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GM 427 truck block questions

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Old 08-03-2012, 05:33 PM
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Default GM 427 truck block questions

There appears to be a few different casting #'s for GM truck blocks over the years, are there any specific ones better/thicker/stronger than others? I have seen mention of partially filling them, at what power/stroke does this become necessary? Would there be any advantage to running a long rod with a 3.76 or 4.0 stroke crank to keep the rod/stroke ratio better for a blower bottom end? This is all assuming installation in a twin engine 8000+lb. 35+ ft hull w/ speedmaster drives. Thanks
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Old 08-04-2012, 07:37 AM
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a 427 tall deck truck block, can be bored .100 over I believe, safely, where a normal 454 std deck mark iv, .060 is about the max for marine use.

Short stroke, long rod, will allow for some high RPM. For a torque producing, moderate RPM supercharged marine engine, Id stick with a 4 inch stroke, or even 4.25 if you need to buy the stuff new. If you use the tall deck, youll need the 6.535 or longer rod.

You need to be careful with using long rods in certain setups. Some times the long rod will put the pin to far up on the piston. Personally, I like using a basic setup, off the shelf stuff.

I just got done doing a pair of mark iv 468CI std deck engines this year. Dart heads, custom roller cam, B&M 420 Blower, 5.5psi of boost. Made 804HP at 6000 RPM.

If its strength you seek, and you wish to use the tall deck truck block, keep the bore size minimal, meaning dont overbore unless you have to, use a set of main studs, good rods and upgraded bolts, a quality crankshaft. Nothing wrong with using a 4.0 or 4.25 scat forged crank, or H beam rods with ARP 2000 bolts. If you were going to a 4.5 or larger stroke, then you want to go with a callies similar.

You can partially fill the block, for added strength, however, I dont think its necessary. For a drag racer, running 7000+rpm, big nitrous shots, and stuff like that, its a good idea. Not needed for what you're doing.

If your gonna use the truck block, bore notching is a good idea. Not very expensive, and helps unshroud the valves for some power/airflow with the smaller bore. Finding 30HP on a N/A setup not uncommon here. Dont go crazy on port sizes on the heads, since you wanna help the small engine make as much torque as possible. Same goes for cam choice. Erroring on the small side is better than being too much here.

Im no means a engine builder, just a weekend do-it-yourselfer. If you are too, and somewhat new to the marine game, just be sure to talk with good marine builders(lots here on oso) and stay away from the drag racers.
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Old 08-04-2012, 08:08 PM
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Thanks for the reply, I figured using a tall deck block would be a good way to run longer rods w/o moving the pin too high. Whatever overbore is necessary for cleanup is all I would do, and the only reason I asked about filling the block is that I saw it mentioned on this website. If there is a problem with main webbing stiffness with big stroke/power I'd like to know. Would it be pointles to rebuild them with a 3.76 stroke and run a little more boost, or should I just buy 4.25's? I figured a shorter stroke with longer rods and a better rod/stroke ratio would be more durable with a little more boost but I'm no engine builder either. I'd still like to know if there is a difference in the various 427 tall deck casting #'s too.
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Old 08-06-2012, 06:16 PM
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I've not seen positive results on filled blocks (partial or otherwise) on a boat. The motor generates too much heat with the long periods of max load for a partially blocked cooling passage to remove.

Drag racing - okay. Boat drag racing - okay again.
Boat - not okay.

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