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.