454 .030 over 460ci
RHS Iron heads
119cc chambers
320cc intake runners.
Intake valve 2.25"
Exhaust valve 1.88"
Bore 4.280 (.030 over)
Stroke 4.00"
Compression ratio 9.5:1
High rise dual plane intake manifold
750cfm Edelbrock spread bore carb
Gil aluminum exhaust manifolds w/ stainless risers (exiting down through the prop)
1.7 rocker ratio (roller)
Hydraulic roller cam

Need to know which cam to chose. Thinking either 168791 or 168731 Crane.
I am open to cam choice suggestions. I am not greatly experienced with proper marine combos.
Here is a link to the Crane specs
http://www.cranecams.com/140-143.pdf

Game plan is to develop 425-450 hp at crankshaft
Top RPM target 5300-5400
Cruise RPM target 3200-3600

I would take it to Facebook/Passion for powerboats, etc. This place is a ghost town.

Do you have these Gil risers??? As far as I know, Gil never made a riser like you describe.

It should easily make over 450 hp on the dyno with headers. It would be lucky to break 400 hp going thru prop.
IMO CR is a bit too high and carb is a bit small.

Think of your engine as funnel. No matter what you pour in, only so much will go out.

There are 1000's of engine build threads on OSO and the first step is always to have good exhaust.

The risers say mercury racing on them. Don't know a part number though. They are polished stainless steel

Merc Racing risers are usually Gil.
Where are your pistons in the holes? Zero deck? -.020"? +.010"?
Are they flat-tops or dished or pop ups?
You say 9.5:1, but that depends on everything from valve relief depth to head gasket fire ring diameter.
Need to know where the pistons are at TDC AND the cc rating of the piston (+ for dish, - for dome).
Then we need to establish whether you are running a "squish" motor (with less than .055" between the piston and the flat part of the close chamber head).
Are your bores notched? With big valves and a .600" cam, they need to be, else you choke flow off on the ouside of the valve curtain.

A 731 will deliver your 425-450hp that you are looking for, but I agree the carb choice is smallish.

Thanks for the response. I didn't buy the pistons yet so I can't provide that info. Didn't chose them yet either. Just thinking to use 9.5:1 as a goal ratio.
I do have a very nice set of flat tops that have only two fly cut reliefs (not 4).
Bores are not notched. Something to consider?
btw now that a dozen people have told me to go through hull I think I'm gunna take that route and get a silent choice system.
Also the flat tops I have do not go past deck height.
Probably more than .055" between piston top and head with a gasket in place.
Maybe .020 below the deck.
The valves do have pockets but it is my understanding that it's 119cc with that taken in to consideration.
I don't plan to prop the boat past 5300 rpm. For that reason I was thinking the 750 cfm was enough.
Is that incorrect?
Hey thanks again.
Kurt

Definitely have the block zero-decked with whatever pistons you choose (piston compression height same as cylinder deck height).
This will allow you to use the inexpensive .039" FelPro Permatorque gasket and end up with .039" squish height which will allow you to run more compression and still be safe from detonation. Once you go past .060 piston to head, you lose all of the turbulence benefits of having a squish motor. The result is that you either have to run overly conservative ignition timing, higher octane, or both. Compression ratio is what makes power, but detonation kills power and hurts parts. You want to run as much compression as your fuel octane and motor combo will tolerate at optimal ignition advance. If you have to pull a bunch of timing out, then it will run poorly. Running a squish motor can allow you more than a full number of compression vs a non squish (if 8.9:1 is all you can run with .100 piston to head, then you will be able to run 9.9:1 with .037" clearance).
A 731 cam bleeds off a lot more compression than something like a 454 magnum cam would, so if that's your cam choice, you can probably run 9.75:1 and still run pump gas and normal timing curve.

Combustion chamber cc never includes valve reliefs or factors in head gasket thickness or piston deck position. If you have 119cc heads, then that is normally just the heads.
Figuring compression ratio isn't difficult, but it requires that you know the bore, stroke, combustion chamber cc, headgasket thickness, headgasket fire ring diameter, piston compression height to deck measurement, and the piston relative volume (+ or - cc).
Changing any one of these variables has an effect on the CR (some not much, some a lot).

Bore notching is good for 5-15 hp on a 454 with big valves. Ask your machinist about it.
Also, when mocking up your motor with pistons to check the deck ht, be sure to do it with all 8 installed and caps torqued (you can use old bearings). There is usually some variation front to back and side to side. If you are running reconditioned rods, the lengths may vary a few thousandths rod to rod. New rods are typically the same. Your machininst should square the decking cutter to the main bearing bore, but it is not unusual to have to take more off one side than the other to average out the deck measurements.

Damn. Sounds like you've done this before. Lol
Thank you for the info. I'll take all of it in to account.