[OCMDGTO]

I've always had HEI on my Pontiacs and ran .045 gap on my iron headed 400 with AC plugs. On the engine in my sig I've been running Autolites and they come pre gapped at .025 but need checking. Fuel lately has been 90 octane non ethanol

[78w72]

Originally Posted by lust4speed

The photo is from the MSD Digital 6 Plus installation manual. Only interesting thing here is they only corelate compression levels to recommended spark plug gap. I keep my cast iron headed engines at 9.2 to 9.3 compression and find that .045" gap works best. Basically the wider the gap the hotter the spark and that helps in lower compression engines. My old drag engines in the 80's were over 13:1 and we kept the sparkplug gap at .035". High pressures and wide gaps don't work that well. Note that GM basically went to the .060" gap on their ridiculously low compression engines which were sometimes under 8:1.

Anyone remember the old Victor sparkplug cleaner and tester? The test part was really to sell customers new plugs. The coil in the tester was rather anemic and one could quickly see that the main difference between a plug firing and not firing was how high the adjustable air pressure was set. The mechanic would leave the gap huge on the old plug and would make sure the new plug was gapped tight. Old plug would fail to spark about half pressure while the new plug could fire with full air pressure. Of course you could take the old plug and put a tight gap on it and open the new plug up and get exactly the opposite result. I'm sure that machine sold thousands of new plugs whether they were needed or not.

Anyway, back to gapping plugs. There's something to be said for a wider gap cleanly firing off a low compression mixture -- and also something said about a high compression engine running race gas not needing or wanting a large gap.

Edit: Opps, Vixen and not Victor

i was just going to post the msd chart, they say .050-.060 for "up to " 10.5. i assume thats with one of their coils or another big stand alone coil & im sure the CD makes a big difference vs a stock hei.

i saw this vid on youtube thats interesting, while its not very scientific & is only showing the spark at very low rpm from him spinning the dist gear by hand & no cylinder pressures, it shows a huge difference in spark with the box vs without, on both a stock coil & a msd one.

https://www.youtube.com/watch?v=vMpPtWtTS_s

thanks for the replies, i will be reducing the gap on the NGKs in the E heads today to .035. i pulled a couple yesterday & they are at .045-.048, my gap gauge is one of the older style with metal tabs of different sizes & both .045 & .048 will slide in with just a little more resistance on the .048, i will have to find my round ramp type gauge or use a feeler gauge to get more accurate & consistent this time.

[78w72]

Originally Posted by Don 79 TA

only thing i noticed on gap sizes as i've tried narrow to wide is when i would go past 40 on the gaps i would get break up at higher rpms under load (runs)
almost like rev limiter kicked in
bring the gaps down, and no issues
had this experience with various cars/toys too (i know... just my luck)
and i've had this with various ignition setups and various MSD boxes/coils despite the MSD instructions on the wide gaps
latest test was with the MSD7a and MSD8 boxes with the red and blue HVCII coils
oddly the only one without an issue with over 50 on the gap was HEI....

just my experience

right now using Brisk LGS-T plugs so i dont worry about gaps/indexing
screw-em in and go....

funny you mention this, i have been dealing with some cutting out or misfire on some low 11sec runs at the track, & have always assumed it was fuel related but havent got out enough to do further testing. its hard to pinpoint where in the run or rpms it happens as it occurs at different times & not on all runs, my lowest run of a high 11.2 it didnt cut out noticeably but other 11.3-11.4 runs it will do it at half track or a little earlier, usually top of 2nd gear at higher 5500-5800 rpms. but it has also done it on some runs i let my dad do in the mid 12's so hard to narrow it down to rpm.

i have played with fuel psi on the robbmc 1100 pump at 7-8.5 psi for a few runs & also tried a mallory regulator set at ~7.5 & turned the pump up to ~10psi.. no change still happens. next step is to attach a fuel gauge so i can watch the psi on a run & see if its losing psi, but after reading about what too big of a spark gap can cause im wondering if it is ignition related to too big of a gap & a stock hei with a close to stock in cap coil?? that will be added to my list of testing as well as trying out the 6al box i have.

[HWYSTR455]

Those types of gremlins are exactly why I gave up on HEIs, can't tell you how many coils, caps, rotors, and modules I went thru. I did everything right, too. Boils down to the modules themselves, and no modules worked flawlessly for any length of time, they all failed. Pretty sure I posted here on all that too.

If you just cruise around all the time, you probably will never notice, but once you beat on it more than once or twice a summer, it will rear that ugly head.


.

[78w72]

Originally Posted by HWYSTR455

Those types of gremlins are exactly why I gave up on HEIs, can't tell you how many coils, caps, rotors, and modules I went thru. I did everything right, too. Boils down to the modules themselves, and no modules worked flawlessly for any length of time, they all failed. Pretty sure I posted here on all that too.

If you just cruise around all the time, you probably will never notice, but once you beat on it more than once or twice a summer, it will rear that ugly head.


.

i agree gremlins are frustrating. actually the car/engine runs excellent in all situations & i drive it very hard quite often on the street but none of the symptoms have ever happened on the street, which is why im thinking the track cut out issue is mostly or all fuel related when under hard launch & G forces. if it was ignition or gap related it would happen at the higher rpms on the street too... but the fuel issues due to track G forces dont happen on the street because the street tires just spin before any real traction is made. i do have a 1/2" pickup & run full 1/2" or -8 line from tank to pump & pump to carb.

im still amazed how easily the car does low 11 sec runs even with its limitations i mentioned, but the gap was definitely too big & im hoping further testing will identify the issue at the track.

[Formulajones]

Originally Posted by 78w72

i was just going to post the msd chart, they say .050-.060 for "up to " 10.5. i assume thats with one of their coils or another big stand alone coil & im sure the CD makes a big difference vs a stock hei.

.

I wonder how long they've promoted that info and/or if anyone has bothered to report back their experiences with that. I feel like it's more regurgitated info from the HEI days in hopes to sell more MSD stuff.

3 MSD boxes here. 2 of them with billet distributors, one a stock points distributor. All 3 have the zapper MSD coils.
One is 11:1 with iron heads. The other 2 are aluminum heads, one with 10.2:1 compression and the other with 10.84:1 compression.

They are all gapped at .035. No misfire problems with any. The stock points distributor with a digital 6 box and MSD coil goes to 7000 rpm. The other two see 6000-6500. 2 of them run Autolite plugs and the other one has NGK.

[HWYSTR455]

Originally Posted by 78w72

i agree gremlins are frustrating. actually the car/engine runs excellent in all situations & i drive it very hard quite often on the street but none of the symptoms have ever happened on the street, which is why im thinking the track cut out issue is mostly or all fuel related when under hard launch & G forces. if it was ignition or gap related it would happen at the higher rpms on the street too... but the fuel issues due to track G forces dont happen on the street because the street tires just spin before any real traction is made. i do have a 1/2" pickup & run full 1/2" or -8 line from tank to pump & pump to carb.

im still amazed how easily the car does low 11 sec runs even with its limitations i mentioned, but the gap was definitely too big & im hoping further testing will identify the issue at the track.

Gotcha, that makes sense, I would probably come to the same conclusion. In my case, it was always after X period of continuous run/drive time, and slowly the time window kept shrinking. The final result was the car wouldn't run or start, it would sputter like it was going to start, just never would. It would sit for a while, and then would start and run like normal again. It was eating coils, which was the fault of the module, and it was no matter what coil or module i used, and yes, I always used the low resistant bushing. The coils would get so hot, you couldn't put your hand on it.


.

[78w72]

Originally Posted by HWYSTR455

Gotcha, that makes sense, I would probably come to the same conclusion. In my case, it was always after X period of continuous run/drive time, and slowly the time window kept shrinking. The final result was the car wouldn't run or start, it would sputter like it was going to start, just never would. It would sit for a while, and then would start and run like normal again. It was eating coils, which was the fault of the module, and it was no matter what coil or module i used, and yes, I always used the low resistant bushing. The coils would get so hot, you couldn't put your hand on it.


.

that would suck... luckily i have never had any problems with the coil or modules on this car, have ran the stock GM 990 module sun tuned tested for me & currently have the MSD under cap module. never had any module or coil issues on the heis ive ran for 30 years either but did have an original pick up coil leave me stranded a few blocks from home a few years ago from the wires breaking after 40 years of use, no signs or symptoms just died out of the blue like i turned off the key.

[ta man]

Originally Posted by 78w72

i agree gremlins are frustrating. actually the car/engine runs excellent in all situations & i drive it very hard quite often on the street but none of the symptoms have ever happened on the street, which is why im thinking the track cut out issue is mostly or all fuel related when under hard launch & G forces. if it was ignition or gap related it would happen at the higher rpms on the street too... but the fuel issues due to track G forces dont happen on the street because the street tires just spin before any real traction is made. i do have a 1/2" pickup & run full 1/2" or -8 line from tank to pump & pump to carb.

im still amazed how easily the car does low 11 sec runs even with its limitations i mentioned, but the gap was definitely too big & im hoping further testing will identify the issue at the track.

I've had loose wiring at the track cause issues.
Interesting on the .035 gap recommendation by most...something to try

[78w72]

Originally Posted by ta man

I've had loose wiring at the track cause issues.
Interesting on the .035 gap recommendation by most...something to try

i am reducing the gap to .035 to see if i notice any improvements in starting or high rpm running.

what gap do you run & what ignition system do you have?

[ta man]

Originally Posted by 78w72

i am reducing the gap to .035 to see if i notice any improvements in starting or high rpm running.

what gap do you run & what ignition system do you have?

MSD 6AL, MSD Blaster 2 Coil and a junkyard GM HEI.

I've ran .045 for years.. The smaller gap has my interest peaked for sure I'm at about 10.8 to 1 for compression. Any slight aid in tuning always helps for sure.

[78w72]

Originally Posted by ta man

MSD 6AL, MSD Blaster 2 Coil and a junkyard GM HEI.

I've ran .045 for years.. The smaller gap has my interest peaked for sure I'm at about 10.8 to 1 for compression. Any slight aid in tuning always helps for sure.

thanks. im curious about the smaller gap too & hope i see some benefit over what im at now.

i think with the CD box & a bigger coil you are probably ok at .045, my main concern is the stock HEI & close to stock coil i have may not be cutting it with 10.7 comp.