Spark Plug Recommendarions
 

Alright, I read spark plug threads until about an hour after I should have been in bed last night.....

Chevy 502's.
Weiand 177's
Stock Chevy Heads
Stock JE700 pistons
Marine Kinetics Cams - 235int and 241exh - .646int lift and .629exh lift
Holley 850's. I believe they were 92 and 86 jets with a 6.5 PV
I believe we settled on 9.1 Compression
I always run 93 octane
I run the V-6 module so my base timing is in the mid-20's at idle.

How I use: idle a LOT (probably 60-70%). Rest of the time is spent at a casual cruise (36-3800 rpm). Would be unlikely the boat sees 4500rpm all summer; if so maybe once or twice for a moment or two.

I ran NGK5673-8 last year.

I guess my real question having boned up last night is should I be running the 5671's, which appear to be 1/8 longer, or should I stick with the 5673's, and should I possibly run the slightly hotter "-7" plug based on my extensive idling and propensity to drive like an old woman.....

Any thoughts?

The 5671 is more than a 1/4 inch longer than the .460 reach 5673. .750 minus .460 is .290. Not sure where you got "1/8" inch longer, but that is incorrect info.

Your stock heads do not take a .750 reach gasketed plug like the 5671. That would be an incorrect plug, and the plug would greatly protrude into the chamber, surely causing a problem. The stock iron head calls for a .460 reach tapered seat plug.

9:1, iron heads, that tiny blower on a 502, I would be very careful going to a hotter plug than the -8 NGK.

if you have unmodified stock heads&pistons the compression is 8.750-1.

you can run the -7's.

What size pulleys are on your blowers? And You say your not going to ever take the motors past 4500 rpm? What pitch props you running? Did you prop the boat so it can't go past 4500, if you do that it will kill your motors.

For reference, the 525SC mercury, and 600sc mercury, and 800sc mercury, ran a MR41T spark plug.

The NGK "-8" heat range, would be comparable to that plug.

In my opinion, if you are fouling "-8" NGK plugs, its not the plugs fault.

Just because you arent' running "hard", doesnt mean there isnt alot of combustion heat, even at only 3500-4000rpm. Especially in a 35 Cig. Throw an EGT gauge on it, and I'll bet you're seeing 1300+ degrees at cruise.

Have you done any plug readings? Is there any specific reason you want to move to a hotter plug? Are you running an intercooler?

http://www.ngksparkplugs.com/about-n...rk-plug-basics

No, no reason I necessarily WANT to run the hotter plug, I was just concerned as to the way in which I use the boat that it might behave slightly better with a hotter range.

The plugs were fouled last year but I believe that was from blown power valves so I unfortunately don't really have a good baseline.

Upper pulley is 3.5, lower pulley is 7.

No, it is not propped for for 4500. I recall that Bob and I settled on 5,400 rpm when he cut the cams. I haven't played with props yet and am just running the 26B1's I had last year on it.....

You wont see any power gains, fuel mileage, or much of anything going to a -7 plug.

How many rpms can you turn those 26 bravos ?

I think Mild Thunder answered you question above, and I agree, as I would NOT go hotter then an 8. I would add that, I really think you would benefit by boost referencing the carbs.

Why idle around all day and drive like grandma? Maybe forget the plugs and put a sail on that thing :D