[nubbzcummins]

Been having some spark knock since day one using 87 octane under high load at varying rpm ranges. Watched it with scan tool timing advance would get low pulling a hill and would suddenly shoot up when it started to knock. No knock with 89 octane though.

Well I did not want to waste an extra $1-2/ tank on fuel so I found some plugs one heat range colder to test out. Been running these colder plugs for severial thousand miles and mileage seems better. Even hit a 45-46 mpg tank which is my highest ever and seem to be averaging more singe change of plugs. I noticed no more knock on 87 but I was like there is no way it is helping mpg this much and must be in my head or driving style.

So I did an A-B-A-B test today on a 10 mile stretch of road severial times each direction. With oem 6 heat range plug I averaged 52 mpg north and 53.6 mpg south. With 7 heat range plug I averaged 53 mpg north and 54.6 mpg south. This is around a 2% gain from plug change alone!

With colder plug it would hold 2 deg more timing pulling a hill resulting in more power and less downshifts to 5th on some slower inclines which causes real time mpg go down. This was at 50 mph cruise control only. Baro was 98 kpa and 86 deg ambient temp with iats running 107-113 deg. Been averaging over 40 mpg with stock plugs and over 1 mpg more with colder plugs. Been a while since I updated my fuel log though.....