Indexing spark plugs, and the effect of gap size

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

 

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.

 

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.

 

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.

 

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?

 

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.

 

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.

 

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!

 

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

Pete

 

Indexing plugs creates flame front balance. Opening the flame front towards the incoming charge creates a minimal increase in explosiveness and also a more complete combustion. BUT when you are discussing an extreme high compression engine it is imperative to COOL the plug tip to its maximum. The only way this is done is to transfer heat to the cylinder head and then into the water jacket. IF the plug tip extends into the combustion chamber (exposing threads into the chamber) this portion will NOT be proper cooled. ===pre-ignition or detonation. This, as we all know, is an engine KILLER. Sooooo on these engines indexing is not an option it must be done.

 

Ideally in a perfect world a wider gap should produce a hotter spark, better fuel burn. If your ignition is not set up for the wider gap, or you are not running the proper fuel mix or enough compression you will not see a real benefit from it and maybe you will see a loss from it.

Ignition should not be an add on every component choice when building an engine should be made in respect of other choices. If you fail to keep ignition in mind when building an engine or have an older engine that you want to upgrade the ignition your best bet is to contact an ignition pro and have them build an ignition for you. example; when I needed ignition for an old race motor that I lucked into I contacted a professional distributer builder that I know, he got bore, stroke, static compression, quench, cam shaft specs including opening and closing events and induction. He then matched the ignition to the mill.

Now indexing plugs, that is total race chit, period. Please pay attention, you do not index plugs to keep the piston from smacking the electrode. OK you missed it so I will say it again, you do not index plugs to keep the piston from smacking the electrode. If your piston is smacking the electrode it is time to figure out what you have done wrong.

You do index plugs to clock the electrode and create an equal flame path for all cylinders. For our purposes in general the benefits will be measured on .001 of a second increments in 1320 feet or .01 MPH increments in 5 miles. Will your street motor benefit from it? Sure but nothing that you can prove or feel in the seat of your pants.

 

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 500 hp engine, you'd be lucky to get 5 hp. While there are exceptions, the bottom line is that without a dyno, gauging success will be difficult. And in Many cases there is Absolutely NO DIFFERENCE seen!

 

Contrary to many folks beliefs, Higher Compression and forced induction, along with richer mixtures require a SMALLER plug gap. The amount of energy to light the mixture in these cases goes up and widening the gap also requires more energy. Combined, you can run out of coil energy real fast. And at higher sustained RPM's in particular you will experience misfire as the coil and ignition system can't keep up with the requirements of the engine. And if you running a Mag, you will run a pretty small gap - .018-.020" is pretty common. The higher combustion pressures can blow out the wide gap.
The reason that GM went to a large gap on the HEI has everything to do with emissions. The ultra lean mixtures in the late 70's and early 80's needed a larger gap and more energy to light the mixture. With a small gap the time to jump the gap is shorter than a wide gap and the amount of flame kernel is smaller, so the lean mixture just wouldn't fire with a traditional .025-.030 typical gap. So going out to .045-.060" and more allowed these weak suck mixtures to actually fire

 

that may be why top fuel cars run 2 mags.

If one is willing to experiment with plug gap it is interesting to see what actually works best with a particular engine, and ignition setup. For example on my 355 with the ignition that I run I discovered that all around driving town and country it works best @ .044 that is after several tanks of fuel and plug readings as well as mileage notes. But I also discovered that tightening the gap a little makes better passes with the exhaust open and the carb fattened up a bit. There is no doubt some theory that explains or refutes my findings, but I have tendency to not care and just stick with what works. A friend helped me design a new set of headers and I may have to change everything with the change. LOL

 

A long, long time ago, I played with indexing on my then-new 70 Z/28. The motor was 11 to 1 from the factory with a 780 Holley carb. The EGR was disabled. Indexing the plugs where the gap was towards the intake valve resulted in a much more consistent plug color while indexing the ground lug towards the intake valve resulted in a darker plug reading. Of course, indexing the plugs didn't improve performance at all but I did waste enough time to drink perhaps 3 more Coors during the task. The motor was sensitive to plug life....you could tell a difference between 6 month old plugs vs. brand new plugs in the crispness of the acceleration. This was in the pre-HEI era. IF I changed plugs once a week, that resulted in 3 Coors/week X 52 weeks/year = 156 more cans of beer. Your mileage may vary.

 

Index plugs to keep piston from banging gap shut --a band aid if you did not correctly notch pistons when you built the darn thing
Big gaps will NOT make more power --never! Period! If you think so then dyno your behind off working every gap you can and results will be when oil gets hot the engine will make a few more HP NOT because the gap was bigger--It is a MYTH run .035 -.040with good ignitions and tighter with a mag
The rest is Bullbleep This is what I have found out racing 30 years and dynoing every week for most of my life
Flame travel is when I bring my grill to your hose for a cook out

 

I got to ask who dynos a mill that is not up to temp? I really need to know so that I can scratch them off my list of respected mechanics.

 

Indexing the plugs will not guarantee a given horse power advantage.
If done right (a coupla different schools of thought !), it put the "open" side of the gap out where it will do the most good. Reliability of burn and same burn, plug change to plug change.

But in reality, it's one of those...small things...that when a few of these little things are added together, you'll get power.

Gap depends on many things. The coil/ignition box will set what it will "reliably" fire, gap wise.
Remember too get in the .060" range or bigger, a degree or two of ignition advance will help bridge the gap of the time it takes the spark to bridge the wider gap. Bill Jenkins figured that out MANY years back.

Mike

 

i have a 500hp 383 stroker in my camaro with MSD and pro billet dizzy. i put new plugs in at the start of last year and forgot to gap them out of the box. car had a hesitation / stumble from time to time at various rpm's after that and i chased the carb etc for a while. then i thought to check the plugs and noticed a couple of the gaps were awful small. gapped all the plugs threw them in and bam the issues were gone! there's my two cent story to add to the pile

 

1. dyno tests with my 517 hp 377 SBC produced 4hp difference at 6600 with indexed plugs (the rpms we ran the engine)
2. wider gap to .050 made no difference
3. the big difference came when I used J-gapped plugs set at .035, 8 hp from 5k up. J gapping is cutting the ground electrode tab back to the middle of the center electrode. I use a Dremel to cut it then file a sharp edge. the sharp edge allows the spark to jump easier and un-shields the center electrode

.....more speed secrets from Rick

 

Rick,
Way back when I knew a guy that filed his ground wires religiously, even on new plugs. He always said that sparks jump better from square edges.

I am going to give the J gapping deal a try. It won't cost me anything and if it works for you it may work for me.

 

In my experience if you increase the voltage on an HEI you can open the plug gap a lot if you combine this with adding more fuel you will gain HP. Performance Distributors in Memphis has help me with this. They tell me I have not reached the potential of the engine yet.

 

Look you ...ANY engine that is dyno'd is brought up to temp before first pulls--anyone that has the least bit of experience knows that as you pull back to back and the oil gets really hot that numbers will creep up a tad Geeez! Big plug gaps being better is a MYTH You set plug gap depending on type of ignition , compression and fuel --but bigger will not show you any gain--it WILL wreck ignition parts such as coils, caps, rotors etc and do it fast

 

I think in my tractor pulling days I ran across a plug group that was factory j-gapped but don't remember group or brand..Yeah, no help.. ..

 

I used to index the plugs in the flatty on my power wagon. Not sure if it made anymore power but with the gap towards the cylinder it made an audible difference in exhaust tone and a more consistant and idle rpm.

 

If your engine is a pig dumping raw unburnt hydrocarbons into the atmosphere there should be some gains [lost hp recovered] from using a fatter spark.

Increasing the gap requires more energy from the power source [and alao allow for losses in the system]
From a battery every time you double the voltage the amps are halved so the energy stays the same , so there needs to be an outside source of energy added which is the alternator [that consumes hp]

Spark plug placement is more important in engine design than actual spark plug indexing.
Think about it.... the explosion in the combustion chamber puts equal cylinder pressure on the piston as well as the roof of the cylinder head [this pressure builds up]
So how is pointing the electrode up vs down going to change cylinder pressure. The plug is surrounded by an air/fuel mixture on the compression stroke.

Spark plug placement in relation to gas flow is the real science [part of engine design]

 

Spent a lot of time indexing plugs, the combo I was running was hard on plugs, don't know that it made any more power, but I had fun doing it, i had more time than money, and I knew it was right.
J Gap is interesting.

 

The J gap method works for me on the blower motor. Noticed it was a little crisper after J gapping.

 

Anybody know anything about these?

http://www.briskusa.com/brisk_multi_spark_plug_multi_spark_spark_plugs