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Old 03-22-2007, 04:09 PM   #1
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Indexed Spark Plugs -- Nice...

After reading (on this site) about what spark plug indexing is, how easy it is to do, and that it can give you minor FE and performance bonuses, I decided it give it a try. Naturally, my independent mechanic took longer than expected to order the index washers (it wasn't an immediate/crisis thing, so it got low on the priority queue). However, he finally got those indexing washers in, and got around to indexing the spark plugs yesterday (he was already in to replace a faulty door closed switch that was triggering "door open" alarms in the car, so I had him complete the plug indexing at the same time).

While one day (without instrumentation) is far to little time to detect what (if any) FE gains I might get, I was surprised how noticeable other effects were. So far I've noticed the following:

The engine just seems to run "smoother" (not that it was bad to begin with, but...). In particular, the engine previously sounded/felt to me l (especially as low RPMs, such as idling), like the timing between the 4 cylinders wasn't quite "matched" (close, but not quite on). Not that this was enough of an issue to be a problem for me, but I none-the-less noticed it. OTOH this seems to have gone away today (hence the statement that the engine seemed to run "smoother").

I noticed a slight increase in power, in all gears (I have a 5-speed manual transmission, throttle body injected, Honda CRX). Not the the car ever had a problem with power (it always had good get up and go, relative to a lot of cars), but it still seemed like I had slightly more power when I tried (out of curiosity) heavy acceleration.

Surprisingly (at least to me), I found that I could use more throttle at slower speeds after the plug indexing (maybe due to the "smoother running engine", above?). This seemed to be true in all gears, but naturally it was most noticeable in high/5th gear. I was actually able to drive with light throttle/acceleration, going only 20MPH in high gear today WITHOUT any noticeable lugging while doing it (and no I was NOT coasting while doing this, but instead very lightly accelerating in 5th gear)! I suspect this feature is likely to save some fuel in the future, if only because it allows me to shift into a higher gear sooner...

It's too early to tell if the pattern will last, but it seems as if my car now gets to lower RPM idle quicker than it did before. If so, this will clearly be a minor FE savings, as higher RPM idle burns more fuel (at least during those times you are idling). Of course, when I thought about it, this behavior made sense, as my car's ECU is setup to adjust the idle speed to what it thinks is necessary to cleanly keep the engine going. So if the engine is running "smoother" (above), than it makes sense that a lower RPM could keep the engine running. And I wouldn't be a bit surprised if the car's ECU was picking up on that smoother firing/running engine, and lowering the RPM (for idle) more than it has in the past.

There was only two things done to that car yesterday, and only one of them (the spark plug indexing) should have any effect on how the car drives (the other "fix" was replacing a faulty "door open" switch, that was resulting in "door open" alarms going off). So, since nothing else was done to the car yesterday (we didn't even clean and regap the plugs, just pulled them out and indexed them as we put them back in), I attribute all of the changes to the spark plug indexing. And that surprised me, as I wasn't expecting that level of changes, just by indexing my plugs!

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Old 03-23-2007, 08:13 AM   #2
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I'm somewhat sceptical of the 'indexing' or rotating the plugs to align the electrode gap in one direction or another. If a given plug in a given hole is off by 330 degrees from the desired direction, and the plug is 'backed-out' by nearly a full turn, does that not mean that the projection into the cylinder head is reduced by nearly one full thread pitch? I would think that making the spark receed might place it outside any stratified mix area and might hurt performance more than it would help.
And if repositioning the spark by the amount of one thread pitch has negligible effect, then why is "aiming" the electrode (the spark is still vertical anyways) claimed to be so benefitial?

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Old 03-23-2007, 10:33 AM   #3
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I think it can have an effect theoretically.

If the negative electrode on the plug is in the way of the incoming air and fuel mixture, it could perhaps have a negative effect.
1992 Civic CX with D15z1 swap.. work in progress
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Old 03-23-2007, 10:43 AM   #4
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I've been luckly. I done this on 3 different cars and have not had to use the spacers. I've just moved the plugs around and it works out. This is one of those things that It might not show up but when added to other little things the next thing you know you FE has gone up and you don't really know why. Best of all it's free and only takes 10 minutes to do.
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Old 03-24-2007, 06:45 AM   #5
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Originally Posted by Lug_Nut View Post
I'm somewhat sceptical of the 'indexing' or rotating the plugs to align the electrode gap in one direction or another. If a given plug in a given hole is off by 330 degrees from the desired direction, and the plug is 'backed-out' by nearly a full turn,
Good point about the indexing washers affecting height of the plugs. However, IMHO the important thing is what does the various movements (both plug height movements and angle movements) do to the positioning of the spark?

And considering that we are talking at most a .064 inch height difference (i.e. at worst the thickness of the fattest index washer in the set), we clearly aren't affecting height much when we use the washers to index the plugs. OTOH when we don't index the plugs, the plug electrode could be at any angle at all, and thereby having the spark preferentially go out (from the plug) at any angle. As a result, it seems reasonable that plug angle (affected by indexing) is likely to have a much more dramatic effect on spark positioning than the minor height change caused by the index washers. If so, then indexing is still a net "win", even if/when you "lose something" do to changes in plug height.

Of course, we all know that plugs work "well enough" no matter how you position them. And that makes sense too, because as soon as you ignite the fuel/air mixture the ignited fuel will ignite fuel around it, and so on. So a spark anywhere in the fuel/air mixture (i.e. anywhere in the cylinder) should work.

The only issue then, is what (if any) gains (over more "random" plug positioning) can be had by adjusting where (in the cylinder) we put those sparks, and are those gains (if any) "worth it" from a cost/benefit standpoint. And on that level, I was able to find (using google) a number of reports/studies suggesting that spark position (i.e. "indexing" the plugs) does make a small but often noticeable difference in burn characteristics after you ignite the mixture (potentially affecting both FE and performance a small amount).

And since "indexing plugs" is cheap to do (maybe 5-10 minutes extra time when changing the plugs + the cost of a few index washers), I figured I would give it a try. And while the effect on FE (in my car) is still "up in the air", I have already noticed some (other) positive effects (mentioned in my previous post) in how the car drives. So it least in my car, I not only got an effect, but the effect was actually much more noticeable (to me) than I was expecting. i.e. I was expecting that indexing might help a very little (and was unlikely to "hurt" at all, even if it didn't help). However, what I got was several positive effects of the indexing, that turned out to be much more noticeable than I was expecting. Granted, each effect wasn't huge (and the car was running fine before the indexing, these were just bonuses/improvement), but considering how cheap indexing is, I really don't need a large benefit to justify the minor cost/hassle of doing it!
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Old 03-24-2007, 09:46 PM   #6
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if you look carfully at your spark plugs you will notice one side gets hotter then the other, or what looks almost like a shadow on the plug from the hot gasses, and from one side of the plug being cooler and leaving a very very small amount of soot on that cool side of the plug, you want the spark to be on that hot side I would think, right? either way, it's a strong enough theory that when you replace the spark plugs on the Honda Insight, that you have to check the indexing stamp on the head to tell what spark plug you need for that hole so the electrode lines up.

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