murfman

I recently purchased a 27' Magnum sedan powered by twin Small block Chryslers I am a die hard Mopar guy so its right up my alley, but I have a question that should pertain to all small block boats. I have 2 360 Magnum (Magnum Mopars in a Magnum Marine that just has to be right!) motors from 1999 Dodge Rams that I'd like to build for the boat with Edelbrock Performer RPM intakes, Edelbrock Marine carbs and Hi Tek exhaust manifolds and some 270 degree cams. I am also thinking about Scat 4" stroker cranks, and this is my question, are the cast cranks up to marine service? Or should I just go with Forged cranks? Obviously I'm stretching the budget with the rebuilds, and I'd like to keep some money in reserve to raise the X dimension, rebuild the Volvo 270 drives, and add Trim and external steering. Don't most of the Mercruiser chevy small blocks run cast cranks?

murfman

Oh' I'm shooting for 400 HP per motor, RHS/Indy make nice cast Iron heads for the Magnums now, or I could go with a set of Eddy aluminun ones. Either way, i'm adding a closed cooling system to the boat as well.

ThisIsLivin

2 years from now you will want 500hp and will be mad at yourself for not getting the forged crank. Trust me I know, I had a good forged crank with a 4" stroke and now I wished I had put in a 4.25". Top end stuff can be done in the boat, bottom end can't. Spend the money on the bottom end now and do the top end in stages.

vintage chromoly

iTrader: (3)

In my opinion, the constant loading and unloading that a marine engine sees warrants a forged crank.

oodcillasan

Top end stuff can be done in the boat, bottom end can't. Spend the money on the bottom end now and do the top end in stages.

murfman

Yeah that's what I've been thinking.

tpabayflyer

Mopar guy here as well but I am starting to get some knowledge on the BBC after 2 builds now! I built a 318/390 with a cast 4"crank and ported magnum heads and it was awesome! smoothest motor I have ever built! I did use the cast crank but I did use forged pistons and scat H-beam rods..... About 450 HP and ran great........ But marine engines are under constant load/unload conditions and I am of the thinking that a forged bottom end might be the safe way to go if you plan on making some more power in the future..... my wellcraft cruiser has a 7.4 MPI with a cast crank and forged pistons and they all came with cast cranks and 310HP stock. I may have 400HP now and 50 hours on the build and its A-OK..... thousands of them running around with cast cranks and you don't hear of them failing hardly ever.....

Back to mopar, I have heard of the small block cast 4" cranks failing but it was almost always in motors making way over 500HP and revving to 7,000rpm...... My gut tells me you would be just fine with a cast 4" crank 408cube motor making 450/500ft/lbs torque. I would definitely use some forged pistons and H-beam rods with good ARP bolts. I would also definitely use a wide band to tune your carbs and keep timing on the safe side...... I assume you will be using the stock style hydraulic roller cams but don't go too crazy with duration...... You should be able to make an easy 450HP with a pretty mild and dock friendly cam.... With 900HP on tap in an older 27ft boat sounds like a handful to me!! You should be able to surprise a lot of folks when you put the hammer down!

Ina slightly off topic rant that is mopar related, I wonder why the Gen 2 Hemi's were never used in our performance boats I will never know? Deep skirted, cross bolted mains with big valve heads are a proven recipe for power. Mild 528 motors make 640HP easily and 900HP 572's NA.....

murfman

They made them, but I've never seen one, I thought I read somewhere that one of the old Bertram race boats had hemi power at one time



Or this one:

murfman

THe race Hemi above was from 1966 and the 6 pack was from 1970

murfman

Back to the cranks I was thinking forged all along, but most of the Marine performance "Crate" small block chevy motors come with cast cranks. As for the cam I was thinking a Com cams HR264 cam 264HR 264 274 .512 .512 112°