ken sampson

iTrader: (1)

I am going have motors built and had 3 quotes all with diffrent cranks. I do not know enough in the area. They will be merlin alum blocks with brodix alum heads closed cooling system. The HP goal is about 700 to 750 with 9:1 compression so I can run on any fuel. Can anyone help with the crank selection? The stroke is 4-3/4"

campionchase910

I'd go with a Scat crank, give them a call.http://www.scatcrankshafts.com/Scat_Crank_Anatomy.asp

GPM

You should be asking someone like Eddie Young, what deck height, what heads, 4.75 doesn't leave much for rod length and piston in a 10.2 block.

Boat1

The heads you have selected have a large intake valve therefore they want a large bore to minimize the shrouding effect of the cylinder bore on air flow. I would use a 4.625 stroke crankshaft to increase the rod length, and this allows a larger bore. I like Callies crankshafts and Oliver billet rods.

Young Performance

Thanks everyone for the vote of confidence.

Agreed on the larger bore. However, I would stick with a 4.5" crank. This will allow you to run a 6.7" rod and a 1.25" comp. height piston. I don't like running a 4.625 or 4.75" crank in a 10.2 block because of the steep rod angle. At least with the 4.625" crank you can run a 6.635" rod with a 1.25 piston. With a 4.75" stroke crank, you have to run a 6.535" rod.
I would go with at least a 4.560" bore, preferably a 4.600" bore. This will go a long way in unshrouding the valves and helping them flow. This will leave plenty enough meat for future rebuilds since you can go all the way to 4.625" bore.
As for parts, I use a Lunati Pro Series crank and Oliver billet rods. They are all but indestructable. If you are looking to save a few dollars, you could use a Howards crank. They make a really nice piece that I have used to well over 1000 hp. I would not use the rods right now. They are having some issues with them.
There is nothing wrong with Scat cranks in a 4.25" stroke. I have used them many times. However, I don't use them any larger than that. When I go to 4.5" stroke (which is the vast majority of what I build) I use either a Howard or Lunati. I don't trust the Scat in a 4.5" stroke. It has nothing to do with power since I have used them to 1000 hp. I just don't like them in that large stroke.
Give me a shout if I can help. Good luck.
Eddie

campionchase910

Eddie you surely are a 1st class act and everyone here certainly appreciates your knowledge and expertise. Thanks for sharing.

Young Performance

Well thank you very much. It's my pleasure to help whenever I can.
Eddie

ken sampson

iTrader: (1)

My only concern about a bore of 4.600 is that is the largest Merlin recomends on there Alum block. This leaves a wall thickness of 0.65 Thats like only a 1/16" thick. seems thin to me.
I guess the 4.560 is safer with a 4.625 stroke to keep my cubic inches. Any last thoughts.

Skatermac

Sonny Bryant Cranks has no equal very pricey, but with lack of a better word perfect in fit and finish. I agreed with Eddie on the rod angle getting it to steep will cause undo stress. 302 427 and 557ci motors have good combos and rev smoothly. My 557s uses 4.375 stroke. Which ever stroke you decide on choose a rod length matches a known rod angle combo that will fit in the block with your deck height. custom pistons will be needed and cam grind. Paying attention to these small details make all the difference in the performance of the finished product.

ezstriper

any stroke of 4.5 or longer you better use a good crank....race with several guys who have broken cranks...not pretty after that...callies is a good one for the $$