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Old 09-28-2006, 06:45 AM
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Detonation 351c Help

I consider myself to be a fairly good mechanic but I have come across a 1970 Mustang with a 351 M code 4bbl, apparently the motor has been rebuilt Holly 670,small cam, roller rockers, factory manifolds with duel exhaust, the heads were converted to run unleaded gasoline. Unfortunately I do not know what the compression ratio is.

This is my problem, in first gear and only the first gear, I have an extreme amount of detonation, once the car goes into second gear the detonation completely disappears, I have an aftermarket distributor electronic, I have even removed the vacuum advance, and set the total amount of timing at 24 degrees, the car has absolutely zero power, but the detonation does go away in first gear, unfortunately at 24 degrees total advance the car has absolutely no power. If you take the car up to a total of advance of 28 the car once again begins to detonate severely but only in first gear. I've also played around with the centrifugal weights, I've tried several different spring combinations with no results. I've also installed a 3200 stall speed converter, under the conclusion that my compression was extremely high, this made a big difference by allowing me to get up to a higher RPM but there's still detonation, but only at half to three-quarter throttle, I also installed a lower temp thermostat with cool spark plugs, this seemed to make a difference also. Unfortunately I still have an unusual amount of detonation for a naturally aspirate motor. The problem is the only occurs in first gear and this is what has me totally confused, I can go up a hill at full throttle with four people in the car, and second or third gear, even light throttle, and still the detonation, I have installed octane booster but the detonation still occurs just not as much. However if you time the car out to about 28 to 38 degrees total advance, the car is incredibly fast and runs up to 6000 RPM without missing a beat. I've also begun to play with some basic carburetor settings. But so far not much of a change.


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Old 09-28-2006, 10:56 AM
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I wonder if you might have a problem with your tranny? It seems odd you'd have detonation in first, when the engine has the lowest load on it, and yet in 2nd and 3rd heading up a hill it's fine.

If you've been having severe detonation, I'd recomend pulling the heads and inspecting them, detonation can do some serious damage and you should see how bad things and if you need to replace anything.

Actually even before pulling the heads I'd do a compression check. With iron heads, you should be around 140 psi. If your seeing 170 psi or better, you definately have too much compression w/o running high octane fuel.
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Old 09-28-2006, 11:18 AM
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I have an aftermarket distributor electronic
Do you have (or plotted) the mechanical and vacuum advance curves? Does the vacuum diaphram have an adjustment whereas you can back off the advance gradually? It is on port vacuum signal, correct?
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