Offshoreonly.com - Prop, carb, rpm questions..

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Prop, carb, rpm questions..

Long story short, I pulled my 454 mpi and swapped to a merc n/a 575hp 540.

What I had

prop- labbed 24p solas 3 blade
454 mpi 310hp
GPS- 66.2
RPM- 4300

What I now Have

prop- 28p 4 blade
540ci 575hp
GPS-68.4
RPM-4500
range- 4800-5200rpm

The boat still has a ton of bottom end. It comes out of the hole hard. I think i need a 30p. I think a big part of my problem is that the motor came with a quadrajet on it for some reason. will a bigger carb make that big of a difference? I expected to see very close to 80 if not faster. Boat is a 01 sunsation aggressor 25ft.

By the way I tried a 26p and the 28p with the same water conditions today and the boat was no faster top end and the RPMs were the same. I did have one more person with me... Anyone have any ideas?

I think max rpm on a 540cu in/575hp should be closer to the 5800 mark. That being said. Try the biggest prop you can get ahold of. When it won't let you reach peak rpm deduct 200-300 rpm for each step in pitch needed to reach max rpm. In other words if 32P prop will max you out at WOT at 5000rpm and max rpm is 5400 you need to go back 2 steps to a 30P. or reverse it for the opposite. If your maxing out rpms at 5400 and you still have more throttle to go then you need to go up in pitch. That Chester might flow 650-700cfm. Trash it. I would say an 850cfm Holley might do it. Without knowing more about your engines. I'm no expert but a 3 blade 30P might work better maybe even a 32P The 4 blade might be knocking off a couple mph top end. Watch the hole shots, you'll be grenading the drive next. More info is helpful. We don't mind dooing the reading and guys can give more accurate answers and advice with alot of info. Maybe I helped? Others will chime in and correct my bad info.

First things first.......was the engine put on the dyno to absolutely verify the hp or was this number just thrown out there? You are way off on something. Either the engine is not making the power, or something is terribly wrong with the hull. I would be surprised if the engine made 575 hp with a Roachcather. Those carbs can do it, but you need to know what you are doing.
Did you change the drive when you changed the engine? Could it have a different gear ratio? If you are making 575 hp and have a 1.5:1 gear ratio, you should spin a 28 a bunch harder than 4500 rpm. Personally, I don't think the engine is making the power. I don't know anything about it, but that is my best guess. Give us a little more info and we will go from there to try to help you out the best we can.
Eddie

Quote:

Originally Posted by Young Performance (Post 3187881)

I appreciate all of the info. the motor is a factory built mercruiser 540 575hp N/A. They built them from 86-92 I didnt dyno it, but it spins a 28p 4 blade like its nothing. Im just lacking on the rpms. I put on a brand new 750 duoble pumper today and saw little to no gain. The hull is flawless ( stepped hull btw) and the outdrive hasnt been changed.

I need to check my fuel pressure as well. The motor is rated at 4800-5200 from merc. Im not 100% sure that the internals and intake (dual plane) are stock, but the guy i bought it from told me that it was. Its a torque monster for sure, but im just not getting the rpms... I figure getting the rpms to 5000 is the first step, then tryin a 30p. Any mor ideas?

Roachcather :lolhit:

Did you change exhaust??

I have the same problem with my newly build 540 na. Only pulling max 4500rpm with 28p 4b. OSO members advised me to add performance exhaust because the stock 454 exhaust is restricting the hp. Its on back order.

Ive got stainless marine manifolds. Figured i wasnt to the point of needing headers yet.

Quote:

Originally Posted by surmirage (Post 3188368)

What boat do you have and whats your top speed?

Quote:

Originally Posted by SickSation (Post 3188484)

What boat do you have and whats your top speed?

Baja h2x 24.5ft. Best top I got was 70 mph @ 4500rpm, 2 people with 40gal(1/2 tank). I have 850 carb dual plane intake. Stock 7.4mpi exhaust. When my carb wasn't jetted right it was pinging and I was turning 4500rpm also but only 65mph. With the right jets it picked up 5mph. Everyone thinks the stock exhaust is holding back at least 400rpm or more...now its waiting game for new gen3 stainless marine exhaust. Make some hard pulls and stop to check color of plugs. See if your fuel is correct. Clean the carb.