Spark plug test success! (A-B-A-B = 2% gain with lower heat range plugs)
 

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.....

My only question is which of the two vehicles listed was this test done on? Great results

I'm guessing since he was seeing 50+ mpg during the tests that this was in the 2013 Accent, not the minivan.

Thats pretty interesting results. I was looking into why this would happen/make a difference in mpg. I came across an interesting thought that the ethanol in our fuel was causing a need for lower temp spark plugs to prevent detination. I don't know if that is the cause but I am interested in maybe trying this out at some point. I can only find E10 gas here.

Im curious at what point the increases fall off if you went another step colder?

I opted for 2 steps colder ngk's on my car but didnt do any real testing to have data to back up my fuel economy but was able to advance my distributor almost all the way...

Did the plugs you removed show any signs the heat range needed to be changed?

Isn't that backwards?
 

I thought the presence of ethanol increased the octane, not lowered it. I understand you can jack the compression on an engine that only ever runs E85 up to crazy high levels with no risk of detonation. I don't know what the ignition thresholds are like for ethanol but I reckon if it's harder to ping, it's probably a little harder to light, too. I'm just taking a flyer here, but I would think ethanol would benefit from a hotter plug, not colder.

OP is taking advantage of his computer's programming to maximize economy with a colder plug, which permits more advance and therefore a more complete burn.

Correct...the van falls behind the accent by ~20 mpg.....

You can only go so far with a plug change as it is only one area that can have too much heat and cause knock. Lowering thermostat temp may help you get a few more degrees of timing out of it by making cylinder walls cooler... Each setup is different so test away!

As in what kind of signs? Color looked good imo. Not wet/dry fouling noted. Ceramic body of plugs from hex head to maybe 1/4" toward top of plug was turning a burnt rusty brown color though. Coil side of hex not threaded end... Not sure if this is normal or sign of too much heat? Around 30k on oem ngk iridium plugs...