Spark Plug Gap: Points vs. HEI Does It Matter?

camaro73lt

Veteran Member
Feb 6, 2003
966
Hanover, PA
The previous owner installed a GM HEI in my 73 stock engine (350 4bbl). Should I use an increased spark plug gap instead of the .035 "points distributor" gap? I read that a wider gap would improve performance and fuel economy (burn).

Would appreciate any advice on this. Go to .045? Etc.

Thanks
 

Fme

Member
Apr 23, 2013
95
USA
I respectfully disagree. In my opinion you negate the advantage of hotter spark when you increase the gap. I would use the AC Delco plugs that your car calls for and set the gap according to your application. Of course, that is provided that the heads you are using are the stock configuration for your car (I.E. the spark plug seat).
 

1980RS

Veteran Member
Jun 17, 2006
6,381
MN
With points I always ran a .035 plug gap. With an HEI most of the time .045 is fine and .060 will get you the best fuel mileage as I have found in testing. When the HEI first came out in 1975 the plug gap from the factory was .060 and the cheap rotors Delco had burned holes through the middle of them. GM put out a service bulletin changing the gap to .045 and that seemed to take care of the problem. Do not run a .060 gap with nitrous the spray will blow the spark out, I know I tried it once. I know run a MSD 7AL2 on my Camaro with a Blaster 2 coil and the plug gap between .032 and .040 and have had no mis-fires except for getting bad AC Delco plugs last year.
 

muscl car

Veteran Member
Lifetime Gold Member
Jul 8, 2001
23,898
Gallatin Tennessee
With points I always ran a .035 plug gap. With an HEI most of the time .045 is fine and .060 will get you the best fuel mileage as I have found in testing. When the HEI first came out in 1975 the plug gap from the factory was .060 and the cheap rotors Delco had burned holes through the middle of them. GM put out a service bulletin changing the gap to .045 and that seemed to take care of the problem. Do not run a .060 gap with nitrous the spray will blow the spark out, I know I tried it once. I know run a MSD 7AL2 on my Camaro with a Blaster 2 coil and the plug gap between .032 and .040 and have had no mis-fires except for getting bad AC Delco plugs last year.
I'm running .040 gap also with my MSD 7AL2 and no issues
 

Bandit723

Veteran Member
Oct 1, 2016
4,266
Waupaca WI
Autolite doesn't recommend adjusting the plug gap more than . 008" in either direction. Beyond that slight adjustment, the ground electrode and center electrode won't align properly, hindering spark plug performance and drastically reducing plug life
 

G72Zed

Veteran Member
Sep 8, 2015
4,072
Canada
The last 20-30 engines I have done, ranging from a 750hp pump gas Hemi to a basic Iron stock 360hp GM 350 2bbl crate and everything in between, MSD/small cap/HEI they all liked a top quality plug like GM/NGK's out of the box, not touched or gapped.

Copper/Platinum/Iridium, they came in at .039 gap, and NOT scratched was key in making the tq/hp, IMO, for me the right timing and heat range is the critical points, not the gap.
 

Chevyforever

Veteran Member
Jan 4, 2011
1,478
Ontario, Canada
From MSD tech bulletin,
What is the best spark plug gap for my application? The spark plug is the point in the ignition system where electrical energy is converted into heat, consequently, the larger the gap the greater the amount of heat available to light the air/fuel mixture. However, too large of gap combined with increased cylinder pressures can put excessive pressures on the initial voltage needed to ionize (cross) the gap. Finding the optimum plug gap for your application is best determined by experimentation because there are so many engine variables to consider. An MSD Ignition Control has enough output power to consistently fire wider spark plug gaps on a performance engine. As a starting point, follow the engine builder or manufacturer’s recommendation for the plug gap. With that, you can experiment with increasing the gap until the best performance is found. As a rule of thumb, it is recommended to increase the plug gap by .005” - .010” followed by testing and tuning. Keep in mind that larger spark plug gaps also place increasing demands on the secondary portion of the ignition system including the distributor cap and rotor, coil wire, and spark plug wires. They should all be in top condition and checked periodically during the race season. Remember that electricity takes the path of least resistance to a ground so if the gap is too large the spark may short to another point with less resistance.
 




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