Hot or Cold Plugs?
 

What type of heat range plug is better for marine?

IMO, I think it is the cold plug due to the constant load on the motor. Hot plugs would be for constant idle like found in a car with stop and go driving. I notice the ACDelco rates cold as #2 with the cross reference NGK is rated at #6.

You should run what the mfg of your engine specifies. If you have a custom build share the specifics and we can give you a good idea what to run.

Running the coolest plug that does not foul out at idle is what you want. On a basically stock engine, run the plugs recommended by the engine manufacturer.

Something kind of interesting about spark plugs that we found is on a paul pfaff blower motor with 1285 hp we kept going colder and colder, because a bosch racing tech from grand am racing gave us a big spiel about how plugs can be a source of detonation on performance engines so you want to go as cold as you can without fouling the plugs. In our test we went from an NGK 8 to a 9 to a 10 to finally a surface gap plug which he says technically it has no heat range so it the coldest plug there is, and what we saw on the dyno was zero power difference between the plugs. Then since it was an motec injected engine we set the idle mixture to a proper running mixture at idle and let it idle for 30 minutes simulating a pass through the harbor. After the long idle we then solidly hit the throttle with load on the dyno to act like getting on plane and listened for any misfire, and to our amazement none of the plugs fouled at all. Now I run the NGK 10 with no issue. They also said if you run to hot of a plug, parts of the plug will glow causing detonation. Get this, he also said the biggest cause for engine failure in nascar with plugs is over torquing the plugs because it separates the porcelain from the steel housing which is how the heat gets out of the plug into the cylinder head. They said this was such a big problem that they spent a ton of money on a plug tool that breaks away at the perfect torque so as not to overtighten the plug. They also said they run surface gap plugs almost exclusively in their nascar motors because they are so on the ragged edge. Thought all this was kind of cool.

I was talking to an old boat racer at JOMAR performance and he used to cut down the arm on his plugs so they wouldn't cause detonation. I buy the Autolite racing plugs with the shorter arm for the same reason. I have idled down a river for 45 minutes and didn't foul them, and my carb was running rich at the time. I am running the MSD 6M box with their new HVC II coil, which has some amazing current and fire time.