• Welcome to ClassicBroncos! - You are currently viewing the forums as a GUEST. To take advantage of all the site features, please take a moment to register. It's fast, simple and absolutely free. So please join our community today!
    If you have problems registering or can't log into your account, please contact Admin.

Spark plug gap

Teal68

Contributor
Bronco Guru
Joined
Mar 28, 2013
Messages
2,565
Loc.
Inlet Beach
If using a 98 Explorer motor with GT40P heads, but with a 600 cfm Holley, Duraspark ignition and Pertronix Flame Thrower coil what do you think the spark plug gap should be?

Originally I had them set at .054" like the factory Explorer motor but got to thinking .042" might be better suited to the ignition system.

Thanks,
Tyler
 

Justafordguy

Contributor
Bronco Guru
Joined
Sep 26, 2009
Messages
6,253
The spec for a 77 with Duraspark is .044, I think I would try that.
 
OP
OP
Teal68

Teal68

Contributor
Bronco Guru
Joined
Mar 28, 2013
Messages
2,565
Loc.
Inlet Beach
Thanks guys. Your input is the same thinking I had and was I was shooting in between at .042
 

DirtDonk

Contributor
Bronco Guru
Joined
Nov 3, 2003
Messages
47,275
Back when I had more time than money (or sense), and because I started off with older sixties cars and trucks, I would spend time creeping up on the right gap setting by starting a little low, then working out to a little higher than spec.
I always found a sweet spot with whatever engine combination I was working on, and it was usually within a couple of thousandths of factory recommendations. But with updated ignitions, it was usually on the higher side of stock.

That said, once I got more and more V8's with harder and harder to reach back plugs, I pretty much changed to a "set it and forget it" mode. Basing it on my own experiences, I'd go with stock to a couple thou bigger and try never to touch them again until it was absolutely necessary!

But hey, back then it was all fun.
I could lean over a hood all day long and not end up with a sore back.:(

Sorry for the old whiney story. Meant to say you've got good advice and looks like you're already on the right track. But as you see, you usually go with the ignition, not the heads by themselves. So better to go with somewhere in the Duraspark range.

Let us know how it works out and the difference (if any) between the larger and smaller gaps.

Paul
 
OP
OP
Teal68

Teal68

Contributor
Bronco Guru
Joined
Mar 28, 2013
Messages
2,565
Loc.
Inlet Beach
Hey Paul,
So how did you know when the gap got too big? Ran poorly?
Thanks,
Tyler
 

DirtDonk

Contributor
Bronco Guru
Joined
Nov 3, 2003
Messages
47,275
Yep. Pretty much.
But it wasn't always to the point of running poorly, before you can just feel it not running quite as well as before.
I have gotten them opened up enough to where the engine would run perfect under light load, and fall on it's face under harder acceleration or higher rpm levels. But more often than not you could tell you were beyond the sweet spot with a certain gap.

It was harder to tell you'd reached any limit by going smaller though. Yes, you could get it to that "not quite as good" level, but I don't think I ever tightened a gap up enough to say it ran like crap or fell on it's face under load.

And it's also very engine-dependent. Or more appropriately perhaps, "combustion chamber" dependent. Some engines have a pretty good leeway for plug gaps, while others are very finicky and like only a narrow range.
Same goes for plug types, but I'm guessing that this depends as much on the ignition system as it does the combustion chamber.
Some (especially older vehicles) don't seem to take well to the fanciest high-zootin' root-tootin' plati-unobtanium plug tips, while others thrive on them for 100k miles and longer.

Lots of the guys with the Supercharged Buicks and Pontiacs swear by one type of plug, and swear at others. With only one or two brands being acceptable for all-around performance.
And that doesn't have as much to do with gaps as it does the brand and material.

But our Windsor family engines aren't quite that finicky with brands.

Paul
 

Broncobowsher

Total hack
Joined
Jun 4, 2002
Messages
34,821
Larger gap the better, up to a point.
Failure to jump the gap under load, high cylinder pressures. In boosted applications it is often called "blowing the spark out". If the cylinder pressure is 150 PSI, there is 10x as much air in the gap the spark has to jump through. So that .042" gap is like trying to jump a .420" gap. As cylinder pressures rise, and the gap gets wider, at some point the spark won't jump the gap. But the bigger the gap, the more molecules get zapped.

Weak plug wires will leak the spark as well. The rotor button has been known to melt down as well. Spark jumping to the wrong terminal in the cap (why they came out with the big cap shortly after electronic ignition came out).

The .030 gap is generally good for points ignition. Just not that powerful of a spark
The .040 gap is good for electronic ignition. The choke point in the power is no longer the points.
The .050 is good distributorless, no rotor issues. Only plug and overall power issues.

As cylinder pressures rise (high compression, aggressive cam profiles that peak more cylinder pressures, boost) will require less gap. Low cylinder pressures will run accept a wider gap.

Ideal gap is a hard target to nail down. Plugs wear and the gap widens, the sharp edges of the electrode round off and that makes spark harder to light off. So a little less then ideal gap is typically needed to account for aging.
 

DirtDonk

Contributor
Bronco Guru
Joined
Nov 3, 2003
Messages
47,275
Good point. That's something I never messed with on car engines. Just used the original factory recommended range.
Used to fiddle with heat ranges on the motorcycles, but from reading and other's experiences, I never really varied the range more than one level either way that I an remember. Never seemed to need it and was having too much fun riding instead of tuning.

And even when messing about, usually settled on the original spec heat range anyway.

Paul
 

RODRIG3911

Contributor
Sr. Member
Joined
Jul 9, 2007
Messages
968
Loc.
Tucson
I'll add that I've noticed that the 302 loves regular old cheap autolite 25 plugs... Well, at least my engine haha.. I've tried lots of fancy plugs and nothing runs better the the autolites.. Got those autolites gaped at .042 with MSD ignition box, coil and dizzy and it's happy
 
OP
OP
Teal68

Teal68

Contributor
Bronco Guru
Joined
Mar 28, 2013
Messages
2,565
Loc.
Inlet Beach
Update....wow, what difference! Was fine originally, but I guess the gap grew beyond ideal since I started so large. .042 now and much happier!

Now I have to get mine running better!
 
Top