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Discussion Starter · #1 ·
Just wondering what heat range plugs people are running in there N/A 540's ?
I've got a iron headed 8.75:1 540, mild solid roller, it's maybe making 600hp. I run the timing at 34deg.
I just got it ready for the season, set the lash , cap and rotor , timed it and changed the plugs. I must have had a brain fart or something , because I bought a set of MR43T's and took out some MR42T's.
I could have sworn I always ran the 43's. What does everyone use ?

Thanks
 

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Only way to really know is to read the plugs. Being too cold is safe, but could be a hassle with plug fouling. Being too hot will make for clean plugs, but could be bad if they don't shed enough heat and the motor gets into detonation. Check the NGK website for a Merc 540 Bulldog, then drop down one in heat range colder and you should be pretty close. I am assuming since you are running a solid roller you also have a decent suporting cast of components which will put you a decent amount above the Bulldog, likely making it a good idea to run slightly colder plugs.
 
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