Indexing spark plugs

[Kohhal]

Originally posted by fritz_269 on PureHonda.com

The general idea behind indexing is that the ground electrode (the L shaped tang) on the spark plug effectively 'shrouds' (hides) the spark on one side. Thus combustion does not start evenly around the plug, but off to one side (the side away from the ground electrode).

We want to use this knowlege to provide the best flame front propagation that we can. So we 'index' the plugs such that all the ground electrodes are facing the same place in every cylinder. But, exactly where they should face for maximum power is different for every head. It depends on the flow and turbulence characteristics. Indexing was mainly useful for two valve per cylinder heads where the spark plug is offset greatly to one side. You can see in the picture below how a plug can be very badly positioned in a Ford 2-valve head, shrouding the spark from most of the combustion chamber.

This can make a difference of around 1-2% peak hp.

In a four valve per cylinder DOHC pent-roof setups indexing will probably make very little difference at all since the spark plug is in a near optimal position no matter which way it faces.

In a four valve per cylinder SOHC pent-roof, I think indexing might be ever so slightly useful, since the spark plug goes in at an angle to avoid the cam, it would be possible to orient it to unshroud the spark as much as possible. Not the best picture below, but you can get the idea. You'd want to keep the ground electrode close to the top of the chamber, getting the most unshrouded spark towards the center of things.


Indexing is accomplished by marking the plug body with a marker (sharpie), and placing very thin washers underneath it until the mark lines up in the preferred direction after it's all torqued down. It's an iterative process, and every plug will require a different number/thickness of washers. Moroso makes some nice brass washers for just this purpose.


PS> You can also "read" the position of the plug directly off of a removed plug. On a 'normal' plug (meaning the engine is running about stoich) there will be a thin, dark, vertical line on the ceramic tip. That line points directly at the point in between the exhaust valves. Sometimes it's hard to see, but it's pretty easy to find on my lude.

Read your plugs - it's fun and easy.

[DemonicAccord]

those are some CLEAN valves!

[JD@af]

Originally posted by DemonicAccord
those are some CLEAN valves!

Indeed they are. Looks like the valves and head are either brand new, or reconditioned prior to being used.

Good info, MasterG. Even though my engine is DOHC, I'll look for that dark line on the tip next time I check my plugs.