If I have replaced the original 74 points distributor with an electronic HEI unit, do I still gap plugs at .35 ?

Thanks and happy new year!

 

Most points distributors call for .035 and most HEI's call for .045

 

Standard Motor Products recommends .035" for both.

 

One advantage from the HEI is being able to run a larger gap. My 77 manual recommends .045 for the HEI.
You can experiment with yours. If the compression is lowish (9.0CR or less) you may even be able to get .050 or more without misfire.

 

Did the recommended spark plug change when the factory went to HEI?

 

I have a 74 also and removed the resistor wire and installed an HEI. I gap the plugs to .045 .

I bought the distributor from Skip White with a lifetime warranty and I want to say it was under $50. They had great customer service when I needed a replacement module.

I see that they have tach drive units for $71. I ended up converting my tach which cost more. Anyhow, the unit has no problem with the .045 gap.

best regards,
John


ps. Of course its from China.

 

.........

YES, points or electronic, .035....... the same to you

 

Good question, you might want to go here for information on your year Corvette:

https://www.gmheritagecenter.com/gm-...tion-kits.html

For 1974, the last year with points they suggest ACR44T plugs and a of gap .033 - .038



For 1975, the first year with HEI they suggest ACR44TX plugs with a gap of .060

 

here is a great post by Lars on this....
Quote:
Originally Posted by Ironcross View Post
I use .035 on all of them that includes the electronic ignitions with L88, and LS7. And the lone 426....
Quote:
Originally Posted by Solid LT1 View Post
GM originally recommened 0.060" gap on HEI ignitions in 1975-77 but soon found out those large spark gaps created multiple problems with ignition wiring/rotor buttons and other ignition parts. Large gaps are not commonly used today a gap of 0.032-0.035" with conventional ignitions and 0.040-0.045" running a MSD type ignition are generally used by most engine builders. If you like your plug wires, run 0.040" spark plug gaps
Quote:
Originally Posted by FB007 View Post
Correct answer. Although like Iron Cross there, I use only .035". Larger gaps do squat.
The guys quoted above have the correct answer. A few points regarding HEI, MSD and plug gap:

The HEI was developed and released as part of the emissions control package in 1975. The engines at that time were low compression engines, and they were set up to run very lean. A lean mixture is harder to "light off" than a "correct" mixture, so the HEI system was used so that a much wider plug gap could be used. This would ensure that the lean mixture would ignite and burn.

When a wider plug gap is used, there is more resistance across the gap, and more energy is required to jump the gap. Increased compression will also increase the resistance across the gap. The wide plug gap could be used on the smogger engines because the compression was low. If you use a wide gap with high compression, you can get the resistance across the plug gap so high that the plug simply won't fire: the HEI spark or MSD spark will jump at the point of lowest resistance. This can become the plug wire boots, the inside of the distributor cap, or right through the plug wires if plug gap resistance (from the combined effects of gap and compression) is high enough.

Keeping this in mind, it is important to note that you cannot run an "HEI Plug Gap" (some gaps in the mid-70's were in the .060" range) on a high compression engine, even though you have the HEI system or an MSD. You will, in fact, get plug misfires from the high energy spark jumping any point of lower resistance rather than across the plug. For this reason, most high performance applications with HEI or MSD use plug gaps in the .035 - .040" range. This assures that the spark stays at the plug and does not move to the inside of the cap or goes through the wire insulation... If you have a modest compression engine, you can run a wider gap, but you have little, if anything, to gain. If you want to assure that your high perfromance engine is firing at the spark plugs, run plug gaps in the .035" - .040" range.
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Read more: https://www.corvetteforum.com/forums...#ixzz3wBlCboMd

 

Did "Standard Motor Products" spec and build the engines????





NO, all they do is manufacture the replacement ignition parts............

 

I think this statement needs to be qualified. "larger gaps do squat....." under what conditions?

What does the combustion mixture consist of at idle on a cam with reasonably high overlap and a 10.0CR?

And is that mixture easier or harder to ignite than that same cam/engine at 4000-4500 RPM?