[ASKED] Indexing Spark Plugs
 

Every year or so the topic of indexing spark plugs comes up on the Forum. Some say don’t bother as it makes no difference at all, others say it will add a couple of horsepower and to position the spark plug gap towards the intake valve or position the spark plug gap towards the exhaust valve. I know that when you are designing a new engine a lot of super computer time is dedicated to combustion chamber design and spark plug placement. Could you please ask the engineers who are involved in combustion chamber design if they would comment on indexing spark plugs? We would like to know if it makes a difference, if so how much of difference (a few, 5, 10 horsepower?), and what is the correct orientation of the spark plug in the combustion chamber. If possible a picture or drawing showing the correct orientation of spark plug in the combustion chamber would go a long way to eliminate any confusion coming from a written description.

I don't know if it would make any difference or not. But if there was free horsepower available I'd take it. Good question.

I have always wondered if indexing plugs was worth the effort. Looking forward to the answer.

Indexing has been proven to work. I lot of work for a very small return.

Indexing on a traditional port injected engine was a good old hotrod trick for a little bump, but on a DI engine, im not sure how much can be gained...

An old hot rodders trick for sure. I was told about this decades ago. I've never seen a dyno test to show that it provides a HP increase but it makes sense to think that unshrouding the spark is a good thing by making sure the spark has an open shot at the center of the cylinder. It would seem that the flame front would progress straight out instead of being split by the negative electrode. I used to do this years ago and it's simple enough. Use a Sharpie and make a line on the insulator that aligns with the negative electrode. Then screw in the plugs and find the one that keeps that line ideally straight up, but I would also be OK with it if it is was within 45 degrees (22.5 degrees either way) by eye.

As far as I'm concerned, an edge is an edge and if the competition doesn't have it, so much the better!