# Does it Pay to Index Spark Plugs?



## SlowGoat (Apr 29, 2011)

Whats are peoples thoughts? Does it matter or does it not matter? Is there benefits to Indexing?


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## Poncho Dan (Jun 30, 2009)

How do you mean... like pushrods? No, just gap them all correctly.


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## BearGFR (Aug 25, 2008)

SlowGoat said:


> Whats are peoples thoughts? Does it matter or does it not matter? Is there benefits to Indexing?


Does it help make each cylinder consistent with all the others? You bet. Does it make enough difference in how the motor runs to show up on the dyno sheet? Maybe, maybe not -- it's not going to make a huge difference in any case.

Is it worth the time and effort to do? That's really your call. I index mine because I want to make sure everything is as "right" as I can make it.

(and for Poncho Dan: indexing spark plugs means making sure that the open end of the plug gap points as directly at the intake valve as you can make it. It's done by putting a felt tip pen mark on the insulator, installing and torquing the plug, then adding as thin as possible shims between the plug and the head so that the mark points to the intake valve. The idea is that it "exposes" the incoming air/fuel charge to the spark as much as possible.)

Bear


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## Poncho Dan (Jun 30, 2009)

That's pure sillyness. What's going to be more important is the consistency in resistances between all 8 spark plugs, wires, and coils. Exposure to the intake charge isn't going to mean a damn thing as far as combustion flame front is concerned. Now you're talking about how/at what degree the thread engagement of the tap was done at the factory compared to the position of the intake valve and respective charge (as well as that of the plug mfgr). It won't net you any more power/benefit. I'd bet the title to my car on it. Dyno back to back runs with/without the shims and see what I'm talking about.


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## AlaGreyGoat (Jul 6, 2006)

Depends on the head design. Early sb chevy high comp heads shrouded the
plug. The groung electrode would almost hit the head casting. Newer heads went to the angle plug design to fix this and get the plug more into the center of the combustion chambers. I index mine within 180 degrees and can usually do it with 2 sets of plugs.

Larry


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## BearGFR (Aug 25, 2008)

Poncho Dan said:


> That's pure sillyness. What's going to be more important is the consistency in resistances between all 8 spark plugs, wires, and coils. Exposure to the intake charge isn't going to mean a damn thing as far as combustion flame front is concerned.


Really? Then explain this:
Eaton Balancing Spark Plug Indexing

...and this thread
Indexing spark plugs ?? • Speed Talk
(especially the response by #84Dave » Sun Jan 14, 2007 12:31 am)

I know it's a very common practice among race engine builders and teams --- and most speed shops carry shim kits that are made for doing it.

So, I'll stand by my original statement. There IS a benefit to doing it. Whether the amount of that benefit justifies the effort involved is up to the individual. 

Bear


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## Poncho Dan (Jun 30, 2009)

:lol: *Prove the HP gains* is what I'm saying. Factory torque spec (14 ft/lbs or just past finger tight) VS 180° past that (ignoring stripping the threads). 600hp vs 601hp? Not worth the effort. Total B.S. IMO. If it was so important, why isn't GM or anyone else interested in getting the best 1/4 worried about it? That's like splitting hairs over 91 VS 93...


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## AlaGreyGoat (Jul 6, 2006)

I don't just screw the plugs to where I want them. The are torqued to spec.
I mark the groung electrode position on the plug and put it in. I look at it
and if it's not close to where I want it, I try another plug. Usually get what
I want within 3 trys. Plugs and head threads are not all the same.
Plugs in LSx motors are easy to get to, so it' not a big deal.
Is it worth it??? To me it is.
If 93 octane fuel isn't any better than 91, why not just get 85 and
save some money??? I'll get the 93 any time over the others.

Larry


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## svede1212 (Nov 1, 2005)

I don't know if it helps or not. It certainly couldn't hurt. I think with high compression and proper combustion chamber swirl the spark is probably pretty well exposed to the combustion mixture regardless tho. That's one of the ways a tight quench helps in that more gasses get pushed into the chamber and not just in the area of the whole top of the piston.


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## Poncho Dan (Jun 30, 2009)

Again, if it was important, why was the subject not reviewed in my ignition & fuel class? Does plug position change the firing line waveform? Does it contribute to knock? Does it even show up in a cylinder power balance test (rpm drop)? No.

And you're gonna see 91 sold in places that are higher elevation than 93. Down Milwaukee way, we can get 93. Elevation is 600-900 feet. Up north you can only get 91, where elevation is 1600+. Is it worth the 200 mile drive down for 93? Probably not.


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## danfigg (Sep 27, 2009)

*re*

I think it would only matter if there was a single side electrode type (ground strap)spark plug. I've seen spark plugs that have four of them so you would not be able to index these type of plug but this is what I found----- Indexing your spark plugs is another way of gaining you a few extra horsepower. The object is to get the gap of the plug facing the intake valve, or at least facing the open area of the cylinder. If a spark plug's ground strap is facing the intake valve or the cylinder, it kind of blocks the flame travel, which impairs the burn path thus causing a loss in power. It can be a 6 - 8 horsepower difference in some engines by indexing the plugs.


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## jetstang (Nov 5, 2008)

Yes it's worth doing, like everything else that can gain 1 hp.. When you lose a race by .001 of a second and know you didn't do it, then it was worth it. Exposing full spark to the gas initiates combustion earlier than going around the electrode. Any obstruction will slow combustion and delay timing, so that cylinder will have lower EGT-Exhaust Gas Temperatures than the other cylinders, thus, not optimizing that cylinder and getting the full burn. That's why all the companies are putting out the plugs that expose the electrode. Old racing trick is to cut off half the electrode to expose the flame to the fuel. Bear is correct.


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## AlaGreyGoat (Jul 6, 2006)

Some high comp engines with dome pistons had to index the plugs to keep the
dome from hitting the ground electrodes. It wasn't for performance, but
a necessity.
Larry


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## zx14man (Apr 26, 2011)

We should all use the NGK plugs for the Mazda Rotary engines..They dont have ground electrodes, they spark to the side of the body of the plug...Now if we use these then the indexing problem/discussion is over...LOL...


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## jetstang (Nov 5, 2008)

Some people spend 40 hours porting a set of heads for what gains? Naturally aspirated motors are making 900 horse power out of 350 CI.. You don't think they consider this kind of item? If your motor is spinning 6000 RPMs, how much time can you spare as the cylinder is firing 3000 times a minute, so it is firing every .005 of a second, or 50 times a second!
You guys are right, just keep buying $1400 headers, and $400 CAI's and ignore what is actually happening in the engine..


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## Poncho Dan (Jun 30, 2009)

Yeah, I don't know what's going on inside a motor, I'm only going to school for it. 

Show me the difference in the firing line backprobed with an Oscope, then we'll see if a difference is made. You DO know how to do that, right?


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## MrGoat (Oct 23, 2010)

Knowing how to do that takes a google search and 10 minutes so I wouldn't prop yourself up on that ability. Knowing how to analyze what you're seeing on the oscilloscope and relating it to the question of plug indexing is more difficult. 

If you do know, great, I would like to know what you have to say.

As far as whether or not it pays, I would say that unless it's easy to do, you have huge HP, and you're racing for money than NO, it does not.


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## AlaGreyGoat (Jul 6, 2006)

I have 40+ years as an electrician/technician and know what an oscope does. It doesn't show anything about what is going on inside the combustion chamber of an engine. Also, don't see many of them to hook up to a
60-80Kv high tension coil output.

Larry


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## jetstang (Nov 5, 2008)

So, with a Hemi motor what is the benifit of spark plug placement over a BBC, both are hemispherical valved motors, BBC is a semi hemi. No, the new hemi is not a Hemi by the way.


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## AlaGreyGoat (Jul 6, 2006)

A true hemi has the spark plug in the center of the combustion chamber allowing the flame to expand the same distance outward 360 degrees.

Larry


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## MrGoat (Oct 23, 2010)

AlaGreyGoat said:


> I have 40+ years as an electrician/technician and know what an oscope does. It doesn't show anything about what is going on inside the combustion chamber of an engine. Also, don't see many of them to hook up to a
> 60-80Kv high tension coil output.
> 
> Larry


Larry,

Directly, it shows the voltage that it takes to fire a spark plug. By looking at the oscilloscope, you can capture the event of the firing spark plug. Through some equations and possibly voodoo magic (AKA I don't know), you can derive a pressure curve/trace (I think lamborghini uses this technique as an 02 sensor in the combustion chamber or something along those lines) By integrating (measuring the area underneath) the pressure curve you can find power and then compare one to the other.

I think that's what Dan was asking for.

Personally, I'd take years of knowing people to index plugs for better performance over equations and oscilloscopes any day.


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## jetstang (Nov 5, 2008)

AlaGreyGoat said:


> A true hemi has the spark plug in the center of the combustion chamber allowing the flame to expand the same distance outward 360 degrees.


True, but the Hemi has the plug between the valves, while the BBC has the same valve configuration, but the plug is offset. On the Hemi you can aim/index the plug to point directly at the intake valve starting combustion quicker than most motors.. The Hemi was king of the day, and still the chamber design of choice for Top Fuel and all the top classes of drag racing now, 40 years later. We don't index the plugs on my buddys' Pro Mod Hemi, but we change plugs every round, so it would be a PITA, and it makes 3500 hp as is.



MrGoat said:


> Personally, I'd take years of knowing people to index plugs for better performance over equations and oscilloscopes any day.


Exactly what I am saying..


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## Poncho Dan (Jun 30, 2009)

Nobody has produced hard evidence proving it works. Waveforms, a dyno graph, track times, etc. Show it to me. I'm more than interested to see proven results if it works. I'd rather know why/how than just take someone's word for it.

Ever heard of Ion Sensing Ignition? It uses the same principle, monitoring combustion through waveform. You can indeed see whats going on inside by using the scope. Not just on this system (in this case the ECM monitors it) but on all systems, including the old points style. Especially when you're monitoring all 8 cylinders at once.


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## jetstang (Nov 5, 2008)

Here's the 1st one I opened from NGK spark plugs:

4. Indexing

This is for racers only !!
Indexing refers to a process whereby auxiliary washers of varying thickness are placed under the spark plug's shoulder so that when the spark plug is tightened, the gap will point in the desired direction.

However, without running an engine on a dyno, it is impossible to gauge which type of indexing works best in your engine. While most engines like the spark plug's gap open to the intake valve, there are still other combinations that make more power with the gap pointed toward the exhaust valve.

In any case, engines with indexed spark plugs will typically make only a few more horsepower, typically less than 1% of total engine output. For a 500hp engine, you'd be lucky to get 5hp. While there are exceptions, the bottom line is that without a dyno, gauging success will be difficult.

Spark Plug Installation Instructions


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