Hello everyone, this is my first post as I am a new member to this site... I own a 18ft Donzi with a 350 that has a cracked block... Has a alpha SS 1.5 gearing. My father has a fresh 400 sbc that is .40 over that he has been saving for sometime now and would like to hand it over to me. Reason for saving this particular block is that its the 509 casting which is rare and stronger then the other 400 blocks.

As of now the lower end is all stock on a fresh rebuild but would like to change the heads from the old 76cc smog heads, buddy of mine has a set of World Products sportsman 2's 200cc intake and 64cc chamber size laying in his basement that he would sell to me for 400 bucks that have less then 1k miles on them. I have flat top pistons in the block now, so what pistons would I need to get to make this work on pump gas 91+ octane. And, what cam selection would you guys recommend? I would like a 5100-5500 rpm range for WOT.

If there is something I am missing please let me know, thought this would be a good father son project and a great learning experience and any of your guys help would be wonderful!

Thanks!!

Watch the compression. Don't know what that head would make it, but I wouldn't go over 9:1. 8.5:1 would be better.

As it stands now with the heads that are on it, the CR is 9.6... Kinda why Im not sure what pistons to get.

Lots of compression ratio calculators online. Go to tractor supply and get a 100cc syringe from the horse doctor area. Get a piece of clear plexiglass big enough to cover the piston hole and put a small hole in the plastic for your syringe. Seal the plastic to the block with some grease. Use some vegtable oil and fill her up (might need a vent hole to get the last bit of air out). Count the CCs used and plug them into the calculator. By varying head gasket thickness in the calculator you can come up with what you want. I'd keep the pistons and get thicker gaskets.
Check your heads the same way, then you'll know what they are for sure too.

Someone more educated can correct me if I'm wrong, but at 9.6 to 1 compression, I'd buy the cheapest pistons you can find, because you're going to be rebuilding soon! I'm just kidding, but 9.6 does sound high. Perhaps with that light of a boat, it could live, but that doesn't sound like a good combination for a 91 octane marine engine.

with the flat top piston you have now and the 76cc heads would be about max compression for that 408..if you drop down to the 65cc chamber cylinder head you will have around 11.5 to 1 compression and you will have to drill the cylinder heads for the steam holes that are between the cylinders on the block..just pick a nich mild cam like the isky stage one marine cam of a comp cam stage one xtreme cam and run it with the 76cc heads for now..f you need more later then i can tell you a few combos with stroling that engine and gettin 525 hp on pump gas..

So with the 76cc heads and the mild cam.... how much hp should i get out of the motor then? Just estimate for me.... 260? 300?

I can always go with some different heads other then the sportsman heads. Im looking for 350-400 hp.

Be sure to drill the heads for the steam vent holes. Match the pattern in the 400 head gasket. That was the main reason a lot of people had trouble with these engines when they started building them. Since the bores are Siamese, steam gets trapped below the head and the engine overheats even though it has the right water pump and thermostat, and everything else is perfect.

With a good set of aluminum heads (AF/ Brodix) proper cam, and decent exhaust 500hp and 500ft lb are achievable (on a dry dyno with no acessories)

400 is a walk in the park with the right parts.

UD

With aluminum, can I go with 64cc chamber size?