Ok a little history. I have been restoring a 88 Sunsation Rocket it had when I bought it a blown up 330hp 7.4 7 of the 8 rods were seized. so I had the motor work done and rebuilt the motor. it's now a .030 over 9.5 to 1 compression what 454. it had the peanut port heads still so I removed them and replaced with a set of 049 heads. the cast factory intake came off and a Edelbrock performer 2.0 went on I had a cam laying around and old erson cam. can't remember the specs to much but remember looking it up and it was real close to the one I have in my other big block Camaro which is a 550 lift 292 duration on a 110 lobe sep if I remember this one is a 110 lobe sep and alittle smaller like 486 lift and 287 duration but like I said can't remember to much about it . the boat had a 650 holley on it merlin marine swapped it out for a 822 holley and cmi split tube headers. the prop is a beat up mirage plus 25 pitch which is out getting reworked as we speak. once I got it all back together the max rpm I can get out of it is 4000 that's full wot checked at the carb. I have read on this forum about the performer intake falling off at 4000 rpm. I don't think it's an ignition issue since the merc setup was replaced with a dui HEI ignition. so what I have learned in the few hrs of driving the boat is on gps I hit 57 mph at 4000 rpm trimmed just shy of rooster tail and the boat was still running wet. could a messed up prop cause that much of an issue? my thinking is I'm not getting bow to lift so the boat is wetter which is more drag on the boat making the motor work harder to spin the prop limiting the rpm. so really theres 3 questions
intake limiting the motor?
some issue with the prop ?
cam selection could be limiting the torque to spin prop past 4000?

sound about right ?

here is a video of it running WOT
https://www.youtube.com/watch?v=35BvVmy8T1E

https://www.youtube.com/watch?v=z1dR1T9JuDc

I know something isn't right cause here is the boat 2 owners ago on lake St Clair in Michigan. this is with from what I can gather a factory 330 the cmi headers and his for sale add said it had a 750 holley on it . same prop too.

https://www.youtube.com/watch?v=_JfB791ttIc

this one you can hear the cam

https://www.youtube.com/watch?v=YkKti9wS-6o

looking out the back

https://www.youtube.com/watch?v=oZnUpWIjq0o sounds like more than 4000rpm to me but tach said 4000 gps said 57mph.

any help is appreciated

Put a smaller prop on it. The motor is running out of power at 4000. This is where hp and torque are crossing. You need less prop if you want more rpm.

^^^^Yup^^^^

Also, no cam info, actual comp ratio vs what piston man states, no installed spring psi info, no valve size info along with any bowl/porting work, if fuel psi checked, carb tuned for engine.....etc,etc is GIGO for info.

Your engine probably isnt making much, if any more power than a stock 330 merc.

Check ignition timing at full advance. You need 30° to 32°.

as stated I gave the cam specs to the best of my memory. the heads are factory 049 castings nothing done to them. the carb was jetted to the motor by Merlin marine and he also at the time checked the timing and said It was dead on what it should be . I found the doc that had the cam specs
542 lift
228@50
296 duration
111 lobe sep
107 intake center
2200-5500

with all of this said propping down isn't the plan the boat use to spin the same prop for the past 28 years of it's life I don't feel propping down is a solution if anything I want to prop up factory props were quicksilver 26 pitch then later they went to the mirage+ 25 pitch.

the part of this story you have to understand is I use to work at Sunsation I built this boat new and like I said 26 pitch props came factory.

That 25p M+ is a huge prop for what it is. I've heard a lot of folks talking about how hard it is to get to spin up, and if your new motor makes more power but further up the power and it might be struggling to get up to more power. That's only food for thought.

Did you check fuel pressure at wot? Is the fuel line big enough to handle bigger power? Also those DUI ignitions need a minimum of a 12g wire going to it. I had a top speed issue with my boat and that was one of the culprits.

the only 3 variables I'm not sure about is would a prop scrub off 10mph being beat up . the cam I have isn't a marine cam are the specs to far out for marine use and the performer 2.0 intake seems a lot of folks have had issues in boats with that intake running out of air at 4000. my prop will be finished this afternoon. so I should be able to get some more info this weekend. I do have the old cam that came out of the motor I can always put it back in also .

I have been building BBC for many years I have 3 right now one being an 1970 ls6 if closed chamber heads would work good in a boat I would put it in the boat I have so many extra parts laying around it funny. for the record this motor should be atleast 400 hp I would guess to say closer to 425hp.



my ls6 in the Camaro




the heads and intake on the boat now came off of this motor it's a 2 bolt

used the factory wires for the distributor hmm I wonder.it has a cheap fuel pressure gauge that was on it when I got the boat If I remember it does drop when we were tuning it upbut don't remember how much . it has the stock block side fuel pump . you can see it in the pics I think.

It seems the only known variable is your GPS speed of 57 MPH.
As Scott has pointed out - GIGO - garbage in/garbage out.
Your speed equation has too many unknown variables.

Are you using the analog tachometer from 1988 also?
I installed a digital tach from Aetna Engineering and never looked back.
http://www.aetnaengineering.com/mode...cd-tachometer/

Yes a chewed up prop will slow you down. They will increase prop slip greatly.

Start doing your speed runs in an analytical way or you are wasting your time.