HI i have a 350 880 block lt1 pistons standard crank and rods alloy heads and a 600 cfm carb hei dizzy whot plug gaps would you run ?

 

Stock

 

thanks

 

Yup... .045" ish max

 

I've been searching the Forum looking for similar info. Although mine is the 305 Vin "F" motor in an 85 IROC, I found a Campaign Label under my hood. 85-C-22 Campaign says, the plug gap is now set to .035". The only reason I can think of that this setting was chosen by GM back then, is Chevrolet was having a problem burning a pinhole through the "Black" OEM distributor rotor. The result of the rotor failure, was a short in the HEI high voltage to ground and either a misfire or no start condition. By reducing the plug gap, possibly the stress was reduced and they had determined the rotor failures would be resolved. Since I'm attempting to remain period correct, I may split the difference and go with about a .040 gap and stay with the HEI ignition with the better quality Cap and Rotor of today.

 

The recall campaign notice is here:

http://www.corvetteactioncenter.com/...coil-1130.html

It looks like GM walked back on the the high output coil and replaced it with a standard coil. The standard coil required a smaller spark plug gap.

 

Very nice. Thanks Paulo.
Now reading that Campaign really takes me back. Looks like they needed to do everything they could to cool down the top of that HEI distributor.

 

When HEI first came out, I made a point of carrying around a rotor for it in my car, for the express purpose of assisting damsels in distress by the side of the road. Of which there were quite a few. I bet I swapped out at least 4 rotors for young female drivers of disabled cars, and got dates out of the deal, in the mid 70s. (my GF at the time, now my wife, bought a 74 Monte Carlo in about 77, some time after I met her, and enjoyed that failure at one point...). Seems like Cutlasses were particularly susceptible but Regals and Grand Prises were common victims too.

They spec'ed a gap of .060" in some applications for awhile. The only noticeable effect of the large plug gap was, to raise the high voltage that the plug wires, cap, & rotor had to deal with. Regardless of whether the increased gap made the engine run better, which it didn't really seem to, it drastically increased the stress on ALL of the insulation materials. The dielectric withstanding voltage required on all of those parts increases with THE SQUARE of the change in the plug gap. I.e, double the gap, and you QUADRUPLE the dielectric breakdown withstanding stress., or to put it in other terms, QUADRUPLE the probability of a failure or QUARTER the average lifetime of the parts. Not hard to see where that's gonna lead.

They changed the non-xxX part # plugs to where they came out of the box, if not shipping damaged, at around .042 - .045" gap, where they had used to be .035". Basically if you bought the non-X part # and just took them out of the box and checked them to make sure they hadn't been dropped or something, they worked fine. You could still get the .060" plugs with the X in the #, if you were one of those people that thought that the FSM was "the last word" on everything; but if you were one of those others that actually stayed informed about the latest developments, you'd buy the TS instead of the TSX (.060" gap) part #.

Use a gap of .040" - .045". Like they come out of the box if not fornicated in shipment. That's plenty. Don't go higher. Don't go too much lower either, not below .035" or so. Of all the things that make a car run better, big plug gaps are WAY down low on the list. Just do what works, and STAYS WORKING, and don't try to outsmart yourself.

 

After running a little Marvel Mystery oil down the holes.....car's been sitting for 25 years now....I'll be running the R43TS AC plugs in there. Couldn't find the R43CTS plugs anywhere. Everything is either GM or AC Delco (if available) going back into this car.

I guess I just wasn't sharp enough to carry a spare rotor back then to rescue all of those beautiful women out there who would have rewarded me had I just used my noggin. I was too busy thinking about what needed to be done on my car to have it ready for the drag strip on the coming weekend. My first real quarter mile car was a 68 Firebird.