Indexing spark plugs

[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.

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.....

So, simply you are not "some people" because many others do need to do this.

[Alaskaracer]

If the pistons are prepped properly, they will not....there will be a small relief for the plug....and using the proper plug as well. If they are using an extended tip plug, they are wrong, period...those have no place in a racing engine of any kind...

I'm not an expert, but I have yet to see a single engine built by a professional builder that needed the plugs indexed. Not one. And I've seen plenty from various builders, including the big names....the fact of the matter is, the last time I saw an engine that needed the plugs indexed it was using OLD double hump heads and HUGE domed pistons....plug hit on one side of the relief, so instead of working the pistons, they just indexed the plugs.....scotch tape fix so to speak...

I said I originally indexed mine, but found later there was no need....and that was the only the second time I've ever done it. First time I was a kid and didn't know any better...

[Bumps440]

If the pistons are prepped properly, they will not....there will be a small relief for the plug....and using the proper plug as well. If they are using an extended tip plug, they are wrong, period...those have no place in a racing engine of any kind...

I'm not an expert, but I have yet to see a single engine built by a professional builder that needed the plugs indexed. Not one. And I've seen plenty from various builders, including the big names....the fact of the matter is, the last time I saw an engine that needed the plugs indexed it was using OLD double hump heads and HUGE domed pistons....plug hit on one side of the relief, so instead of working the pistons, they just indexed the plugs.....scotch tape fix so to speak...

I said I originally indexed mine, but found later there was no need....and that was the only the second time I've ever done it. First time I was a kid and didn't know any better...

May I ask the cubic inch size of your engines? Have you ever built a small engine and fought for every .1 of a cc to try to gain compression. It is a battle trying to fit domes and chambers and valve notches to hopefully get what you are after. So if I have to index a plug I do it. Then again back in the day we removed the ground strap altogether and the plug sparked to the dome. It did work, but I cringe thinking about it. I am talking 265 to 302 inch SBC. I am retired now so all that is just a memory.

[Alaskaracer]

My stuff is a 12* BBC, 665" deal, aluminum rods...The last small block I built was a 350 chevy, basic rebuild with a few upgrades, so nothing special. The big thing is, if you're building a racing engine, you're gonna likely do custom pistons. I won't buy a shelf piston anymore because I can get custom for the same price, and they are made to fit my chambers. I take a mold of the chamber or take the head to the manufacturer to digitize. Most professional builders now will do the same thing. They may offer "builder packages" with "matched parts" that are "shelf items", but they have gone though the time and effort to make sure everything fits the way it's supposed to. That's just the nature of the beast. Like I said before, if the engine is built properly, they won't be needed. Even if you're fighting for compression. There are other ways to gain it, such as thinner head gaskets, angle milling, less deck clearance, etc....I get it, it was needed at one point, but isn't anymore. There are just too many options out there to make the practice unnecessary, and there are no gains from it worth the effort....

If it matters, I'm at 16.5:1 static, and have 67.8 cc chambers....4.600 bore...5" stroke. I have to worry about rod stretch, piston rock, and rpm, as i turn this thing upwards of 8000 rpm...I've had witness marks on the pistons from the valves, but that's just how I run my stuff....I have very little room for error, but having to deal with indexing plugs isn't one of them....

[hoodeng]

Spark plugs dont spark, they ionize. A turbulent chamber and charge will propagate pretty evenly regardless of ground strap position.

Years ago i used to see heads for plug thread repair due to owners cranking that sucker down till the lines came up, very rare these days.

Cheers.

[GARY C]

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.....

So, simply you are not "some people" because many others do need to do this.

Could be that the people with "real race engine" have to index their plugs because they are not "real" engine builders?
I always thought you you should address clearance issue during the build, not after.

I may be wrong but in theory, indexing for piston clearance would put the plug in the worst possible position not to mention the dome being used would be the worst for flame travel?

I only tested plug indexing on the track to see if there was a performance gain but never found one.

[Walter R. Malik]

My stuff is a 12* BBC, 665" deal, aluminum rods...

At that large cubic inch and shallow valve angle you certainly would not encounter any clearance issues trying to gain compression.

Now try an open chamber 26 degree, (rolled to 24 degree), aluminum rod, de-stroked big block Chevrolet at 388 cubic inches like the Pro Stock racers of the 80's were using when they INTENTIONALLY built those engines to where the spark plugs needed to be indexed because they needed every bit of dome they could get.
I would venture that they were professional engine builders. Those pistons and chambers looked like a stamping die with plunged valve reliefs.

[David Redszus]

Years ago I used a Sun distributor test machine equipped with a coil and a mounting rack that could fire spark plugs.
The spark traces were visible both on a scope and visually.

If a jet of shop air were directed to the firing plug, it was easy to blow the spark right off the electrodes, or into the
ground strap.

If the flame kernel were fully developed before approx 10 deg BTC (max squish velocity), indexing had absolutely no effect.

[Alaskaracer]

My stuff is a 12* BBC, 665" deal, aluminum rods...

At that large cubic inch and shallow valve angle you certainly would not encounter any clearance issues trying to gain compression.

Now try an open chamber 26 degree, (rolled to 24 degree), aluminum rod, de-stroked big block Chevrolet at 388 cubic inches like the Pro Stock racers of the 80's were using when they INTENTIONALLY built those engines to where the spark plugs needed to be indexed because they needed every bit of dome they could get.
I would venture that they were professional engine builders. Those pistons and chambers looked like a stamping die with plunged valve reliefs.

That was then....and I said it was needed at one point. But with today's parts, it's a waste of time...and that's been proven time and time again....

All BBC conventional heads now are 24*, except some aftermarket replacements. The plug location is moved in all of them over OEM. I do not consider the SR20 head a conventional replacement though, due to the valve angle and valve train requirements, even though it's technically a conventional head design.