Hello all
Curious as to what others have done with their engines and what advise you may have for me as always I have a unique situation

Spark plug gap called for 1975 all engines is 0.060, first year of HEI
Ok I understand HEI, more energy, more gap, better torgue....so why has every year since called for a smaller 0.045 gap, still the same HEI smog issues, but the smaller gap???

Also since the PO pf my car switched out the HEI distributor and installed a Mallory breakerless with standard chrome tube coil, my ignition should be less than HEI energy, wouldn't this call for smaller gaps than the 0.060 that's recommended for the year???

Any help would be appreciated, thanks
Bill

 

I'm using AC-R44TX plugs in my 75.

Just changed the plugs two weeks ago, along with the distributor cap, rotor, and coil. The old plugs were clean and free of oil, the electrodes were not burned down and all the plugs looked like they should have and were uniform in appearance. They were gapped at .035 by the previous owner.

I know the 75 says to gap at .60, but I regapped mine at .035 and my engine runs fine. That is using a stock HEI distributor.

 

if I remember correctly, opening up the gap a bit was for better "burn" or "flame". I believe racers used to do this somewhat,back in the day.

 

thanks for the info,
ghoastrider1 if I remember correctly, opening up the gap a bit was for better "burn" or "flame". I believe racers used to do this somewhat,back in the day.

My understanding is the same, bigger gap better combustion, but there is a twist, higher compression causes flame out, estinguishing the spark on too large a gap, so you decrease the gap to compensate, or you increase your ignition voltage to bridge the large gap.

Now in the case of a 75 engine 8.5:1 compression means flame out won't be an issue so a larger gap can be used, and the HEI tended to put out more than ample voltage to bridge the gap of say 0.060.

The earlier cars 68-74 all call for 0.035 gap, this is understandable as they were higher compression engines with weaker ignitions thus the smaller gaps.
Oddly enough all later years 76-82 call for a 0.045 gap yet still retained low compression and high output ignitions which would lead one to believe that a 0.060 or larger gap would be fine and benefit torque. but NO the General says.

So my luck, the oddball year with the oddball spec.

75 Red Vert
Glad to hear this works on yours as I'm leaning towards the same gapping. Have you noticed any other benefits, less misfire at higher rpm's or cooler running engine?

TIA

 

Run a stock HEI at .045.

 

I gapped out 75 at .060 with the stock L82 and with the modified eng and MSD. Didn't have any problem.

 

75 Red Vert
Glad to hear this works on yours as I'm leaning towards the same gapping. Have you noticed any other benefits, less misfire at higher rpm's or cooler running engine?

I picked the car up in Florida in September of 06. Immediately drove it to Missouri. Never ran hot, even when tach was reading 3500 with the AC running.

Never cuts on/off even when I "really get on it".

Doesn't sputter at all anytime.

I may open them up a bit to .045, but that probably won't happen until I put my new headers on here in a couple of weeks or so and retune the engine.

 

i work on a couple 75's and i can tell you that the 060 gap was strictly for emissions.the early 75 carbs were murder and the later ones had electric compensators on them because of all the drivability problems we ran into at the dealer and complaints to chevrolet.
and yeah i've been working on them longer than that.
God how i love the smell of gasoline in the morning.

 

Thanks all, I'll go with 0.045 on mine and keep it like the rest of the later cars for now. Maybe this will help clear up a slight miss/sputter at WOT at 4200 on up, the current gap of 0.060 and non HEI set up is likely too much for the system to perform optimally. Will close them up and monitor the results.

Thanks again

Special thanks to general69 for clearing up the 75 only mystery, kindda figured the first year of smog was like shooting in the dark for the automakers.