The Stovebolt Site Forums The Shop Area The Electrical Bay Quick question, spark plug gap 283, HEI

I installed an ACCEL street legal HEI with built in coil, I gapped the plugs to 35 thousands per speck. Have been reading this forum and some say gap should be set at 45 not 35 thousands. It running fine at 35, please explain the added 10 thousands, will it run even better?

HEI requires a larger gap than the stock .035. The suggested gap is .045.🛠

A .045" gap raises the minimum firing voltage a little to let that P.T. Barnum coil get up to about 1/3 of the voltage they advertise. A clean plug gapped at 35 will fire at around 10KV, no matter how much voltage the manufacturer claims their coil can produce. If you open up the gap to 45, it will fire at around 12-15KV. The gasoline won't know the difference, sort of like a deer shot with a .458 Remington Magnum doesn't get any deader than one shot with a 30-06. Gap the plugs anywhere you choose- - - -the gasoline doesn't care.
Jerry

Seems like the hotter the spark the more detonation of fuel per cylinder times 8 delivering more power to the wheels. Wasn't that the whole idea behind the split fire plugs? 12-15kv times 8, sounds a bit more than 10 times 8. The old plugs were gapped at 35, gona gap to 45 to see if what the difference if any will make, thanks again for all the help, Hugh.

Hugh,

If you value factory service manual information over personal opinion here is what the 1978 Chevrolet service manual for Chevrolet, Malibu, Camaro, Monte Carlo, Nova and Corvette specifies:

6 cylinder engines: 0.035"

V8 engines : 0.045"

Please post your dyno test results. All of us would enjoy seeing some actual measurements. In several hundred hours of dyno testing, I was never able to measure any performance difference by changing plug gaps.

Or you can listen to our resident manual reader.
Jerry

Being a firm believer in the points ignition for our trucks I am curious if you ever measured performance increase by switching to HEI. Talking about street driven machines, not race engines.

At normal highway speeds and engine loads, (even heavily loaded commercial engines) there was no measureable improvement with HEI over point ignition, as long as the plugs were relatively new and clean. As the plug gap increased due to wear and/or neglect, the increased voltage capability and the long-term reliability of solid-state coil triggering became a factor. Conventional ignition points simply could not meet the artificially-imposed "50,000 mile service free" mandate in the Clean Air Act, which was the only reason HEI was developed in the first place. Operation of a General Motors HEI system above 5500 RPM required major modifications to the distributor to prevent spark scatter and other problems created by speeds above those the HEI was designed to run. I challenge anyone to show me documented evidence (not opinion) otherwise.
Jerry

Confirms my belief and experience. Thank you

This article documents lower emissions at idle with HEI vs points

http://www.superchevy.com/how-to/67498-hei-coils-modules/

This article compares HP and torque in the 3500-7000 RPM range,, way beyond practical street RPM, with points vs 2 versions of HEI.

http://www.hotrod.com/articles/ccrp-9812-ignition-performance-test/

If HEI cleans up idle emissions it would be interesting to see what happens between idle and 3500 RPM

Deve has done some fuel economy testing with HEI vs points. Here is a link to his article on the subject. Scroll all the way down to the end if you want to see the fuel economy results.

http://devestechnet.com/Home/HEIgnition

Matt

How do you get reliable gas mileage figures from an engine on a test stand?
Jerry