I need help guys, what is the correct way to index spark plugs? Electrode towards the intake or the exhaust or the center of the cyl.? My bench racing buddies and I have different view points on this. Help me settle this please. If it makes a difference we are working on a set of Dart Aluminium heads, 64 cc 2.02 I, 1.6 E.

It really depends on the head design and as a result of design, air flow patterens in the cylinder.  You want the ground electrode pointing at the approaching air flow.  This exposes the maximum air fuel to the plug arc.   If the electrode is in the way, it partly blocks the air flow, and restricts ignition.  This is more important at full throttle and high rpm due to the fact during those time there is a large increase in air flow and velocity. 

When the air flow disrupts the spark, having the electrode behind the spark also give the arc a place to go, and still be effective.  Air can actually blow out a spark plug.  it takes a lot, but it can be done.  The electrons are blown away, and never reach the opposite electrode.  If this happens to a lesser extent, the spark is weak.

None of that answers the question.   It just tells you why it makes a difference.  Most folks will tell you to point the electrode at the intake valve.  For most engines this works best.  But even different Chevy stock heads may have a bit different air flow.   The air flow in the Dart heads may be quite different.   If stock heads have been modified, that will affect the air flow as well.  Keep in mind, machine work in the ports and behind the valves will have just a  much effect as work inside the cylinder.  Sometimes you can index the plugs, then look at them after several many miles and determine the burn patterns.  Look for electrode erosion or deposits.  Then you can adjust the indexing from there.  You could also test track times and do a compairson at different indexing.

In any event, these performance differences will be fairly small.  Mild stock engines will see less benefit than built or performance engines.  On the street it probably won't make enough difference to notice.  On a drag strip, track, or dyno testing, you can see a difference.

Ok - I'm curious about this:

1. do you mark the plug somehow in reference to the position of the electrode?
2. if so, how do you then point this mark in the direction of intake, exhaust or center of the head? 

I'm always amazed at some of the things I continue to learn on this forum.

Thanks for the education!

As Joel points out, you dont want to over or under tighten the plug.  You can cause thread damage to the plug or head.  Or the plug could come out.  The shims allow indexing by changing the position the plug becomes properly tightened.

Also as stated, just use a marker on the ceramic, lined up with the electrode.    The position will be determined by the indivudial setup.  Most of the time the electrode is pointed at the intake, but this is not always the best.

I think 5% is a bit more than optimistic.  Perhaps in an unusual example you could achieve that much, but I think it would normally be much less.  Granted, any improvement is an improvement.

If anyone has ever seen the Champion Spark Plug Tester you will be amazed at what air can do to a plug.   This tester allow you to place a plug into the tester through a grommet.  Just the threaded section is inserted.  You can then look through a window, and into a mirror.  This allows you to watch the spark.  Connect the plug wire, and push a button and the spark starts to arc.  This is a continous arc.  Then you connect an air line, and start turning up the air flow.   Air rushes past the tip of the plug.   As air flow is increased a gauge indicates the condition of the plug.  You can actually watch the spark being deformed by the air.  As you build more air flow/pressure, you can see small amount of spark blow away.  When the flow reached a point, the spark disappears entirely.   At this point you look at the gauge.  It measures how much air it took to blow out the spark, and thus judge how good the plug is operating.

I haven't seen one of these testers for years, but I wish I had one here at the school to demonstrate the principle of air flow in the spark.  It's really quite interesting.

Indexing the plug to different positions in the tester will affect when the plug blows out.  The same thing happens in the engine, as well as the flame travel as a result of the air flow, and how complete the combustion is achieved.

Many head manufactures won't give you a exact position.  The piston shape also makes a difference, clearance as well as air flow, and they have no control over piston selection.  Most of the flow is determined by the head itself, and to some extent the intake manifold.  Lots of factors.

Again, in a street car it probably is not worth the effort.  On a drag strip or track it may payoff.  What will I do in my car?  I'm just going to properly tighten the plugs and not worry about it.

Thank you all for explaining this to me - this was a  very interesting discussion and one I had never given a second thought to.

Now if I could somehow use this information to tune my snow blower or lawn mower ...... look out!

Thanks again!