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Indexing your spark plugs

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Scott
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I haven't indexed my spark plugs an my bikes for quite a number of years, but I used to as it can make your plugs ignite the incoming fuel mixture better, improving your gas mileage and performance depending on the shape of the combustion chamber and the placement of the spark plug in the combustion chamber.

To index a spark plug, you mark a line on the the porcelain with a black marker in line with the side electrode, then screw the plug into the head and tighten it down to spec and observe the relationship of the mark to the intake valves, as you want the side electrode to be facing away from the intake valves and the incoming fuel mixture. The logic behind indexing your plugs is that you don't want the side electrode shielding the spark from the incoming fuel mixture.

Sometimes you'll luck out and have the side electrode facing away from the intake valves and don't have to make any adjustments, but if it doesn't you remove the plug and place a copper washer on over the threads on the plug which will make it lock up in a different position to the intake valve. I still have a bunch of different thickness copper washers from Moroso, and to accurately determine just how thick a shim to place on the plug to get it to lock up in the right position, all you have to do is determine the approximate degree that the plug needs to be rotated, and then using the pitch of the thread on the spark plug you are using, you can calculate the thickness of shim to use to effect the change in the angle of rotation to get the side electrode to line up where you want it.

So on a 14mm spark plug with a 1.25mm pitch, which equates to .050 of movement per turn, if you needed to have the plug lock up 45 degrees farther in rotation you would use a .012 thick washer.

I used to do this a lot on my older Harleys as it was easy to see the alignment mark on the side of the plugs. On the Bandit with it's plugs recessed way down in the head which will keep you from seeing the alignment mark on the plug, you would have to carefully place the alignment mark on the spark plug socket extension.

Summit Racing still sells several different brands of indexing washers, and this winter when I do a valve clearance inspection, throttle body sync and install new plugs, I may just for giggles index the plugs on my Bandit. You can also do a search on the internet and find all sorts of info about the benefits of indexing your plugs.

http://www.summitracing.com/parts/MOR-71900/
 
Indexing the plugs was popular for a number of years.
On modern bikes the ignition systems are much more powerful and predictable than they were a few years ago. Spark plugs have come a long ways also.
Most manufactures say the procedure is unnecessary on modern engines.
Might be fun to play around with..........just for grins as you say.
 
I bought a pack of indexing washers and played with indexing the plugs on my 1980 Yamaha XS850. Could not tell any difference in anything.
 
I bought a pack of indexing washers and played with indexing the plugs on my 1980 Yamaha XS850. Could not tell any difference in anything.

Ya, I couldn't say for certain if it ever really improved anything on the bikes I did it on either.
 
I messed around with it years ago when I was convinced it would work...all to no avail.

I've since had to ask myself: What makes it "better"?

It doesn't ignite the "incoming" charge since it was drawn in on the intake stroke, and then compressed, ignited a little BTDC...so that doesn't hold water.

The shape of the combustion chamber may play a factor, but that has more to do with spark plug position, not orientation. The flame front is the force that pushes the piston down, and several factors would have a much larger effect than the miniscule positioning of the electrode:

1. Timing
2. Octane (complete burn, many people run too high an octane for no reason)
3. Compression (wouldn't an extra spacer pull the plug out of the chamber a bit and cause a (small) reduction in CR?)

All things to think about, and all surely much bigger parts of the combustion process than some undefined effect of indexing.

1. Most folks can tell the difference in about 4* of timing advance, and the dyno surely can. I doubt any change in flame origin would even equal 1* of advance, which you can easily push just for kicks as long as your not past MBT or det-limited.

2. I can tell the difference between 87 and 91 octane in my bikes. I too used to think I'd get better performance with higher octane, but that only applies in cases where either you or an ECU is advancing timing with a knock sensor, and the higher octane allows you to get more advance before detonation. For set, open circuit advance settings/tables, lower grade fuel is better, unless other conditions are causing detonation.

I can however tell when premium fuel is working in newer vehicles, as the lower grade stuff will cause the ECU to "pull" a little timing.

3. The actual compression loss may be small, but I hate to mess with spark plug height in any application. I check to make sure new plugs aren't longer when swapping for obvious reasons, and I'd worry that a short, or shimmed, plug may be detrimental as well. May be a small factor, but certainly bigger than whatever "power" the electrode is blocking.


It seems the only part of this that has any bearing in the physical forces involved would be the spread of the flame front around the electrode. The electrode doesn't apply any real force in the gas expansion as this is all done by the chamber, walls, and to some degree the face of the plug.

So it might be a little bit of a fluid dynamics problem, but then which orientation is the best?

Maybe block the exhaust side to keep those valves cooler?

Block the intake side to keep the least burnt fuel on the intake side? (or vice versa to help spool a turbo...)



Help me understand this, because I'm at a loss.


Either way the theory works out, it's all conjecture until it's measured. I've never actually seen dyno proof that indexing makes a lick of difference.
 
Hadn't thought about that in years.

Not sure I know how to get to the plugs on the Strom?? So many things to play with so little time :huh:
 
Did a heck of a study on that subject back in the late 60's when I was hot rodding and stuff. But, can't remember a thing about all the bennies on doing it. Something about plug tip temps to fractions of power increase. All the top drag racers were doing it mostly. Never heard that much about it on motorcycles even back then. Never did it on my old hot rod '67 Fairlane 500 with the 427 Holman-Moody engine in it either. That thing was a bear to drive and much faster than my '69 Norton 750 Commando Fastback.
 
So just how do you index a plug with multiple ground electrodes?

What about hemi combustion chambers etc?

On a car or race application with domed top pistons and an A-semitrical head I can see the need and advatage of indexing a plug.

But over all gains in an everyday street situation? Certainly not worth the effort.
 
So just how do you index a plug with multiple ground electrodes?

What about hemi combustion chambers etc?

On a car or race application with domed top pistons and an A-semitrical head I can see the need and advatage of indexing a plug.

But over all gains in an everyday street situation? Certainly not worth the effort.

I remember that one. if 2 > the side to the intake valve. if 3. split the difference between 2 to the intake valve. surface gap . don't worry about it as it's always indexed. ;-) :trust:
 
Did a heck of a study on that subject back in the late 60's when I was hot rodding and stuff. But, can't remember a thing about all the bennies on doing it. Something about plug tip temps to fractions of power increase. All the top drag racers were doing it mostly. Never heard that much about it on motorcycles even back then. Never did it on my old hot rod '67 Fairlane 500 with the 427 Holman-Moody engine in it either. That thing was a bear to drive and much faster than my '69 Norton 750 Commando Fastback.

Ya, I was just sitting there the other day when the indexing thing popped into my head for some strange reason, and I 'm pretty sure I'll never put the time into doing it again on any of my bikes, but I thought it was something interesting to throw out there for conversation and I guess I was just thinking about the good old days. :rider:
 
but I thought it was something interesting to throw out there for conversation and I guess I was just thinking about the good old days. :rider:

You brought back memories of the cool cans to run the gas through before it got to the carbs, cutouts for the exhaust, etc. Thanks for the memories. :clap:
 
You brought back memories of the cool cans to run the gas through before it got to the carbs, cutouts for the exhaust, etc. Thanks for the memories. :clap:

Remember the water injection systems for the carburetors that were really popular on cars back in the 50'' and early 60's and they are still made today for cars.

Here's a pretty good read on it.

http://www.dave-cushman.net/misc/mannject.html
 
Very common in late WWII aircraft engines, used almost like nitrous today for a small "boost" until the tank ran out.
 
Water injection was also used on turbojet engines. A "wet" takeoff in a KC-135 was a deafening affair. And the smoke....it always looked like something was horribly wrong with all four engines. More noise than thrust it seemed.
 
Very common in late WWII aircraft engines, used almost like nitrous today for a small "boost" until the tank ran out.

Actually water injection alone doesn't do much for you in terms of a boost besides cooling the intake charge, leading to a denser intake charge. I haven't really messed with straight methanol (just 50/50), but there may be benefits there and this may be where this common misconception comes from.

Where the power is really made is allowing you to dial in more timing advance before finding detonation. This can be really beneficial in boosted applications without an intercooler, but even with an intercooler I was able to ad up to 5* in some parts of my advance table fine tuning the WI and using det cans...and that wasn't a conservative table to begin with.
 
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