08-08-2009, 05:50 AM
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CC Member
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Join Date: Jul 2009
Cobra Make, Engine:
Posts: 416
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Not Ranked
Pretty sure. It's hard to hear detonation as the exhaust drowns out noise from the enigne, but as someone else pointed out, you can feel it. I did try as high as 28 intial, and I could feel it then, BUT only as the rpms came up as the final timing came in too early.
Being a standard transmission car, rpms are usually way up from idle before the clutch is even engaged when launching in 1st, and really, the same thing between other shifts. So as long as the timing curve isn't too steep, a high intial timing is OK.
Detoantion on a slightly too far advanced engine will only occur under load. Try turning your dist for max vacuum and rpm at idle. Then check your timing. It may be getting close to 40 Deg BTDC, but if you try to accelerate at that setting (from low rpms), detonation will definately occur.
Guys who lock their dist to 38 Deg max are getting away with it as by the time they let out the clutch, the rpms are getting to the point 38 is appropriate...or at least close to appropriate. Additionally as it is a light car, the load may not be enough for the engine to develop the most Hp it can for that particular rpm.
However, all this theory aside, I am now certain the "bang" I heard is detonation, but not becuase of too far advanced timing. Crossfire is likely. Probably inside the dist cap and due to bad ignition wires. Suppose one wire has become very high in resistance. Even with the rotor button pointing towards that terminal, the least resistive path to ground may be for the spark to jump to an adjacent terminal, down that "good wire" to the sparkplug. A sparkplug under less pressure (with piston near bottom of stroke) will fire more easily than the one with a piston near the top of the compression stroke, so even this "wrong cyl" spark plug is showing less resistance.
And it doesn't help I am using a Mallory dist which has a small diameter cap. No room for anything bigger due to the front carb being so close.
I am using Accel 95 wires, and although no spark leak was detected with my "midnight" test, the wires are quite old now. I just think of them as new as the mileage on them is small.
Although all of the above is theory, and has not been proven yet, it makes sense and at least I have a direction to follow now instead of running around in circles.
ps: Yes, it is the same CR, although it may be 1/10 of a point higher as I had the block deck and heads re-surfaced (just a slight skim of material removal) due to some small pitting from when my engine was detonating before.
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