Often with cars problems turn out to be the cause of something very simple. In the case of my very annoying misfire at 4500rpm on full boost electrode gap on my spark plugs was the issue.
An old hot rodders trick I heard was side-gapping plugs to allow for more area of the spark exposed to the fuel & air mix. Apparently you can gain a couple of free hp of doing this, I doubted that with a 230+hp turbo engine I was likely to notice a few more horses but hey why not?
In the process I also closed down the gap to 25 tho and I'm quite certain that this solved my misfire problem rather than the side-gapping but it was an interesting and easy thing to do anyway and if you're running a n/a engine every bit counts!
The idea is simple, carefully file down the end of the plug point exposing more of the electrode to the cyclinder. As you can see from the standard one on the left of each pic, the point travels fully over the electrode tip.
File down this end so that half the electrode is exposed as seen on the right of above pic.
- I found the best way of doing this without damaging the fragile electrode was to fold some clean cardboard and slot in the gap. This not only protects the electrode but also makes it easy to keep the file in line with the end. I took my time and stopped to check I wasn't taking too much off and also to frequently replace the cardboard sheild as it wore through. In the end I had 4 cleanly side-gapped plugs and you can notice the difference. However, as I said before, since boost raises the voltage requirement to spark I also closed down the gap to 25 tho, a lot closer!
I learnt that whilst a wider gap ignites the fuel better it places a high stress on the secondary electrical system. This can lead to breakdown of the components. But now, with the filed end it's more open to the chamber and acts like a wide gap but fires like a close gap, the best of both worlds. Apparently the downside is the electrode erosion causes the gap to enlarge much faster. Cheap plugs will need replacing often.
Speaking of brands, I've only ever used NGK. Simon had some experience with Champion and it wasn't good. Apparently he had one spit the whole ceramic centre out leaving just the thread....
Lastly, the other change I made was going a range hotter. Originally I had followed some advice from Vizard and gone 2 ranged colder plug due to a turbocharged system. This meant from AP6FS to AP8's... it seems that although often it feels as though Satan's spanking a demon in my enginebay, the charge in the cyclinder isn't so hot to warrent such cold plugs. So I settled with AP7's and that seems to be doing the trick!