Indexing spark plugs, and the effect of gap size

[Deuce Rails]

I'm looking for some theoretical advice from some engine gurus.

What kind of horsepower gains can be found from indexing spark plugs? (Maybe as a percentage of total horsepower.)

What difference does a wider or a closer gap have on performance? What engine specs require a smaller gap versus a larger one? When should you tighten or open the plug gap? Should you listen to the original (and older) car spec, or follow the plug spec?

Thanks in advance,

Matt

[Bruce Lancaster]

One consideration on gap is that wider gap needs more voltage. Coil actual output is set by the voltage needed for the spark to reach ground, up to ultimate capacity of coil. If you have a super duper 50,000 volt coil, the actual spark will be whatever voltage is required, not the 50,000 volts.
While the longer and higher voltage spark is good in terms of ignition results, there are two downsides: If you are pushing ultimate coil capacity, you may be frying the coil and other parts. The second and more troublesome on old cars not designed for high voltage is that as you build higher voltage by increasing gap, the gap may no longer be the easiest route for the spark--it may decide to jump to ground or possibly another plug anywhere in the syatem.
The GM HEI is a distributor designed from ground up for high voltage with the huge cap to get maximum separation, but even on that the factory had to substantially reduce gap after the first year or two of production because the huge gap spec lead to rapid burn-through of the rotor.
An early engine with a small cap and/or primitive materials in the insulated parts may break down rapidly if gap is increased too much, and almost all older iron will need at least a wire upgrade and wider separation between wires to function well.

[28rpu]

Indexing plugs is usually done to prevent piston interference. Some plug gurus say to not alter the gap as that can lead to electrode failure. As in fall off and do damage.

[Shag]

We are indexing the plugs on my drgaster for a couple of reasons.

1. To make sure there are no problems with piston interference (as was already mentioned)

2. Also to try to get the flame path the same so all the cylinders match as much as possible. We are running mechanical fuel injection and having all the cylinders as close as possible helps sort out the bugs and makes charting tuneing changes easier.

When we went to a Magneto we found we had to close up the plug gap for better performance. Follow the manufacturers advice on your plugs or with your distributor if you are running a newer one.

[Deuce Rails]

Thanks, guys, for your responses.

[ QUOTE ]
Bruce said:While the longer and higher voltage spark is good in terms of ignition results, there are two downsides: If you are pushing ultimate coil capacity, you may be frying the coil and other parts. The second and more troublesome on old cars not designed for high voltage is that as you build higher voltage by increasing gap, the gap may no longer be the easiest route for the spark--it may decide to jump to ground or possibly another plug anywhere in the syatem.

[/ QUOTE ]

Does this mean that a wider gap will give slightly more power, at least up until the coil can't cope, or until the plug misfires?

If not, why wouldn't everyone try to use a really small gap?

[JOECOOL]

Bruce is right on. The performance improvment on a street type car would not even be noticeable if you were to index the plugs.
Run the widest gap you can fire, but the money spent on upgradeing the rest of the system to handle the extra spark could be used somewhere else better.
Performance gain should be about the same as you taking a Dump before driving.

[Bruce Lancaster]

Just ran into the tuning section of Vizard's "How to build Horsepower" series. This guy is a thinker and has a dyno, a junior Smokey Yunick operating more at our level of driving and racing rather than Indy/NASCAR.
He says, after actual flogging on dyno, that there were some gains up to .050, nothing after that, and verified the possibility of various ignition problems at high gappage. Does anyone have 1975 and maybe 1978 GM specs? They really went wild in the first year of HEI, and had to back down. Even with the smaller gaps, HEI always seems to fry the rotor center eventually. The early ones running big gaps actually eroded away entire advance mechanisms from wayward electricity.
I didn't see anything on indexing in my quick glance at the book. I will try to dig out my other favorite philosophers, Jenkins and Yunick, and see if they said anything on this.
I've heard of folkloric/seat of pants reports of better running on flatheads with slightly enlarged gaps, but I suspect you can't go very far with early rotor materials. The crab cap would probably be the most voltage tolerant Ford distributor because of its wide spaced terminals, and wire routing would need pretty drastic changes.

[Fat Hack]

Actually, on small, ONE cylinder engines it makes a far BIGGER difference! I've found that the old Tecumseh engine on my mini-bike is sensitive to spark plug gap, and even to what type of plug it runs!

(Single cylinder engines that only make a few horsepower offer the backyard hack a chance to actually see the effects of minor tuning changes in an engine where little bits can actually be noticed!)

The engine is equipped with a basic magneto, so it can handle a fairly large gap, but it seems to run best at around .032"...unless it gets a load of bad gas, then it likes about a .035" to .038" gap. NGK plugs work FAR better than Champions, and the cheap low octane gas from the local Sunoco station works best.

I've tried different fuels, octane ratings and plug...and have reached the conclusion that 86-87 octane unleaded with an NGK plug gapped at .032" is optimal...anything else and performance falls off noticeably.

I did try indexing the plug once, but noticed no effect. In this sort of application, you can actually feel a few tenths of a horsepower, so tuning is alot of fun. I haven't even messed with the engine's adjustable timing yet...so far it's been a matter of fuel mixture adjustments and spark plug tuning!

I'll be able to pass the bike on to scarpmetalart with a good dose of info on what it likes and doesn't like...and he can tweak it further if he wants!

http://www.jalopyjournal.com/ubbthreads/images/graemlins/grin.gif http://www.jalopyjournal.com/ubbthreads/images/graemlins/cool.gif

[blueskies]

Tom Langdon of Langdon's Stovebolt Six recommends "spark plug gap at .060"-.080" for improved idle & low speed driveability" for the GM HEI distributors he is selling for inline six engines. I set my plugs at .060" and it seemed huge. Seems to run great although I haven't driven it yet http://www.jalopyjournal.com/ubbthreads/images/graemlins/laugh.gif

Pete http://home.rmci.net/blueskies/wires1.jpg