I have Mallory Unilite on my '70 454. When I got the car the plugs were gapped much greater than the spec in the manual. I have gapped them per the spec but I see many of you talking about running a larger plug gap with an aftermarket ignition. I don't seem to have any idle problems or stumble. Are there any performance benefits to the greater gap? Thx. Skip

.035 is perfect for performance applications,the larger gap the bigger spark,but also a greater chance for flame out at higher rpms.

Big gaps tax the ignition components a bit more too, esp the coil. I run a W-I-D-E .043 with an MSD.

I run .045-.05 with a Jacobs ignition and AC R45TS plugs on an otherwise stock 307. No problems and I get about 20mpg on the highway.

Jeff

Nobody is really identifying any benefits. Why run the greater gap then? More spark for the inital ignition? But if you are lighting the fuel off hot enough does it matter? The downside is a missed spark that doesnt make the full jump in the gap. Do hot aftermarket ignitions burn plugs if the gap is too small? Lots o questions.

I think the basic theory is that with a wider gap, you expose a higher percentage of the air/fuel mixture to a spark initially so you burn a higher percentage of the total available mixture. As in my situation, granted, it was totally UN-scientific. But with my 36k original mile 307, fresh points/plugs with .036 gap I got 12mpg on a 2.5 hr ride to Epping,NH. Ran a high 17 sec at the track and then installed the Jacobs with generic reman HEI distributor and .05 gap plugs. It picked up .8 and about 3-4 mph. On the way home, I averaged roughly 20 mpg. This was just the basic fill up at the start, fill up at the finish and check mileage driven.
Would I have gotten the same results with just the HEI and no Jacobs? With more or less gap? Can't say,just relating my expirience.
I read Jacobs book, and it's not too bad on the theory part. Obviously, he'd like you to buy one of his parts but it's not just "shameless" promotion. I also read alot of what Smokey Yunick had to say and as near as I can tell, it's still just alot of theory as to what is really going on when the engine is running.

Jeff

Originally posted by YellowSS:
Nobody is really identifying any benefits. Why run the greater gap then? More spark for the inital ignition? But if you are lighting the fuel off hot enough does it matter? YellowSS,

The benefit would be that since the spark is bigger, it would help you to ignite a leaner A/F mixture. The leaner the A/F mixture the harder it is (generally) to light.

MY thumbnail assesment: If you are just going to throw a Holley on your engine and run it box stock just go with .035" If you are looking to get max lean performance and best fuel economy, go with a wider gap and HEI.

Mike

I like the larger gaps(.050-.080) and yes, I do run my stuff slightly on the lean side. For bracket racing, lean means less dramatic e.t. changes when the air gets better at night.
Electrical energy needs to be present to jump any gap. The particles of air between two points must be ionized to create a spark path. The larger the gap, the more electrical energy must be present to "jump" the gap. Thus the need for high output ignition systems for big gaps. And thicker plug wires and wide-spaced dist. cap terminals to keep the energy going to the right place.
When you upgrade your ignition to produce and control larger amounts of current, the sole purpose is to enable you to use a larger gap so to expose more fuel molecules to the spark so they can be ignited RELIABLY and achieve more complete burn of the fuel present in the chamber.(without misfiring)
This is why indexing plugs and side gapped plugs and back gapped plugs and cutback electrodes have all been tried...in the hope of exposing more fuel molecules to the ignition source and obtain a more thorough "burn" of the fuel present.
In the extreme application, an old stock eliminator racer's trick is to clip the plug's ground electrodes nearly off. This works for ONE pass. It's generally used if the car needs a few hundredths to qualify or if it's a class runoff and you need a little more to get around the other guy. This is REAL hard on the ignition system, though. It will find weak spots in wires, marginal caps and rotors, etc.
I'm no ignition expert, just a racer that's tried lots of stuff over the years. Take this info as food for thought only.

Randy

For you guys recommending .035 just remember that late 70's bone stock HEI ignitions run .045 gaps from the factory. and since bone stock engines don't get fresh plugs as often you know that the stock HEI can fire those plugs reliably with a much wider gap than .045 (electrode wear opens up the gap).

Some factors that affect how much gap you can run --- higher cylinder pressure (higher compression) is harder to light. Leaner mixtures are harder to light. Higher RPMs makes it harder for the coil to supply enough energy fast enough to fire reliably. So if you run a high compression, high RPM motor you may need to close up the gap a little.

IMO if you are running an aftermarket HEI ignition, good wires and a reasonable compression ratio start at .045 and maybe go to .050. Slight power gains can be made with wider gaps for reasons already mentioned. Also, indexing your plugs is usually worth a couple HP too.

Remember we're talking about single digit HP gains in power here -- tops. Its just not that big of a deal IMO.

Originally posted by Eric68:
For you guys recommending .035 just remember that late 70's bone stock HEI ignitions run .045 gaps from the factory.And in that same time period there were Oldsmobiles spec'd at .080". If you look at AC plug numbers and see a 6 or 8 at the end, that's the gap. .060 or .080. If anyone wants to try a wide gap and has trouble getting the ground flat and parallel, you might want to try one of those because the ground is longer on them and gives you more room to "play" with the gap.

Jeff

Like Eric68 said, there isn't any huge power gains here. But there is positive trends. A few horsepower here and there add up to substantial gains. That's what's known as "tuning", an often used but often not fully understood term. Plus, when you change one item, it enables you to go back and change another item and make it more effective. JMHO

Randy

I've been running .55" gap on both my HEI, vortec headed motors with 9.4:1 compression as the lowest. Both have MSD components, and neither have any trouble.