My current spark plugs have a heat range 3 settings cooler than the factory-recommended plug - which allows the timing to be advanced a bit over stock. The plugs also got a "fluff-n-buff" job with a bench grinder buffing wheel before installation, to remove the sharp edges on the ground electrode. Sharp edges tend to encourage detonation, so "smooth" is good.
Detonation is from when you have two flame fronts that collide. Since the initial flame front starts at the spark plug, and you cant burn fuel twice the plug is not a factor in detonation.
The only way it could have an effect is if you had pre ignition that caused detonation ( not a common occurrence). Since you dont have any large holes in your pistons, I'm pretty sure that hasn't happened.
As far as plug heat ranges go, I'm running NGK BPR6EY's on a 2.6 running 14 PSI of boost with about an 8.3 to 1 CR and I have never had a pre ignition problem. I did test 7's and 8's, but eventually went back to the 6's. The 8's were just too cold. The 7's worked OK.
A rule of thumb is 10? of extra timing = one heat range colder. Each heat range change drops the plug temp about 100?C. Optimal temp is around 800?C. Assuming that the stock plugs are nominally at 800?C, three ranges gets you 500?C.
To get a plug to self clean, you need to hit about 450?C. I have tried colder plugs, they took a long time to get up to temp. On my fuel injected motor, cold drivawy was noticably worse when I dropeed to a colder plug.
When I had a carb motor I used to run 12? of timing, that seemed to work reasonably well. In general, the 2.6 is tolerant of extra timing. Tolerant to the point that I would bump the compression ratio 1/2 a point and still run regular. The hemi head does not have good turbulence and as a result, somewhat slow combustion. That tends to make it tolerant of extra timing.
One tuning trick that may help is to use the dist advance springs from a turbo engine. The primary spring is a bit lighter and allows the timing to come in a bit faster. That can get you better throttle response but keeps your total timing at higher RPMs the same.
I realize every setup is differant... That's what Ive found on my engines.
