I agree, my compression is a good thing; I don't think that's the issue. I'm trying to be methodical and scientific about this, so please hear me out.
Since I installed this new build, I had a sporadic detonation issue, otherwise the car ran great, idled smoothly, lots of power, etc. During this sporadic phase, I noticed at least once (even with the hotter plugs) an episode where it had a miss or power fall off on the highway, ran rough, seemed like an intake leak or bad misfire, no detonation or funny noises, just loss of power and I could feel that the engine was struggling, stumbling, out of balance somehow - same as what I'm reporting now, but I shrugged it off as bad fuel, water in the gas, or something, since it was a very short episode (like 10 seconds) and went away. Still, the thought passed through my head that it might be a sticky valve.
Since that time, months later, the detonation issue became chronic. It was so bad, it burned through a head gasket. When I changed the head gasket, it was clear that the damage was in #4, it burned through to #3 and also burned a good way past the metal ring in another spot. You can see that in this picture: http://s785.photobucket.com/user/67dart ... ort=3&o=19
That evidence suggested to me that #4 had a sticky or burnt valve. Also, another indicator that the detonation issue seems to be in one cylinder is that during detonation I don't hear multiple noises out of sync, but rather a regular "rap" "rap" "rap" which to me suggests only one cylinder has the issue.
OK, so I changed the head gasket, and ran a compression test, nothing out of the ordinary except #1 was a good bit lower than the rest. That good compression test got me away from the bad valve theory, I figured it must be something else.
I checked AFR, intake vacuum (good, no leaks), exhaust back pressure (fine), ECU, distributor, exhaust leaks (none), cap, rotor, wires, etc. - all seem normal. Perhaps #2-#6 were running too hot a plug given the AFR at cruising (14.7, but still normal range). Indeed, I changed to colder plugs and dialed back timing about 5 degrees (from 10 advanced), the detonation issue seems to be much better (I still get a hint of it under load). It runs great now, except I am having that miss and/or power fall off more regularly now. After the head gasket change, I drove it for about 100 miles or so without issue, but the miss or dead cylinder or whatever it is comes and goes regularly now, I can feel the power drop and a vibration in the steering wheel; then it goes away again.
The thought occurred to me that perhaps the timing and colder plug adjustments merely are masking another issue. This miss or stumble or whatever it is, along with that picture above, is leading me back to #4 valve(s) being the issue.
Before I pull the head, I suppose one more test that I could run is to install those hotter plugs back into #1-3 and #5-6, but leave the colder plug in #4 and see if I get detonation. I'm betting I won't see it. I don't think it's that #4 is running leaner than the rest, the plugs all look the same after the head gasket was changed. Greenish tint indicates lean condition; I did see a hint of green tinge on #3 and #4 but those plugs were from when I drove home 50 miles with a blown head gasket, where those two cylinders were in direct communication, so I can't make anything out of that. Since then plugs look about the same, AFR in front and rear bank are identical (I don't think one intake runner is leaner than the other two in the rear bank, but I suppose that's possible).
I'm going to pull the head and swap out for this fresh one that I have. It should have almost the same compression according to the head thickness, but I'll measure the compression once I have it installed to be sure. Being as scientific as I can, at least some evidence is pointing to #4 valve(s) as the bad actor. If I do that and nothing changes, at least I'll have another data point.
Brian
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) and put in fresh 91 octane. I wanted to see how it runs on pump gas with the new head. I am sticking with the colder plugs as well. The old head didn't seem to run well below 5 degrees advanced, this one seems OK at TDC or even a few degrees retarded. I can't figure how that makes sense, but at this point I don't care, as long as it's running good.