gta ta

I had a miss So I decided to change plugs and wires. After I was done I noticed the plugs I pulled out were gapped at .035. I had looked up the gap in my chilton's manual and it said .045. So that is what I gapped them at. Now today I noticed the sticker on the hood says .035. Did not even think to look at the hood at the time.

SO question is do I need to pull them all back out and lower the gap to .035 instead of .045?

86 T/A 5L TPI

HalfInchWrench

There are a lot of mistakes in those Chilton and Haynes manuals. Even after all their revisions they still leave the same mistake in print. You require a .035 gap just like the label under the hood says.

brutalform

As long as they were all .035, or .045, I would not pull them back out.

305q_ta86

iTrader: (1)

yeah. Not a big deal IMO. Heck, if your coil can take it, which it should, leave it.

If not, get a new coil.

gta ta

What do you mean if my coil can take it?

In replaceing it are you talking about a new stock one or one of those MSD ones. Actually I was thinking about replaceing it. I priced it when I was looking for a cap and rotor and they were cheap.

I couldn't find a cap and rotor anywere. I don't want those aluminan contacts. I coulnd't find brass anywhere so just cleaned up the old on I bought two years ago while repairing an intake gasket leak.

HalfInchWrench

I wouldn't leave no .045 gap plug in there. The old plugs you took out would be better than that. Stock ign. was never designed to jump such a large gap.

Sure a bigger gap is better if the system can handle it. The stock HEI system can't tho. Do you think the folks that designed it said yeah, this can handle a .045 gap but we will tell every body to gap them less and get worse fuel economy and emissions? They tweaked it as best they could. My LS1 uses a .060 gap but it also has a coil for every plug. I would hardly gap the plugs at .070 or .080 because I am smarter than the folks that designed the system. I wouldn't do it with a HEI system either. Not unless it's been beefed up. All the system beefed up, not just the coil.

305q_ta86

iTrader: (1)

A wider gap = more power and economy. But it takes more power to jump the bigger gap.

My 86 lg4 has a factory sped .045 with the stock HEI.

Mkos1980

iTrader: (8)

Autozones Conrad cap and rotor comes with brass terminals. Thats what I use with my Full MSD setup. No problems after 10K miles so far. I also keep my plugs at .040 since I am trying to beet down some detonation and I hear a shorter gap can lower knocks.

dyeager535

Well, then you should do some more research. The exact same ignition components GM used to jump a .035" gap in Chev motors was used to jump a .080" gap in Olds motors. (check the 80's applications if you don't believe me)

In a stock application HEI will jump any gap you care to run. You just won't see the benefits of doing so in any measurable way. If you don't believe me, try it. I've gone from .035" to .060" on a chev motor and there was no measurable difference in idle quality or fuel economy.

HEI being weak is a fallacy propogated by the aftermarket ignition companies, and thats it.

gta ta

That is what I bought a year ago but they no longer carry them here anyways. My has 4 hold down screws. Everything they have is the clip style.


I decided to compare the 3 spark gap gauges I have and noticed there was a .005 differance in the them. Gap the plug to .035 with one and the other reads .040.


Thanks guys for all the replies. Once again you guys come to the rescue with the right answers! Thirdgen.org rocks!

HalfInchWrench

Sure it's going to work for low rpm's and new plugs. What happens above 4000RPM's tho? The voltage drops like a stone. What happens when you have put 10 thousand miles on those overgapped plugs with a little bit of carbon on them and the like? What do you think the folks who designed the system had in mind for a .035 gap? Why not bigger? It would result in better firing which would lead to iimproved millage and emissions.

Sure there are different gaps and heat ranges required for NOS motors and blower motors and for good reason but I don't think it is all that relavant for this thread.

paulo57509

I don't really understand why the voltage would drop with a wide gap.

The coil provides only enough voltage to jump the gap at the spark plug (and at the cap/rotor).

So, as the spark plug gap gets wider, the coil would have to provide MORE voltage, not less to jump the gap. The spark plug will stop firing when the coil can no longer provide adequate voltage for a given spark plug gap (coil saturation also plays a part in this too).

I think GTA will be fine running an extra 0.010" gap even when spark plug wear is taken into consideration.

dyeager535

Show me an independent dyno test, dragstrip run, datalog, etc, that shows stock HEI dies at 4000RPM. Doesn't matter the gap, that 4000RPM barrier is a myth.

Crap! What will it drop to? Points voltage levels? If it jumps the gap, it's job is done. But now you are getting into coil saturation, etc.

Umm, nothing? Electricity is jumping from one surface to the other...why does it matter how far apart they are? If you've got something going on thats is fouling your plugs, that needs resolved.

You mean like when GM first put HEI on SBC's and gapped them at .060" and then dropped it down the next year? What about gap being .045" on some 305's, and .035" on another, yet again with same HEI components? I can keep going back to the Oldsmobile. GM engineers used the same components to run a .035" gap to a .080" gap. They ran what the engine liked, it has nothing to do with plug wear, components, etc. You seem to believe the 350 (or any SBC of choice) was built around a .035" gap, and that isn't true. Many issues can influence what an engine likes as a gap, most just don't play with it. But even GM changed the gap on the same engines, so .035" has never been set in stone.

[/QUOTE]Sure there are different gaps and heat ranges required for NOS motors and blower motors and for good reason but I don't think it is all that relavant for this thread. [/B][/QUOTE]

I agree. Show me a reason GM was wrong to run whatever gap they wanted with HEI, and why it would be wrong for anyone else to experiment with whatever gap they want?

pasky

Bigger gap = smoother idle. Nothing wrong with a .45 gap on these vehicles.

brutalform

FWIW, My car ran its fastest time with the old 357 engine in it, with a faulty MSD 6AL box disabled.

84 Z-28 350

.45 gap is pushing it allitle on a stock ignition with ~9.5:1 compression, but it shoulden't be a prob if your ignition is in good order, if the compression was lower you could go with a bigger gap. as for the argument dyeager535 is putting up, with the ignition sys being the same for the olds and the chevy, that may be true, but the engines aren't, I'm not 100% sure which olds your talking about but I would put money that it reves allot lower and has allot less compression then one of our chevys, the higher the cylender pressure the more resistance the fuel/air mixture and the more voltage is needed to bridge the gap, also the voltage drops as the RPM's go up due to less coil saturation, starting at about 23-2500RPM(that's the point where maximum saturation happens for a V8, any RPM below that will have the maxumum saturation), then you add the carbon (carbon dose not conduct eletricity well) that will build up on the plug, the wear of the plug, roter, wires and the coil. that's why chevy had diffrent gap settings for diffrent engines.

Apeiron

Might want to check that fact again.

84 Z-28 350

carbon is a seim-conductor...it depends on what it is mixed with and how densely packed the molicules are...the stuff in the combustion chamber will not conduct eletricty well, take a flake off and use a DMM on it and you'll see(done it, got "OL" for ohms on an AF DMM)

naf

iTrader: (7)

Lot of good stuff here but simply: larger gap=better burn, but requires more voltage to jump. More rpms (above coil saturation rpm level) make less voltage available. At some point you run the risk of not having enough voltage available to jump the gap=misfire.

With use the spark plugs will wear (increase gap) and deposits will add resistance, again requiring more voltage to jump gap. So a fresh plug will handle a larger gap. Upgraded ignition components (including lower resistance plug wires) make more voltage available at the plug.

By all means experiment with your plug gap. GM standards are by their nature conservative, but one universal gap will not work for everyone due to differences in ignition components, timing, compression, air/fuel, cam, etc. which all affect resistance between the electrodes or voltage available.

dyeager535

Yes, I didn't really want to get into that, since the same is true when running boost, nitrous, etc., and that just serves to dilute the topic.

Plenty of older Chev's are running 8.x:1, which is what the Olds ran. FYI I'm talking about the Olds 307 and the .080" gap, but even the late 70's 403 used a .060" gap. (still low compression of course) I find it hard to believe that a 1 point jump in compression *requires* the halving of the plug gap, which is essentially the difference between a 305 Chev and a 307 Olds. Perhaps it does, I certainly don't know. If it were true though, anyone running HEI on a 10:1 motor would need a gap of .015", and I seriously doubt that is the case.

My point is that HEI is CAPABLE of jumping an .080" gap. (and I guarantee much larger) Since GM ran so many different engines, and so many different designs (compression, chamber design, etc) there is a large variation in plug gap. Not only that, but now when many people have a totally different engine that was built and installed in a vehicle, playing with plug gap isn't going to hurt anything, and may help.