adjusting the initial timing to 10 degrees, I beat on the car the entire weekend. Was never able to get it to make the noise. Granted that was running approximately 96 octane fuel. The good news is this is the first time that I was not able to get the car to make the noise. . .it quite some time.

This morning I put 12 gallons of 93 octane in the tank. I calculate that I'm down to approximately 94 octane. I beat on it during my morning commute and could not get the noise to re-occur. It was cool out this am (48 F). Supposed to be 70 F today. . .so I will try again on the ride home.
 

glen242

Well-known member
Joined
Aug 30, 2004
Messages
135
Location
Moon Twp. PA
Corvette
1976 Mahogany
Timing

I have run a stock recurved HEI distributor and now run an Accel recurved distributor in my modified '76 stroker.

I never cared or recorded what the initial timing was, only set it for 35* all in at about 2600 rpm. Per Lars' papers, I picked a vacuum advance unit that would give me the advance that I wanted at my 13" of vacuum. Total is about 50* initial+mechanical+vacuum. I ran this setting on my prior slightly modified 350.

My static CR is 10.3 with aluminum heads and a CC XE274H cam. I run 89 octane BP without any problems.

Do you have Lars' papers?
 

glen242

Well-known member
Joined
Aug 30, 2004
Messages
135
Location
Moon Twp. PA
Corvette
1976 Mahogany
Don't forget, the amount of quench you have can determine if you have detonation. I believe ideal quench is 0.038 to 0.045.

Mine has 0.041 quench with my '0' deck block.
 

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