Spark plug gap

Asked by Apr 28, 2014 at 11:02 PM about the 1999 Pontiac Grand Prix 2 Dr GTP Supercharged Coupe

Question type: Maintenance & Repair

I have a 1999 pontiac grand prix gtp,
it has 1.9 roller rockers undersize 3.1
pulley ported and polished heads,
ported and polished blower 3.5 inch
downpipe with cat delete and the
computer has a tune on it as well
from ZZ performance on 93 octane
with kr, I'm looking to replace my
spark plugs but am not sure how to
determine my spark gap with the
modifications that have been done,
the car makes around 340hp and
boost peaks at 14 pounds.

1 Answer


You're going to have to go 'old school' on this. You should be running plugs 1 to 2 ranges cooler than stock as a starting point. Make sure your fuel ratio isn't too rich or lean. Set the gap slightly tighter than stock, but no smaller than .032. Again, this is your starting point. Run the car for 300 miles, pay attention to the way it's running of course and make any timing/fuel adjustments you need to get the best performance. Pull the plugs, and inspect them for heat damage (plug range is too hot) or fouling (too cold). If the plugs look correct, you can begin to zero in on the right gap. Try opening the gap until you get misfires at 3500rpm+. You want the gap to be as wide as it can be without affecting performance. Good luck!

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