260Velocity

To some of the more experienced Marine engine builders here on OSO. I am looking at building N/A BBC instead of going with the 250 supercharged with 2 holley's that i had on the 509 in the past.. So my goal in to make my 260 Velocity run 85 -90MPH. I was told the hull should go 85 MPH with 650 HP, not sure how true that is, but OK..

I am looking at stroking the 509 which is a Mark IV Bowtie block to either 548 or go to a 572 and make very reliable power to push the boat to my goal and have good dock manners. Not too large of a stick in it. The current stick is a Crane 226/236 @ .050 and .587/.610 cam, this is the mild normal cam for the marine engine from what i have seen.

The 572 worries me with the rod ratio on a short deck, but i dont build these everyday like some of you do.. Next would be a Compression ratio to run, ?
I currently have 320 Pro Topline cast iron heads that have been all redone and ready to go. The Bowls and CC have been blended and that is it for port work. I have heard to only go with 9:5-1 (tops) with cast iron heads and marine use, is this the case? Can i make the HP with that comp? or do i need to go to an Aluminum head and up it to 10:5-1 comp?

So if i can get a good 725 to 750 then that should put me where i need to be, i think

Any advise would be appreciated...

Black Baja

iTrader: (2)

There is nothing wrong with a 572 short deck. Withe that cubic inch and 9.5:1 compression I don't see any problems with your 750hp goal using the pro top line heads. You definately going to need more cam than what you have posted.

articfriends

iTrader: (1)

Do you already own connecting rods u plan on reusing?

260Velocity

Thanks for the replies, i have a stock forged bottom end with JE blower pistons i will be trying to sell, it maybe has 30 hrs on it.. the rods are dimple 7/16 with ARP bolts. i am currently looking at a rotating assembly from Lunati with center counterweights. Vodoo Series Crank and rods w/2000ARP's. the pistons i am trying to figure out yet. Thinking of a 4.375 stroke and then i assume (lol) i should use the 6.535 rods to get a better rod ratio out of it. i will be low on my CH though. whats everyone's thoughts?

Black Baja

iTrader: (2)

You need a 6.385 Rod. I wouldn't worry about the ratio. That's old hot rodder mentality and is worth about nothing. Going from a 4.250 crank to a 4.375 crank is 1/8" difference. Which is nothing. Since it's an N/A deal you don't have to buy some crazy expensive crank and rods. Callie's Compstar would be more than sufficient for what you are doing and then some. Eagle and Scat will do but honestly in the long run I think you end up with more into the rotating assembly because of the extra machine work to make it right. The Callie's stuff is on the money.

vintage chromoly

iTrader: (3)

Check out CP for your pistons.
They have a "sportsman" line of pistons called the "bullet series" that are nice.
They retail at just north of 800 bucks and come with the rings.
Another nice feature is that you can get a .200 wall pin at no up charge.

endeavor1

iTrader: (3)

Curious how tight you want to turn a short deck with that much arm in it.

260Velocity

I looked at the bullet series from CP but they are limited to bore and stroke combo's. The 4.375 I guess is a custom piston combo.... It seems many of them have very little selection for the 4.375 arm so I will have to call and see what is available.
As for spinning the 4.375 arm. I don't plan to spin more than 5500 max. It is a pleasure / play boat. Not at all a race boat...
What is a good max RPM for the 4.375 in a marine app. I have a friend running a 4.5 arm in a short deck making 1135 hp N/A. It's an ultra street Malibu, but it only runs 5 seconds at a time. Lol.

Black Baja

iTrader: (2)

6300 rpm stud rockers. 7200 shaft rockers. Piston speed is not your issue with a low deck 572. If you start going over 6500 rpm the oiling system will need addressed.

GPM

I'm not an engine builder, but I am curious, are you running a 509 with a 250 blower now ?