Indexing spark plugs

[travis]

I’ve been reading through a bunch of my old late 70’s-mid 80’s car magazines lately, which got me wondering...

Apparently back then indexing plugs was a BFD, as there was multiple articles on the subject as well as it was pointed out in the write up of many featured cars. Nowadays you don’t really hear about it unless it’s Super Stock or something like that.

So did indexing plugs turn out to be just some catchy buzzword of the day, or is there actually some merit to it?

[Alaskaracer]

You'll hear a ton of good and bad info on this. Some will say point the gap towards the intake, some the exhaust....On older head designs there might have been a little bit to gain, but today's heads, the only reason to index the plugs is to get clearance from the top of the piston to the ground strap on the plug. There is little to nothing to be gained by doing it, and I have yet to see a single pro stock or comp eliminator car do it...if it was worth the time and effort they would. IMHO, it's not needed anymore unless the piston will contact the plug at TDC. IF there are any gains, they aren't enough to make ANY DIFFERENCE in performance....and fyi, on my 1510 hp BBC, I did index the plugs at first. It made zero difference on the dyno, on the track, and in performance overall. I no longer index them at all. I did it at first to ensure the ground strap didn't contact the pistons because I'm running very tight piston to head clearance....but it's all good....

[BigBro74]

I agree with mark -I only do it if there is a clearance issue that can be solved. I do think that back in the day using factory heads there was more to be gained because they had a lower plug position than most aftermarket heads do. To that same thought- I also feel like factory castings are more sensitive to tall piston domes than aftermarket heads with higher plug position - no data to back this up just my own observations though—

[rebelrouser]

When I taught my class on performance engine tuning, we had two identical 350 SBC mule engines about 340HP. I had my students index spark plugs, used plugs to new plugs, many times and we did not see any HP increase. We had a Eddy current dyno and it was very repeatable.

[Walter R. Malik]

Some people with real racing engines NEED to index their spark plugs so the piston dome won't contact the ground electrode.

[Alaskaracer]

Some people with real racing engines NEED to index their spark plugs so the piston dome won't contact the ground electrode.

I have a REAL RACING ENGINE...and no, I don't need to index them......I did at first but found later I didn't....most pistons made now have a small relief in the domes just for that reason.....

[David Redszus]

One factor to consider is the squish velocity and direction.

If the squish band directs a stream at the plug electrode so that the flame kernel is blown against the ground strap ,then a rotation of the plug could be helpful. If squish does not blow at the plug, then indexing would provide no benefit.

If the flame kernel is well developed before peak squish velocity (approx 10 deg BTC), then indexing has no value.

[BobbyB]

One factor to consider is the squish velocity and direction.

If the squish band directs a stream at the plug electrode so that the flame kernel is blown against the ground strap ,then a rotation of the plug could be helpful. If squish does not blow at the plug, then indexing would provide no benefit.

If the flame kernel is well developed before peak squish velocity (approx 10 deg BTC), then indexing has no value.

Does the ignition timing need to be reoptimized to realize any gains from what you say above?

[AC sports]

I remember in the go cart days indexing was worth 1/2 hp. So was cooling your fuel before use.....so I was told. Never tested either theory though.

[Joe-71]

In back to back dyno testing at the 600 hp level, I did not find any gain in indexing the plugs both facing the chamber, and facing the intake valve. Random installation repeated the indexed dyno numbers when done back to back. MSD 6AL, MSD coil, MSD distributor and plug wires, NKG 3/4" reach plugs. Joe-71

[BillK]

Travis,
I had to do it in the Small Block I had in my Camaro but only because the domes would hit the electrode if I didnt. I think for the average racer the performance increase, if any, would be hard to quantify.

I used to buy 3 or four boxes of plugs at a time to find enough that would work

[novafornow]

Do you have something against index washers?

[BOOT]

I've done it but I change my spark plugs too often tah worry bout it most the time. I kinda fell it goes in the category with gasket matching, CCing chambers and some other feel good stuff that any guy can do.

Still I feel there may be some flame travel element to it...

[steve cowan]

I’ve been reading through a bunch of my old late 70’s-mid 80’s car magazines lately, which got me wondering...

Apparently back then indexing plugs was a BFD, as there was multiple articles on the subject as well as it was pointed out in the write up of many featured cars. Nowadays you don’t really hear about it unless it’s Super Stock or something like that.

So did indexing plugs turn out to be just some catchy buzzword of the day, or is there actually some merit to it?

I helped a friend some years ago who raced superstock here in Australia, 358ci,10000rpm prostock truck engines lbs per cube weight break class.he purchased his engines from Patterson's in USA,they did not index plugs,we tried different indexing positions?? No evidence to prove either way,I am sure the engine builder had tested all that stuff years ago.

[emsvitil]

I haven't noticed any difference.

But I only do it enough to avoid a BAD orientation.

Or in other words, all the plugs strap orientations are about the same badness or goodness................