There is no way Advance auto or any other chain auto parts place would be able to help wit this one . The only real modification done to the 403 is the #7 seven heads . I understand the reason for different heat ranges but how do you pick the correct one . The motor is running fine but with the extra compression and HP i am guessing i would need a colder plug according to what i am reading . This is something off a engine builder site . I would guess that i would be ok by going down one heat range colder by what is in the article but would the gap still be the same as stock. .
• Increased compression ratio: Higher compression ratios mean higher cylinder pressure and temperature. Once again, you’ll need a colder heat range to rapidly transfer all that extra heat to the cooling system.
As a rule of thumb, you can expect to require one heat range colder than the factory-supplied plugs for every 75-100 horsepower you’ve add with your modifications, according to Champion Spark Plugs
