I have a newer (~`93) 350 SBC crate engine in my `80Z. I also have a MSD Ignition box. The original engine sheet says .060", but a have another sheet that says .035 or .045, I can't remember which. I think I have them gapped at about .055-.058. Does anyone have a suggestion as to what the correct gap would be? Is the difference in gaps due to HEI vs. something else?
I looked at the gap numbers for a stock `80Z and a `93 Caprice, which is what one sheet says to use for parts interchange, and they are both below .060.

.035 with cars with points.
.045-.060 cars with HEI.
You could go to the local gear head store and look up the tune up specs for a 93 SBC.

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Mark D
1970 Z28
http://members.rogers.com/mlvd/

The lower the compression ratio the more gap you will need, if the C/R is 8:1 to 9:1 than a .045" gap will do for an HEI or .035" for points. Once the C/R gets above 9.5:1 you need to lower the gap .005" for every point of compression above 9.5:1.

Lots of smogger engines had to run up to .060" gaps because of really low compression and very lean mixtures for emissions reasons.

Thanks.

My compression ratio is 9.1:1, so maybe I'll try a smaller gap (.045) and see if that works any better.

Actually, the reason for big plug gaps in smoggers is the reverse-because of tightening emissions and lower compression ratios, GM was forced to go with what they would call "High Energy Ignition", which was just a more powerfull spark. MSD requires a wider gap because of the increased amount of several spark currents available per firing-this means you can take advantage of a bigger gap to light off more of the mixture for a more complete burn. Too small of a plug gap combined with a powerfull aftermarket ignition box can create what is referred to as a flamefront-this is where the initial "spark" produces so large a flame kernel that it actually pushes the fuel away momentarily, resulting in a mis-fire. How to tell how big of a gap to go is trial/error. It also depends on others factors such as plug type, amount of resistance in your wires, etc. I use Taylor 409 Race Wire (10.4 mm @40-50ohms resistance), NGK U-groove plugs, Performance Distributors HEI coil (60,000V)w/module and I run a .058 gap as opposed to the factory setting of .045-so it depends on several things as to the perfect plug gap-very motor is different.