Detonation question

[Tommy]

Now that I've finished breaking in my new engine, I'm ready to fine tune the timing. For the record, I'm running a 351W with aluminum Victor Jr. heads, hydraulic roller cam and roller rockers. My concern is that I won't be able to hear sounds of detonation over the side exhausts if I advance the timing too much. Is there any way to tell if the timing is advanced too much if you can't hear the engine ping?

[Jeff Frigo]

Setting timing by ear and listening for detination is not the way to do it. Detonation actually starts before you can hear it. If you have a compression ratio of 10.5 to 1 or less, you should be able to run 92 octane gas without having any detonation.

The correct way to set timing is on a dyno (engine or chassis) or at the drag strip. Start at 30 degrees total advance and measure the hp on the dyno. Then go to 32 degrees and measure again. Keep doing this until the hp goes down. When this happens, you can fine tune down to a one degree increment.

If you go to the drag strip, do the same thing. Keep increasing the timing. The car will get faster and then eventually slow down. Again, then you can fine tune the last degree.

If you have driven a car at the strip, you may not have the experience to be consistent. The chassis dyno is the way to go. Will cost you $200 - $300 to totally dial the engine in. May sound expensive, but think about how much you have invested in your car. Well worth the money.

If you have 11 to 1 comp. or higher, then you will need to run 100 octane or higher. You shouldn't try to reduce the timing in order not to have detonation. You will loose hp this way, and it is difficult to tell when detonation actually starts.

[A Snake]

Tommy,

That's both a simple question and a complicated one. First, if you can hear detonation, you're way past the point of damage. Detonation, also known as pre ignition, is simply the fuel/air mixture igniting before the piston reaches TDC.

Several things, including timing, effect when and if this will occur. All of the following, in combination, effect detonation. Compression ratio, camshaft profile and timing,distributor advance curve, fuel/air mixture ratio,(influenced by size of carb to cu. inch displacement and rpm range) Last but not least type of fuel and octane rating. Add to the mix weight of car and rear axle ratio. (light car low gears allow a little more freedom to advance timing)

Old rule with octane: Higher the compression, the higher the octane. Higher compression needs slower burning fuel.

Now with all of this in mind and since I don't know your engine specs, I'd suggest calling the manufacturer of the camshaft. Give them every detail I've just listed and get their recommendation. Tommy, timing is not a random chance thing. Wrecking an engine will be the result. No, it may not blow up in your face, but one of two things will happen. The first you already know about..Detonation. It will crack pistons, break ring lands and if it's bad enough, melt the tops of the pistons. Second, if you're under timed, your engine will obviously not perform up to its full potential.

You might also contact Edlebrock, since you have one of their manifolds. Again, give them every bit of information I've listed.

By the way, as your engine will continue to break in for up to 5,000 miles or more, be aware that it will continue to seal tighter and that means compression will continue to come up. Timing cam be fairly aggressive on a fresh motor after break in and still live. Later that same timing can be in the detonation range.

Remember, detonation occurs long before you hear it.

[427sharpe]

Tommy, if you don't have access to a dyno or want to do something just to be streetable, I have 2 options for you that you can do yourself.
First, make a line on the dist housing near the block and loosen the distributor holddown clamp so that the distributor will move with a little pressure, but not so loose that it is too free (DONT MOVE THE DIST TOO MUCH). Then get the motor started , warmed up and at idle. With the motor at idle, move the dist SLOWLY clockwise until the motor begins to stumble slightly and mark the block where the line on your dist housing is. Rotate the dist counterclockwise until the motor begins to stumble, and mark the block again. Turn off the motor and set the distributor in the middle of the 2 marks and tighten the holddown. You will be very safe here.
Second: do the same with a vacuum gauge hooked up to a main vac line, and adjust the distributor to the highest vacuum yu can with a smooth idle.
Either method should take you no longer than 15 minutes.

[Mr.Fixit]

If you are detonating and DEAF than looking at the sparkplugs will show you that you are chipping the insulator and eating up plugs. Dopesn't anybody pull their plugs and look at em anymore?