do i have to use smthng special?
or go with the previous part #'s?
thnx

lower # = cooler right?

Numbers being higher or lower depends on the plug manufacturer... Ac plugs and Champion... The higher the number... the hotter the plug.... NGK... just the opposite. Smaller number is hotter plug.

I'm running NGK 5673-8 they are two heat ranges colder than the AC MR43T

It also depends on your boost - I run 4-5# and use the MR43T and never have a problem.

I'm running 8 lbs boost

NGK

always worked best for me

15psi boost @7500rpm i use ngk

NGK UR5's. They are the same heat range as an AC41.

They might be the same as the ones Biggus posted the # for. UR5 is the # on the plug. There is a different stock # on the box.

I run anywhere from 10 to 15 lbs of boost, depending which top pulley I run, and use an NGK #6061.
That series of plugs has a huge ground electrode and are designed to be used w/blowers.
The only problem that you may encounter with this particular series is that they use a 13/16th plug wrench, not the small 5/8th's that most use. I turned the outer dia. of the large wrench down and it just fits in the head to put the plug in.
In my opinion NGK is the only plug to use in this application.