In a stock 351W for my year, the proper gap for my plugs is 0.044". However, my motor is not stock. I am using a mustang computer in the same block and added a blower (motor below).

My question is, does any of the above change what I should be using as the stock plugs and gap? The blower needs copper core plugs and I was advised to reduce the gap to 0.040" for the blower. I am running an MSD 6A box as well. Makes me wonder if I could go to a larger gap (0.042") since I have the MSD on there or what benefit that may have?

If anyone would school me on this, I'd appreciate it. I am replacing the plugs today so I have an opportunity to make a change.

 

How much boost are you running? You'll want to decrease the gap as you increase boost in order to prevent blowout. What coil are you running with the MSD box? That will make a difference as well.

 

Up to 10 lbs. boost. Just a Bosch coil for now but need to get a new one of those as well.

 

You can try .040", but at 10 psi, you may be better off in the .035"-.038" range. I certainly wouldn't even try a gap larger than .040".