Spark Plug Indexing

[Mudwheelin]

Spark Plug "Indexing":


Here's an old racer's trick guaranteed for a slight fractional increase in horsepower. Grab a handful of new plugs that have been gapped and are ready for installation. With a Sharpie, place a mark on each plug (on the porcelain part) opposite the point where the side electrode attaches to the spark plug tip. Now (through trial and error) experiment with different plugs in each cylinder until you find one which will orient the mark you made on the plug as close as possible in the direction of the intake valve / intake runner when correctly torqued. Note: don't set the final torque now…just estimate the final position based upon where the mark would be if you added another 1/4 turn (for new plugs).

Unless you're very lucky, you may have to go through a dozen plugs to get a good match for each cylinder. Perfection is not necessary. What you're trying to do is orient the plug so that the intake charge filling the cylinder can "see" the spark gap directly and is not shielded in any way by the shadow of the side electrode attachment point. This promotes fast and complete flame propagation that yields maximum power.

This is tedious and time consuming, but I always take the time to do this for my own cars. Once you've indexed the plugs, you're set for the life of the plugs. Just make sure you always replace the plugs back into the same cylinder whenever you remove them for maintenance. I've seen this trick make a noticeable difference in cars with slight plug fouling problems and slight idle roughness. Recommended!

I just did it to the TB and couldnt be happier. I noticed a difference right off the bat!

[fishsticks]

You sure you're not just seeing a placebo or the effect of changing to new plugs?

I don't really see our combustion chamber design benefiting from this. Hell, I tried this on my LT1 way back and got exactly 0 in performance gains.

[Mudwheelin]

I used the plugs I had already in the truck. If it didnt work for you, you must have done it wrong. Everything about it makes sense imo...I did notice abit more pop off the line.

[The Roadie]

So.......which is best from your combustion physics point of view? Pointing the open area towards the front intake valve, the rear intake valve, or between the two?

A quick Googling, since I've actually never heard of this process, shows some claims of improvement for off-center plug positions, pointing the opening towards the central part of the combustion chamber. We're already centralized. So you point towards the intake valve. But we have two. So a few sites advise to experiment, because the optimum rotational position might be different for each engine, and the 1-3 HP you might gain can only be detected on a dyno.

Honestly, this is so far away from the mission I have for the Roadiemobile that it's not even worth much more Googling. YMMV. But thanks for the lesson because this process AT LEAST is not an instant and obvious pint of snake oil like throttle body spacers, swirlers, and hair dryer turbos. My first impression was that, and it's illuminating to know my first impression isn't always right.

[fishsticks]

I used the plugs I had already in the truck. If it didnt work for you, you must have done it wrong. Everything about it makes sense imo...I did notice abit more pop off the line.

"Everything about it makes sense" if your combustion chambers are terrible at having uniform mixture, and the intake charge is moving so slowly that it's still lingering at the intake valve at near TDC. A lot of the "old racer tricks" apply to "old racers" ie before modern cylinder head design. MAYBE this would help in a max-effort engine with ridiculously high compression (top fuelers)

It didn't make one bit of difference at the track in my car, which has a less efficient cylinder head design than our trucks. Obviously I just did it wrong though, because pointing the open end of the electrode at the top of the cylinder is precision engineering.

PROTIP: Grab some sealing rings for spark plugs and use them as shims, then you don't need "a dozen" plugs.

[Mudwheelin]

Sorry fishsticks, didnt mean to put you down like that. Maybe to some its not worth the 1-3 extra hp. But I like to sqeeze every bit I can out lol. Anyway just thought id share that.

[Diacom]

It's an old tried and true way of helping ignition. When I managed the Checker Auto in Montana, we had a fella who would come in and order some of the indexing washers a few times a year, he had clients that would have him index plugs on their vehicles when they had them tunned up.

He had a few buisnesses as clients and they seemed pretty happy with the results. Now for those that can sucessfully use plugs like the E-3 or Bosch +2 or +4 plugs, I suspect that this will be a mostly useless endeavor. Suprising that you didn't have much success on your track car Fish, but I would guess that a higher compression engine would see less of a bennefit from indexing than what we would see on a lower compression engine. Mainly due to the effiency of fuel burn.

Do I think it could be a bennefit on a TB, possibly, would it take some time to test which direction pointing the opening would help the most. Unless someone took the time to log their indexing with maybe some dyno tests to back it up or some other valid testing, I doubt it would really be worth our time to bother with unless you are trying to have the most effiecent engine.