Technical Spark plug indexing

Spark plug gap facing what direction?

  • Intake valves

    Votes: 4 100.0%
  • Exhaust valves

    Votes: 0 0.0%

  • Total voters
    4

veer90

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Disclaimer: this thread not meant for debates on whether there are any hp gains from spark plug indexing

When it comes to spark plug indexing I've heard 2 different "correct" ways to do it. Gap facing intake, and gap facing exhaust.

Traditionally, domestic guys and NASCAR race teams have indexed with gap toward intake valves. Justification was to promote equal flame front travel on both sides of the plug, more complete combustion, more power.

On N5x Enthusiasts I've heard the opposite, index with gap facing toward exhaust valve. Justification was to promote spark plug cooling under load and protect against spark blow out by putting the ground strap between the intake and electrode tip.

I'm about to install new spark plugs and will be indexing them. 6266, E85, high boost, port injection, NGK 2 step colder plugs, and R8 coils. Which way should I index my plugs and why?
 
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NoQuarter

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I only recently learned this was a thing. My biggest concern is about the change in height of the plug.

The plug won't extend as far into the chamber and that seems like a problem to me.
 

veer90

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I only recently learned this was a thing. My biggest concern is about the change in height of the plug.

The plug won't extend as far into the chamber and that seems like a problem to me.

I think that might become an issue if you stack 5+ indexing washers, they're pretty thin (0.030", 0.050", and 0.060").

Heat range is unaffected since that's determined by the contact between the ceramic and metal thread collar of the plug.
 

JimboFresh206

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Wouldn't towards the Intake valves be for a port injected car?

so on DI... towards the injector? (which is still towards the intake valves)
 

Erichale77

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I would index plugs to face injectors. Perhaps slightly towards exhaust valves if blowout is a concern. If I'm not mistaken, our DI injectors spray downward and not towards either valves so as long as plug gap faces center you should achieve a complete cylinder burn. IMHO
 
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veer90

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I would index plugs to face injectors. Perhaps slightly towards exhaust valves if blowout is a concern. If I'm not mistaken, our DI injectors spray downward and not towards either valves so as long as plug gap faces center you should achieve a complete cylinder burn. IMHO

per BMW documentation on the N54, injector spray cone is specifically designed to not wet the spark plug under normal operating conditions.
 

fmorelli

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I think that might become an issue if you stack 5+ indexing washers, they're pretty thin (0.030", 0.050", and 0.060").
The plug is an M14 diameter with a 1.25mm thread pitch: that's about 0.050" in advance or retard per complete turn. If I was indexing the plugs with multiple spacers, I would want washers that gave me whatever increment of control I'd want over a 360 degrees - 0° to 360°, one full rotation, is 0.000" to 0.050" spacing. So if you want 45° increments, for example, the spacers want to be about 0.008". I've not done it before, but I would imagine putting a mark on the plug and torquing it in place, noting its "o'clock" orientation, calculating the negative rotation angle, spacer thickness respective to the delta per, and index with appropriate washers. In my case, given I have a thickness sander, I'd just fab the exact washer thickness I need for each plug.

All that said, and back to your original numbers, common indexing kits (e.g. Moroso) have huge index values (like you suggested). Why is that?

Filippo
 
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JimboFresh206

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Does it possibly have to do with this originally coming from 2 valve heads, with the plug entering almost horizontally?

The v8 hemisherpical and flat heads only have 2 big valves to clock the plug between, opposed to our 4, with the plug basically in the center of the head.

On my 442 I would index them, but never cared about being "exact" given the size and location of the valves.
 

veer90

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Update: installed spark plugs this weekend. NGK 97506 @ 0.025" gap. Indexed with gap facing toward intake.

Car idles a lot smoother, sounds smoother while cruising, and feels very strong on pulls. No misfires, no timing pulls. Will turn up the boost as weather permits.

indexed plugs.png
 
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SlowE93

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Update: installed spark plugs this weekend. NGK 97506 @ 0.025" gap. Indexed with gap facing toward intake.

Car idles a lot smoother, sounds smoother while cruising, and feels very strong on pulls. No misfires, no timing pulls. Will turn up the boost as weather permits.

View attachment 25126
Any updates ? Did you change gap or orientation ?
 
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