Plug Gap

Doing some maintenance today and new plugs is first on list. I have an MSD 6AL box and the instructions say start with a range of .050 to .060 but when I pulled the old plugs out they were gapped at .030. I haven't touched this motor since I got it back from Compuflow who did the assembly and tuning on the dyno so just want to make sure going to say .050 won't be an issue.

Thanks for the help!

I run ours at 40 to 45.30 seems low.Rocky

Thanks Rocky, I'm setting them at .050 and see how it goes.

Paul

Remember back in the days of the first GM HEIs?. They suggested gaps even as big as 0.060. Then they pulled back a little.

Here's the deal...The spark takes a LONG time to turn into a flame that burns across the chamber. The larger the spark when it starts out, the less time it takes. SO, if you could make a 0.250" spark, that would be super. It just takes a lot of voltage that'd rather leak out of the system somewhere else.

There's no doubt that any decent CD ignition will make a spark for a 0.060" gap in a regular street engine. It will show you any problems with the ignition wires and cross-fire or tracking in the distributor.

My 1993 Suburban with old faithful 5.7 chevy (350 TBI) had 100K+ miles on original wires, distributor cap and rotor. It was completely stock, I don't know if plugs had been changed once or not. I had logged every tank of fuel and knew really close what kind of mileage it'd get going 80 on the Interstate with a vacation load. So I pulled the plugs and found every single one had a very rounded center electrode and 0.060 - 0.062" gap. The wires were whatever chevy had installed when new, they kind of crackled as I removed them. I replaced with new BWD spiral wound wires, new AC plugs and new Accel distributor (original had burned out pickup coil and the ECM does the timing, so no change to the timing) Everything was new, the plugs were gapped to factory specs and we took our next trip. The mileage was exactly the same. There was no difference at all, at least to the tenth of an MPG.

From this experience I gathered that even with the crunchy stock wires and original cap and rotor, the stock HEI system had no problem firing a 0.060" gap, even with worn out plugs.

Your high performance system should be able to do at least as well.

R.

The MSD will fire a moderate engine with .050-.060" but high secondary voltage will likely shorten life of the cap rotor and wires and make leaks/misfire more likely. I run around .040" with beefed ignition components. With stock style distributor I would probably run .035"... stock gap to keep the system reliable.

.040 here too. 50-60 seemed excessive to me.

Yet my stock chevy stuff at 100K miles had no problem firing a 0.060" gap with the center electrode perfectly rounded off (no sharp edges for the spark to jump).

R.

When Bosch was heavily advertising their +4 platinum center sparkplugs I bought eight to try on a 1995 360 V8.

These had 0.066 gaps but the four ground electodes "probably" helped reduce peak firing kV by allowing the hotest, most shielded, or sharpest edged ground strap to accept the most favorable ionizing.

The Bosch+4 plugs definitely pinged more on hot summer days on 85 AKI gasoline and a 205 degree thermostat.

When a Mopar Performance JTEC PCM was swapped in and 93 AKI fuel was used they pinged even more. Not even Sunoco 96 AKI gas would stop the pinging.

My guess is that this indicated that a big gap did advance the "real world" ignition timing.
Perhaps a 4 degree advance or thereabouts.

I found no fuel economy gain with these Bosch+4s in single truck 304 mile tests over the same highway but in fairness to Bosch a much more sensitive test like a SAE/TMC Type IV two truck test is probably a necessity to reliably find 2 to 4% gains like Bosch was claiming in writing.

I seem to recall that for warranty claims purposes automakers routinely retard timing on factory vehicles about 6 degrees from what their in house testing shows a blueprint engine could operate on without ping.

It would be interesting to set 7 old style sparkplugs like Autolite 3023s to gaps of 0.025 inches, then take an Iridium tipped spark plug with a gap of 0.080 inches and try it sequentially in all eight cylinders. You might find both the most troublesome cylinder(s) and also make a guess as to the timing change that an extra 0.055 of gap gives.

keep the electrode edges nice & sharp as it takes more voltage to jump across rounded surfaces. might keep you from misfiring if you are pushing the limits with the gap distance

NGK states to keep gap at .045 max on there regular plugs. If you need a wider gap move to a race plug. I gap my MSD 6al at .045 on my 11 to1cr SB.