float_6969 wrote:I had just done a pretty long WOT stretch when this happened. Like 4k-8k in 2nd, 3rd, and 4th and was at about 6k in 5th. I've never had any hints of detonation or knock. The total timing under load is 27deg. That's not an arbitrary number, but was set on the dyno with a detonation microphone to ensure there was no knock. This is only 12-13psi of boost and about 320whp. EGT's never exceeded 1300F. I'm on E85 with an actual ethanol content of 85%. The issue is isolated to this cylinder. I had plug issues on the same cylinder with the last rebuild. The porcelain cracked and a piece came off and scratched the cylinder wall. The only thing that would effect this that was the same from the last build is this cylinder head. the entire intake manifold is different, the turbo is different, the cams are different, fuel injectors are different, pistons are different, and compression ratio is different. I didn't know that there was a helicoil in it the last time. I didn't even know about it until recently when I went to check the plugs after dyno'ing it and that cylinders spark plug felt funny when it came out, like it was steel-on-steel threads. So I stuck a magnet down in there and it stuck to the threads. Then I got a flashlight and realized it had a helicoil in it. I have read in the past and heard from engine builders that a helicoil in an aluminum head doesn't allow the plug to dump heat as easily as the original aluminum thread because the steel doesn't conduct heat away from the plug as well as the aluminum does. In high performance applications, this can require the use of a colder plug on that cylinder to prevent this exact situation. If you don't do that, it can cause the exact situation that you're describing, but it wouldn't be due to ignition timing, but instead, the plug it self not being able to conduct heat into the head like it's supposed to.
I'm pretty confident the plug overheated from the helicoil, but I'm never one to say I know it all. I'm going to do a compression test and go from there. If the compression is low I'll start inspecting the engine more closely.
I glossed over the helicoil part. I wonder if anybody's done any legit testing on that effect. I can see how the coil would affect heat flux, but I was also under the impression that the majority of heat transfer through the plug was through the seat rather than the threads.
The thing with EGT's is that a low reading in the runner/turbine/exhaust pipe may not be telling the whole story. If you've only got one sensor in the turbine, say, it's only reading an average temperature through the pipe. I don't think those sensors respond quick enough to allow us to observe temperature variations cycle-to-cycle. If only Cyl. 4 is suffering you probably won't see it.
To add to that, preignition and detonation are characterized by an excess of heat transfer into the physical engine components, which is why you see melted plugs and pistons in an engine suffering preignition. All else equal, increased heat transfer into the engine itself would mean less energy going out the exhaust and a therefore lower EGT reading.
It's a good sign the engine fires up and runs ok though! Might have caught it before major badness went down.