I am ready to fit up the last pieces of these 2: SBC's going in my 255 Formula Liberator.

I haven'y fooled around with marine spark plugs for a while so I'm looking for guidance.

The engines are 355 Vortecs running about 10:1, Comp Extreme Rollers with 1.5 needle bearing rockers, Davis Unified Ignition optimized to 32* total advance at 3200. The intakes are marine versions of the Performer Air Gap, exhaust is stock late model OEM Mercruiser. Carbs will be Holley DP's 700 CFM.

So theses days are we still AC 43RTS or something else? I am thinking .045 gap with the DUI which is a tricked out HEI.

Thanks in advance.

I would probably go down in heat range to an AC 41 or 42

With that much compression I'd cross it to the Autolite racing plugs with the short arm. It's less of a detonation risk.

with 10:1 and pump gas ( what octane are you going to run ) i would think the 43 or 42 would be good. I have 455 buick gs with 10.25:1 compression running 32 degrees timing in at about 2400rpms and limited to 32 degrees the rest of the way i run r45tsx but i run 100 octane with now issues few thing to keep in mind with the a/c plugs the lettle M gets you maring plugs MR43ts standard maring plug same as a R43ts exccept the threaded part on the marine is stainless steel to help it not rust. The x at the end gets you extra gap R45ts and R45tsx same heat range just the gap is closer to .050 on the x compaired to .032 on the regular plugs this is nice when running larger gap so the electrode is not at some silly bent angle. the autolite racing plugs do have a nice cut back electrode but very smal gap like .030 and with the short electrode its almost impossable to open the gap up, i run these on my 464 buick motor with almost 12:1 with the same 100 octane av gas part number auto light racing 33 works well but i run .030 gap on those now i dont know if you canfind or if they make an MR43 or 43 TSX but that would give you the best of both worlds if they do if not you would have to figure out which is more important to you the extra gap, or the rust protection of the stainless. the problem with a very streched electode that does not sit very flat( going uphill after the gap is opened up ) is where ever the closest part to the center is is where the spark will occur.....if its nice and flat you will get a even flam start as the plug wears and get the spark coming out towards the bottom like it should not the side. When the flam starts off to side it could cause burning faster on side of cylinder than other and once the flam on the one side is out, the still buring flam with have extra room to expand towrds and you will loose pushing power on top the piston or could even cause a strage type of reverburation that is similar to detation but not the same....but similar bad effects

with 10:1 and pump gas ( what octane are you going to run )

I plan on 93 GASOLINE ...NO ETHANOL. I have a source with in 15 miles of here with 100% gasoline at $4 (today). I have about 50 gallons of that in tank now with Stabil so it should be OK.

I just hope I dont have to use it all next week for generator gas if we get Issac.

Ac delco mr43lts

Treat it like a car engine, I have a stock 5.7Gi from volvo and i swapped out the NGK for bosch fusion of the same heat with no issues. I also upgraded the ignition wires but not the coil yet.

Japanese line to use small numbers for hot plugs and large numbers for cold, Bosch do the opposite, they like high numbers for hot and low for cold...

My plugs are bosch 9 or NGK 5 on the heat range...

I found the followin useful

upto 400hp Bosch 9 or ngk5
upto 500hp Bosch 8 or NGK 5.5
Upto 600hp No Bosch but NGK 6
upto 700hp Bosch 5 or NGK 7

A lot depends on carbon build up and knock, if you notice your plugs are filthy black after a cruise in low rpms go to a hotter plug. Dont let the exception dictate the rule.

I use to seldomly hit the high rpms and chose a temp to accomedate that, i had misfires, excessive black smoke and all around issues. I went to a hotter plug made more power in the mid range and sacrificed a little on top, but no more misfires, cleaner burn and less issues all around...

Do your research properly and you will get a good plug for your application