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 sparkplugs, 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.

 

Hmmm ? I see what you mean... weird.[&:] Ok, first gear is low on RPMs and should be low centrifugal advance but then the engine is trying to move the load of the car so the conditions are right for what you describe.

Ok here is stuff that is inside my head in no particular order. including some questions thrpwn in. Hope it helps


Detonation typically is an abnormal wave front inside the combustion chamber. The normal slower wave front starting from the plug encounters another air fuel explosion caused by one or several of the following. The end result is the sudden detonation or explosin of the air fuel mixture and shock wave that is audible as ping knock etc.

All of these are related and cause the peak momentary pressures in the combustion chamber to rise causing the detonation point of the fuel to be reached. The fuel basically ignites before the spark or in a part of the chamber where the spark has not started the combustion. - Similar to a diesel engine I guess.

Lean mixture - more oxygen hotter burn

High engine load - raises the peak pressure since the piston can't move down fast enough to increase the effective volume of the chamber.

Static timing too advanced - causes combustion to start as the piston goes up and raises the peak combustion chamber pressure.

Hot areas inside the combustion chamber - could be carbon, valves, parts of the head gasket or even edges of the conbustion chamber Also clogged areas inside the cyl head or cooling system.

Low octane fuel - for obvious reasons

If we were talking about an engine with otherwise STOCK carb and ditributor etc, that the heads, pistons and other mechanical stuff had been changed I would look at the following in this order.

1) Timing mark on vibration damper - Make sure that with no 1 cyl at TDC the mark coincides with the 0 degree on the damper. Both rockers on intake and exaust must be up. The rotor should be pointed near to the no 1 contact on the dist cap.

2) Valve timing - A bit harder to do, you need a degree wheel and the specs for the cam.
Note that valve timing will have an effect on the peak pressure mentioned above. Think of valves clsing as the piston is moving up.... the pressure goes ballistic and. POW!!!

The two above suggest that something mechanical/structural was done to the motor to affect the conditions inside the combustion chamber.

Head type or design : The stament that you make about the engine beign fitted for regular scares me. A good head design tolerates a lot of compression. This ios usually acomplished with squish area. It creates combustion chamber swirl and quenches heat at the same time.

This all raises the question of who assembled these particular components and what was the purpose of the particular cam head etc. ( ? )[:-]

Is this a one of a kind mix/match type of motor or a known good combination of pieces?

Now since the motor also has aftermarket carb and ignition both are suspect but first let me point this out. The noise you hear may be taking place in all or some of the cylinders.

Safe to say that the fuel system can contribute to the problem by creating too lean a mixture in all or some of the cylinders...

The distributor may be over advancing timing at some RPM. Since you say it only takes place in 1st gear, it MAY be too much advance at low RPM. I am assuming that there are no engine controls that block advance when the car is shifted out of first gear.

Part of the problem is that you have no starting point or base line since most everything has been fiddled with.

If you time the car by ear meaning set the timing to the point where you get slight or no detonation but still has some power, is it driveable?


Do you know anyone with a distributor machine ? Maybe the local high school. This may allow you to establish a base curve even if based on a similar motor.

Same for the carb - how is it jetted ? What about the accelerator pump shot? Is it enough or does the engine start out lean ?

That's all I can think of for now ...

 

One more thing ...... plugs. It is not something that I have persoanally seen but they say that too hot a plug can actually cause fuel to detonate


good luck

 

Right, plugs. If you have a modified "hot" engine, you may be running too hot a plug that will cause predetonation. For example, the '65 superhypo 289 with solid lifters would require a plug two (2) heat ranges below that of the standard 289 V8 plug. Look into it. With the cheap cost of most plugs I'd give it a try. Pull one plug and visit your favorite parts center and ask for a comparable plug one or two heat ratings below your existing plug. This may solve your problem completely. You can also try platinum that will not foul as fast. Your local Ford dealer may be a good place to start.