persuader

Im new to powerboating ( all other boats owned were ski boats, inboards) and this is my first build and I was wondering what some of the motor experts think of this motor set up im doing.
First off I have a 23' checkmate persuader with a stock 300 horse merc 7.4.
I put a gps on it and got theese speeds.

first run : 62 and some change on gps, with a 23 p quicksilver cleaver.

second run : 66.5 in some chop with a 23p @4600 mirage plus and it held 65 nicely in calm water

Third run:68.5 in some chop, trued up the hull (bluprinted) and added a set of gill magnum choice manafolds and was running a 25p mirage plus @ 4200 and the boat held 66.5-67 in calm water.

the boat takes off from a dead stop to on plane in just a few seconds with lots of bottom end.
After I trued up the hull ( I own a fiberglass and gelcoat repair business) I used less trim. Before there was a rooster tail that was about 60'. after the work on the hull the roster tail was gone and the boat responded much better at higher speeds and felt like it was riding higher in the water.

We pulled the motor last week and tore it apart. I sent the block and parts to the machine shop to be balanced and have the bottom end put back together using the stock crank and rods, only replacing the pistons with new(flat top). The block is getting cleaned, honed, decked.
I am using the following parts for the rebuild:

1968 large oval port 396 closed chamber heads (to up the compression) There getting a valve job and resurfaced.

Cam- crane roller 553/576 214/222

hydrolic roler lifters (retro)

SPRINGS - Outer Diameter 1.540
Inner Diameter 1 1.140
Inner Diameter 2 0.760
Damper YES
Seat Pressure @ Installed Height 150 lbs @ 1.900
Open Pressure and Height 560 lbs @ 1.150
Coil Bind 1.085
Maximum Net Lift w/.060" Clearance 0.755
Average Spring Rate 528 lbs/in

Holly 750 marine carb

Edelbrock rpm air gap dual plane intake

crane roller rockers

crane push rods (13628)

Does this sound like a nice setup for a boat? What RPM's should I run to be safe? and what kind of power should I expect?
I got most of my info for the build from a local high dollar shop. the owner built a motor for his old boat using simular parts, he said it was dinoed around 500hp. Im not sure if this sounds right but what do you guys think??
Thanks for your help
Chris

Vinny P

It sounds to me like you did some homework on this one. But, I have a couple of questions. What is the overall valve overlap and lobe seperation with that cam? Is the stock bottom end ( crank and rods ) forged or cast? I think cast.

I would do some porting work in those heads to get them to flow real nice.

persuader

here is the info on the cam
Part Number: 139721 Grind Number: HR-276-2S-10 IG
Engine Identification:
Start Yr. End Yr. Make Cyl Description
1967 Up CHEVROLET 8 GOOD IDLE, DAILY USAGE, OFF ROAD, TOWING, PERFORMANCE AND FUEL EFFICIENCY, 2600-3000 CRUISE RPM, 8.75 TO 10.5 COMP.RATIO ADVISED. BASIC RPM 2000-5500
Engine Size Configuration
396-454 C.I. V

Valve Setting: Intake .000 Exhaust .000 HOT

Lift: Intake @Cam 325 @Valve 553 All Lifts are based
on zero lash and theoretical rocker arm ratios.
Exhaust @ Cam 339 @Valve 576
Rocker Arm Ratio 1.70

Cam Timing: TAPPET @.004
Lift: Opens Closes ADV Duration
Intake 29.0 BTDC 67.0 ABDC 276.0 °
Exhaust 73.0 BBDC 31.0 ATDC 284.0 °

Spring Requirements: Triple Dual Outer Inner
Part Number 99896
Loads Closed 132 LBS @ 1.906 or 1 29/32
Open 395 LBS @ 1.346
Recommended RPM range with matching components
Minimum RPM 2000
Maximum RPM 5500
Valve Float 6500

Cam Timing: TAPPET @.050
Lift: Opens Closes Max Lift Duration
Intake 2.0 BTDC 32.0 ABDC 105 214.0 °
Exhaust 46.0 BBDC (4.0) BTDC 115 222.0 °

and the bottom end is cast, I wanted to change to forged but found from most of the people I talked with that i would be ok with the cast parts(thats why im getting it balance). and I would have liked to get some work done on the heads but for the cost compaired to the overall gains that the machine shop told me i would get, i didnt see the need for it.

Dave F

Ovals are definatly the way to go for your engine. A 214/222 and guessing a 112 or 114 lsa. you're probably in the neighborhood of 0-4* overlap on that cam @ .050. If you have stock Merc exhaust DO NOT go with more than that.
If you're putting cast pistons in you'll be fine, although the forged is preffered, just don't put a blower on later with that set up.

My opinion, no need to go above 5200 rpm. You should be somewhere in the neighborhood of 400 hp.

If the heads are 1968 becareful of head gasket to the Gen 5/6 block. Valve work is a great idea but don't forget about the exhaust ports.

DAVE

Dave F

well thats what I get for guessing. You have a 110 lsa and -2 overlap.

persuader

Im using gill magnum manafolds, and i dont have any plans to supercharge it.. 400 hp sounds good to me, as long as i can be in the mid 70's(thats where i want to be).
I wouldnt mind doing some head work but i just paid for the boat. I was only thinking of putting a cam in it and now this im trying to keep the build under 3k.
thanks

thanks.

persuader

Anyone else?

aex

I remember the closed chamber heads shrouded the valves and hurt flow . Don't spend big bucks rebuilding those heads .