Just how critical is the plug gap??? I've seen here on the forum several different opinions on what the gap should be ie. (.035, .032 etc). My car is a '76 and the info I have from the GM specification guide shows a gap of .045 with a plug of R45TS.

Just how critical is the gap?? What occurs if the gap is smaller than called out? How about larger?? My engine is not original ( 383) but shouldn't it be close to the stock 350???

 

Good question, I myself am trying to figure which gap to use for my buddy's car. He has an 82, but we took the "cross-fire/miss-fire" out, and replacing it with a carb. So it looks like we will be doing our best to guestamate by trial n error.

 

gap has more to do with the voltage of the distributor and coil than engine size I believe.

The cars, in general, from prior to HEI distributors essentially worked with about 15,000 volts or less. Not a very hot spark. With HEI and subsequent more modern distributors they operate at something north of 45,000 volts. The higher voltage will jump a wider gap. The wider spark ignites fuel/air faster and better than a narrow spark. Hence, the movement from roughly .030" years ago to ".045" or so. My L48 is a 76 and is gapped as you noted...wider than the old narrow gaps.

Lance

 

to answer the question how important the correct gap is, various papers of Lars and others say that you can make good gains by having everything tuned for performance...proper gap, proper plug (I am running new set of the ones noted above), distributor initial timing, advance and curve to kick full advance in at 2500 to 2800 rpms all together can make a noticeable difference in engine performance. None are big things but they are virtually free things.

Lance

 

Well that makes sense about the higher coil voltages. I did not know about wider gaps igniting fuel faster. Thanks for the response, guess I'll check mine and reset if necessary to .045.

 

The real rule is: How good are your plug wires. The electrode air gap increase exponentially increases the ohms of resistance. That lightening from the sky to jump down to earth or vice versa takes mega volts.

So as you increase the plug gap the least point of resistance is where the spark will go. You can open the hood in the dark of old cheap carbon core plug wires and it looks like light show of the spark blowing through the holes on the plug wires.

Good wires and super high powered multi sparks ignitions run fine way out to the .050 -.070.

But read all you want and normal ignitions run fine in the .035 - .045 range

 

With my points distributor I set the gap on my plugs between .032 & .035.
After a year or so (about 4,000 miles ) they are replaced and after measuring them they still measure the same.
I've heard that the gap gets larger as time goes by but I've never seen a large difference.

With your HEI distributor just try .045 with the suggested 45 plug and go from there.

Hotter plugs are generally best for low compression, low performance engines to help burn off carbon.
Cooler plugs are better in high performance, higher compression engines.
I've tried 45 & 44 plugs and I don't see and difference in my Vette.

 

While looks like most were covered here, another thing to keep in mind when going to wide a gap on a low voltage ignition, running high compression. The turbulance inside the chamber could actually blow the spark out like a birthday candle. Dont think the engine will run to good that way.

 

Very important on Vertex magneto which requires .015-.020" plug gap. Although they generate alternating current.

 

Lots of experience here but when I bought my 76 last fall the p.o. had used a diesel truck mechanic to maintain it. He had the plugs set to .030" which is what plugs roughly used to be set to on low voltage ignitions. I had new either 8 or 8.5 mm spiral wound I think it was plug wires put in, new plugs per spec and gapped to .045". I also have advanced the timing to where I'm running it right now at 13 degrees though it could go another degree or two. There were other changes like new exhaust system converting it to true duals, etc. but the net effect is that my stock L48 runs with more power, more uniformly applied in all the gears and got back about 750 rpms at the top end. The two biggest changes were getting rid of the 2-1-2 exhaust and advancing the timing and with the plugs set to what many have found is the right gap area including the factory. Engine runs like a top and at 3,000 rpms total centrifugal advance kicks in and it is a mild kick in the pants.

Just a plain little old, low compression, stock L48 except it's better than that now. I'd guess it actually is probably in power on a net change basis more like its L82 cousin would be. No drag car but nice to drive.

All the little changes helped including good plugs, gap and wires.

Lance

 

The gap on the plug has to be jumped by the voltage potential in the ignition system. And, it has to be jumped repeatedly all the way up to engine redline. The GM HEI system upped the voltage and the robustness of the ignition [compared to the prior points system], so the gap could be widened. Now, you can leave the spark gap at .035" with HEI and it may work just fine. However, if you put a larger gap in a 'low potential' system, all you get are a lot of misfires. With HEI, the wider spark gap better utilizes the additional potential in that system, providing for a larger, higher potential, and more reliable spark to fire off every fuel charge. The ignition system's capabilities define how large a gap can be applied and still get reliable ignition. Wires and coil are big players in that "game".

 

There's a wide spread belief that a wide gap is "better". But gap has pros and cons. Some that come to mind:

Wide gap: Better at igniting a lean mixture (main reason OEMs went to a wide gap in the 70's).

Narrow gap: Better at igniting a dense mixture. Dense mix = high compression. Better recovery time (meaning it works better at high rpm). More resistant to plug fouling. Easier on plug wires.

Many think any high performance engine needs a wide gap, but that's not so. For example: My DD is a '05 BMW M3. 8000 rpm red line. Makes over 100hp/liter. Coil on plug (lots of voltage available). Plug gap? 0.028".

 

You'll get lots of 'experts' who claim they can feel a seat of the pants difference in performance by opening or closing the gap or by indexing plugs etc, accompanied by an elaborate technical thesis of how this all comes about.

These are generally the same type who believe in miracle-in-a can additives, racin' gas, basically all sorts of engine black magic voodoo.

Don't waste your time.

 

The larger the gap the better in most engines as long as you get no miss fires. One thing that is miss understood is that no matter how many volts a system potential is you only use what it takes to jump the gap. Say your plug only take 3KV to fire, if you got 30,000+ makes no difference in this case as it only takes the 3KV to fire. But still having a reserve is a good idea because of different resistances in a system caused by age , dirt, poor insulated wires and such. Yes like others have said here run the best wires you can afford and get the timing curve right for your engine.

 

Well I should be good then, new Accel wires, new plugs, new Accel corrected GM dist cap, gap'd at .045.

Thanks everyone for your input. At 51 still learning.

 

Interesting on that one, as the spark is only for a brief instant, so maybe could vary from what part of the waveform you're pulsing on. For many here, common household 120vac outlets really have peak waveform values double 240v for brief periods. Not a magneto expert, but hard to believe these aren't rectified, as spark voltage will be jumping all over the place. Kinda think they put out pulsed dc like basic lawn equipment engines myself, but what do I know.

 

You are right they do put out DC. But you know it is not really how much voltage it puts out but how much amperage a mag puts out as well as any other ignition too.


Also the reason for a small plug gap on a mag is because of the slow rpm's produced at cranking speeds. after an engine is running at 1000 or so rpm a mag will fire a very larger gap, like .200 or better, but then you can't start the engine like that!

 

Lot of myth and misinformation in this thread.

 

You are spot-on accurate there!

 

I've been using the same NGK racing plugs for quite some time now. I have mine gapped at .045 with MSD box.

The funny thing is the other day I went to order from Jegs and they now have the exact same plug pre-gapped at .043", and you can still get the original ones that you can set where you want