Advanced Engineering Tech - Spark Plug Indexing...for you hard-core guys!

Hey guys,

I'm looking to head to the dyno next week to finally get my car tuned (383 LT1, AI heads/cam + all bolt-ons) by the guys at Second Street Speed. I'm going to switch (per my tuner) to a colder plug (TR6s) from the TR55s I have now.

My plugs in the car now were never indexed, so who knows where the gap is positioned. I'm going to index them before I get to the dyno.

My question is, where are you pointing the gap? Most people just point the gap between the intake and exhaust valves, but would it make more sense to point it towards one valve or the other? I know on a lower compression engine it wouldn't matter much, but I want to make sure I have ALL my bases covered. Also, if I'm being too anal about it, just tell me. Heh...

Any thoughts? Thanks in advance. :cheers:

http://www.fordmuscle.com/archives/2000/07/indexplugs/

Not sure if you already saw this, but does this help? (i know, i know, its a ford site. lol)

i was wondering if it was possible to index tapered seat plugs?

from ford link: The object of indexing spark plugs is to ensure that a spark plugs ground electrode is not positioned in such a way where it is shielding the spark from the fuel/air charge or impeding the flame front as it travels away from the spark plug into the chamber.

well when the spark plug fires, the piston is anywhere from 40 degrees to ~5 degrees top dead center depending on engine load, rpm, etc. at which time the air/fuel charge is compressed and is a homogenous mixture. Pointing the spark plug gap in a certain direction to not shield it from the air/fuel charge is bullshit.

as for thinking the groundstrap electrode impedes or alters the way the flame front propagates and how the mixture burns, well that seems like it has a little more logic to it but if that is true then bosch +2 and +4 plugs should then work better with that ground strap electrode design... and the +2's should be better than the +4's since there's less ground strap to "impede the flame front as it travels away from the spark plug into the chamber". But don't tell bosch this, it might blow their whole advertising scheme that the +4 plugs are better than the +2.

as for indexing tapered seat plugs, just like the article in link above describes you either keep trying different plugs till they happen to be indexed correctly, or you buy some indexing washers for tapered seat plugs and depending on the thickness of those you then reduce the amount of reach the spark plug has into the chamber which can affect combustion... i would argue that for a 1/4 or 1/2 turn of the plug that's going to equal 0.012" or 0.025" if the 14mm spark plug thread has a 1.25mm pitch, and for a 0.025" of reduced reach could have an affect, whether good or bad i don't know.
would it be better to initiate the combustion closest to the piston, as in having the spark plug spark as close to the piston as possible, or better to have the spark closet to the head?
Or maybe, is it best to have combustion take place with the entire charge burning as fast as possible? This would give the highest pressure from the air/fuel charge turning into a gas via combustion, and would yield the work done on the piston. So if that's the case, having the spark at either the piston or head end the flame front has to travel the entire length of the chamber; placing the spark in the middle of the combustion chamber would give the flame front the shortest distance to travel and should allow combustion to happen as fast as possible.

Every write up I've ever seen as far as the angle has never shown any difference on a dyno. They all say "myth busted." What does make a difference is the depth of the plug into the chamber and they only find that by trial and error with spacers.

I believe Car Craft did this once back in '04ish, and it showed some gains, but it was on a 600+ HP car.

I've only heard of people indexing the plugs on older motors with smog era heads. The plugs are usually indexed so that the opening faces the intake valve, I did this with my pontiac 400 w/112cc heads and it seemed to help, but I put in new plugs and wires at the same time.

One thing that it could possibly help very slightly is detonation. If the ground strap is turned farther away from the hot exhaust valve it could maybe stay cooler and helf prevent detonation on a high compression engine.
Just a thought.

http://www.carcraft.com/techarticles/spark_plugs_tips/index.html