NGK vs AC Delco

[RoccoP]

I have a 2003 Mercruiser 350 MPI and was looking to replace my spark plugs after around 200 hours. The current plugs are AC 41-933 so I thought I would try NGK's which I have used in my previous boats. I ordered a set of NGK ITR4A15 5599 installed them and took it out for a run. The engine would not go over 3000 rpm's with full throttle. Removed the NGK and put old AC's back in runs perfect. I have never seen anything like this before, the engine sounded like it was starving of fuel. Just a heads up.

 

[greno]

That is pretty extreme. Haven't seen plugs make that much of a difference.
The NGK must be super cold plugs.
Either that or you had a couple plug wires switched.

 

[Alumarine]

I looked at a couple of cross references that show the NGK plugs to be a stock no. 5146 or part no. PTR4B-15 for the AC 41-933's.

 

[scoflaw]

Appears that is not an iridium like the 933

 

[Alumarine]

I think the 933's and 5146's are platinum and the 5599's are iridium.