I am looking for opinions on spark plug manufacturers for the Mercury Racing 575 SCi. I am currently using an NGK R5673-8, but just had 2 of them fail at about 40 hours of use. I was expecting to get a little more time out of them. What are you all running in your 575's? Thank you for the feedback.

The plugs you are running are too cold. With NGK plugs, the higher the number, the colder it is. You should be running a 7 series plug. In you case, NGK R5673-7.
Eddie

Eddie Merc is still recommending R5673-8 thru out all 575 sci engine serial runs. Just wondering is plug fouling the issue as to why you are saying -7 instead of 8. Thank you for your time

Will post this as well we have seen a few bad plugs coming from NGK as of this year that have never been experiencing before. Not large amounts but one here and there.

Factory plug is the NGK R5673-8. Merc# is 33 - 8134214

That's all I have for info. Have done nothing with these motors other than some ported throttle bodies.

Edit in: BUP sent his post faster than me. LOL. Time for more coffee.

40 hrs.....that's great I have run both the NGK and MR41T plugs (same same) Recently I started using the AC R42TS plugs (1 range higher AC, 2 higher NGK) and it seems to be a lot better. However, I chickened out before a recent poker run and put some NGK-8s back in cause I knew I would be WOT all day

It may not be just the plugs, the stock tune startup enrichment is retarded, AFR from 8-9 until fully warm. The stock tune is rich all around. Also scan for intermittent engine coolant temp and MAP sensors issues. I have had one of each over the years, not enough for the code, just readings that didn't make sense. Both issues caused fouled plugs.

I hear you I don't feel like working on boats today but will have to and I don't drink coffee.

Great plug cross chart here http://www.dragstuff.com/techarticle...reference.html

I dont have 575s but im running NGK 8s in my blower motors with just shy of a hundred hours so far

The last set of 575 SCI we did I used the R5674-8, it's a colder plug; but the center electrode is extended, instead of shrouded. Being the piston is inverted (dished) you can expose the spark into the chamber. I would worry more about the head gaskets burning out from heat, than running a colder colder plug. If you are fowling plugs at 40 hours, you need to start looking elsewhere. If being 2 plugs, better check ignition wires, sounds like a lead issue, unless you are seeing oil on spark plugs.

I've had much better luck with the 7's since those engines are so rich. As long as you are running the recommended 91 octane or better, you will have no issue at all with the 7's. It will run cleaner and is less likely to foul plugs at idle with the colder plugs. Now this only holds true for stock engines. In 4Bus's instance, he's changed pulleys (I believe), so it all depends on the boost he is running as to which plugs I would run.

As Mark (MER) mentioned, you may be in need of an ignition system tune up. If the plug wires are more than 3-4 yrs old, change them out. Same for the cap and rotor.
Eddie