View Full Version : spark plug gap with HEI

06-27-2012, 05:57 PM
My 396 has a hei in it,( bought it that way). i was wondering since the motor came with points stock, the spark plugs were gapped for a points distributor and the motor now has an HEI dist should i gap the plugs at 45? any thoughts? thanks.also, has anyone had experience with E3 plugs.

06-27-2012, 06:01 PM
I wouldn't go over 40

06-27-2012, 06:05 PM
Thanks rod,

i was wondering what would be a good gap size.

06-28-2012, 06:03 AM
I think there's more at play here, I've been reading up on this since I'm still playing with my ignition system too. From what I read, things like using an MSD box or the like, a higher voltage coil, type of plugs you're using, etc., these could all affect the proper plug gap.

06-28-2012, 04:01 PM
yes the HEI and higher spark systems ask for larger gaps, but the systems dont take into account that many plug manufactures in the last few years have shortened the electrode arm and when you open it up past 40 the tip is no longer covered by the arm, they have shortened the arms to already create a hotter/higher spark for the poorer fuels, so in the systems I have tested here at the shop DIS, 6A or 6AL boxes, cheaper street fires and others and they have work perfectly set at between 35-40

06-29-2012, 10:38 AM
good info, thanks.

06-29-2012, 02:02 PM
My advice is since you may not know for sure-trace what should be a 10 or 12 gauge stranded wire at the HEI BATT terminal all the way back to the firewall junction block to verify the previous owner properly replaced the stainless resistance wire from there-if not, NAPA sells the correct style GM junction spade terminals to make it correct-if it was done correctly, then at least we can start there.

For what you're running a good gap would be in the .042-.045 range if we are dealing with a stock or lightly modded 396. I do a lot of tuning with regards to water/methonal injection and water cells so plug mods are far ranging but it's interesting that what water fuel tuners had been doing for the last 10 years now the aftermarket is picking up on-such as plugs with small 1/64 sized hole in the ground electrode directly over the center electrode acting as a "spark beacon" of sorts....but, unless you have a reason to run any other gap like serious ignition mods, alternative fuels, etc. then I vote for a .045 gap.