Didn't the early GM HEI start off on .045 then it was decided to go back to .035?? I'm old so I could be wrong.
I think I read it was .060" or even more, and they had problems right away with rotors burning through. They went to wide plug gaps with the trend of pollution emission mods, trying to light off very lean air fuel mixtures. The general thing to keep in mind, distributor cap, rotor and plug wires are only capable of a certain amount of secondary or high tension voltage before they crossfire between terminals or misfire between cylinders, arc to ground etc. Distributor point ignitions aren't necessarily going to handle excessively wide gaps, even or especially with hot coils installed and electronic modules, and the coil itself will run hot. The cap and rotor materials won't handle it, plus the terminal spacing too close. With the modern cap & rotor he's talking about it would be interesting to see what the drag racers have found, whether there is much if any advantage.
If this is a drag race car, I keep opening the gap tell the mph falls off then close it back down a little.
Pretty much what Truck64 said. With higher grade / new components, .055"/.065" works. A wider gap smooths the idle a little. For higher rpm, keep the gap down to .035"/.040". As has been said, the wider the gap the better the cap, rotor, wires must be to properly direct the electrons without having the gap jumped at a place you don't want. DO NOT tightly bundle the plug wires and tie-wrap them together if you don't want any crossfire..! Reading the MSD instructions will help too..! Mike
Thanks guys for the input guess you gotta muck with it till it’s good .thought there might have been a gap that works best ,so test and tune it is. Cheers
How does one own a 750 horse 455 and ask about plug gaps? (Not being a jacka.. just curious) There's a lot to know about what alls there to make that much horsepower. Sent from my SM-G900V using The H.A.M.B. mobile app
(raises hand) I KNOW! I KNOW! Money. Thats the answer. If you have enough money you can buy your way into stuff you dont know about. Ask me how I know haha
I think (I hope) the OP meant 750 CFM Holley. I have a Mondello built 455 w/ aluminum heads and 11:1 compression and it still only makes just south of 500 HP and 650 foot pounds of torque. Making 750 HP out of a normally aspirated Olds would be a task.
I gapped my 1st set of NGK plugs in my SBC to .040, now I have a fresh set and everything tells me .035 for the gap, which I will try. See what the manufacturer recommends first. Or what Pontiac recommends for the 455.
Jim Lindner at Bubba's Ignition has talked about this in other threads here. He claims, changing the type of ignition, say installing a high output coil, doesn't really change what a particular engine requires. The energy required to jump the plug gap stays the same, it only uses what is required, and no more, the rest is left as a reserve. So installing a "40,000 volt coil" doesn't apply 40,000 volts. If the plug gap is widened from spec it does take more voltage to fire. As plugs wear they take more voltage to fire. A longer spark is beneficial to lighting off leaner fuel air mixtures. But it also puts other components at risk of burning up or arcing to ground, and misfire under load and/or high speed. I bet the dyno guys know, or drag racers. They widen it a little bit at a time till the power falls off. Seems to me we're better off using stock spec's and ignition components will last longer and less likely to leave us stranded by the side of the road.
I gap to .035 for the street, makes it easier to start and you don't get as much 'erosion' of the electrode, very reliable.
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