View Full Version : Old performance trick/mpg +

07-05-2008, 09:38
-Spark plug indexing and side gapping-

I thought I'd bring it up, i was explaining it to my roommate out here in afghanistan, he's a ford drag strip junkie.

Indexing the spark plug is the technique of aiming the side electrode towards the outside of the combustion chamber.

The concept is getting that pesky spark flame from the plug to perform at its best for burning fuel. First I'll start with indexing portion. If you picture the inside of a combustion chamber, the object of this is to point the spark towards the inside of the combustion chamber by pointing the side electrode towards the outside. This can be done with indexing spacers/washers, ask any automotive parts store or summit online. The kit comes with different sizes ie. 10mm 12mm 14mm. Mark your spark plug as follows so you know where that side electrode is:
use trail and error to correctly aim the side electrode away from the center of the chamber by way of the black mark. Therefore giving you this on the inside of the combustion chamber:

the following is what you don't want, when the flame of the spark has to travel around the side post to ignite the center of the combustion chamber:

With the side electrode pointing away from the center of the chamber as the spark ignites the flame will not have to travel around the post ignite the fuel cloud in the chamber.


Since the combustion chamber isn't perfectly flat the spark can use more help by making more room to travel.
Pictures explain alot, first picture is a stock spark plug obviously:
Now we take this spark plug, not some fancy bosch 4+ or splitfire, but a regular Autolite, and shave back that top electrode to expose more area:
Now this is what we get, the flame travels without restriction to fully ignite the combustion chamber:

I've seen different numbers for different for different vehicles. Larger gain on larger horsepower engines. On average 2-5% hp/tq gain, 1-1.5mpg gain. Personally my jeep got 1+ mpg definitely and I never dynoed it so i got no hp/tq numbers. I've been working on the proficiency of my jeep not just horsepower. But this technique has been used by racers back to the 60s. Its a proven technique, google it and you'll see. Spark plug indexing and Spark plug side gapping.

07-05-2008, 11:15
That's pretty interesting. Especially the shaving back of the electrode. I may get a wild hair and do this when my jeep's up again.

07-05-2008, 11:20
Used to do this on my car.

07-05-2008, 12:50
My jeep, wife's xterra and the Truggy all done, Gas prices are hurtin' anything to grab another 1+mpg. As much as i despise the deployment, it is a year with only the wife spending money on gas. Its up almost a dollar since i've left......just thought I'd share the info. Doesn't take very long, and if your really cheap; (I did this on the truggy), the spark plugs are only .99 cents so I bought 5 packages and I tried every plug till I found a plug that just went in in the correct postition for each cylinder. I did it with no washers and returned the rest of the spark plugs. Course I didn't side gap them till I found 8 that clocked in just right. Maybe took an hour and a half...to find the right spark plug for the right cylinder. Hour and a half vs. 14.99$ and the wait of shipping...

07-05-2008, 13:56
Thanks Chris. Good info. And what the hell? You just got home a few months ago. I take it the back is okay too?

07-06-2008, 08:55
great info. I did the indexing in an old Chevy I had. I'm interested in the side gapping too though.

02-10-2009, 02:29
Never heard of that before. Very interesting and helpful. Will def. Do this on all the plug changes if I remember. Anyone interested in being my personal sticky note? >)