Non-Factory Spark Plug Gap Setting

Hi All, Well, I'm almost done with the engine swap and got my built-up V8 in the Jeep. I got to thinking and have a quick question. After modifying the ignition system, is there any logical method or rule of thumb for changing the factory-recommended spark plug gap? I did a lot of hot rodding and a few drag races when I was a kid but there's gotta be a better approach than, "Let's open the gap a little more and see how it does." Engine in question is an '86 Chevy 350cid V8 engine with [305 heads], TPI, HEI ignition, headers, high voltage coil, "performance" 8mm plug wires. If anyone can shed more light on the effects of the 305 V8 on the 350 block as compared to Vette heads (Sorry, no further info on those other than they were from an '86 Vette and I couldn't have 'em:), I'd appreciate that as well. I think the head casting number is 462624 and the flow could be better. Best Regards, Jim P.S. I know the Jeep's not a hot rod and don't intend to ever drive it like that. But, when the guy I asked the TPI about said, "There's no way in hell you can do what you're talkin' about; TPI, computer, to That transmission in a thirty year old Jeep? You don't know what you're doin'. No way." I had no choice:) I'm sure it'll manage the trails just fine.

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L.W.(ßill) Hughes III

Thanks Bill. I've looked through several cast number charts and have found about three different stories. One site said "58cc chamber/305 head/'76-'78/not enough flow for larger cid engines" and another echoed your info. Even still, that site said "1.94 in/ 1.50 exh" valves. Having a real time trying to get multiple sources to agree but have slowly figured out some of the errors. I've a 273/282 cam in it and was gonna go with .035-.040 gap on the spark plugs but got to wondering if I can present a better spark by opening it up. .045 sounds better if you think it's not too much. Thanks again for the tip. Jim

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