700hp scarab
 

Why cant i get my 1996 26' Scarab to go over 70mph? It went 70mph with a 500hp motor and now it has a 700hp 540ci and it still wont break 70? I have tryed a 25 p, 26.5p, 28p, and a 30p prop nothing will bring the boat over 70mph. Could it be the outdrive ratio keeping the boat restricted. If this boat wont break 70mph it will be for sale! Please help!!!

Is it getting enough fuel, did you use stock fuel pump,carb or is injected

Has motor been dyno'ed is it really 700. Reason i ask is might boats engine was built to 600 by previous owner and its not even close more like 450-500

fuel system
 

it has a areo A1000 electric fuel pump 1/2 fuel line a quick fuel 1050cfm carb and the fuel pressure is 7-8 at idle im wairing on a sending unit so i can hook up my fuel pressure gauge on the dash.

Are you using GPS speeds? Just trying to think that maybe your speedo is off and you really have gained some speed. I would think you would get mid 80s with that HP unless your prop is slipping or some other drive related issue not putting all the power to the water.

fuel system
 

it has a areo A1000 electric fuel pump 1/2 fuel line a quick fuel 1050cfm carb and the fuel pressure is 7-8 at idle im wairing on a sending unit so i can hook up my fuel pressure gauge on the dash.

dyno result
 

yes the motor has been on the dyno i was there to witness it it made 698hp at 5800rpm and also i have a gps speedo and i checked that with a hand held gps

what RPM are you turning in the boat now ? hull could be wet and way heavy or a bad hook from bad trailer set up...

its turning 4900rpm

Are you hitting the rev limiter,on a stock merk rev is 4850, if it is fire power it is 5350

your 900RPM short of your peak HP. What does the dyno sheet say for 4900RPM's? What are your TQ #'s. If the motor is built to spin 5800 the you need to spin it. Just watch that outdrive if it's a Bravo 1 it's not going to like spinning close to 6K RPM

I was thinking rev limiter too.
Have you checked the timing?
What's the correction factor on the dyno sheet?

I have a 23 Nova with similar engine/dyno specs. The Nova ran +70 gps with a ZZ502 and a labbed 25P. Now runs low 80s gps with a stock 27P and bravo 1 at about +,-5500 RPM. Engine's not even breaking a sweat but do worry about the B1 LOL

Who dynoed your motor? 700 hp is obtainable N/A from a 540 BUT motor would have to have ALOT of cam and outstanding cylinder heads, nothing run of the mill. What Cylinder heads do you have? I have a friend who also built a N/A 540 that made over 700 hp (at 6800 rpm's) but the cam was a giant solid roller and sucked for marine use, I borrowed him several props,he previously ran a 26 labbed, boat would not rpm even close to the cams peak until he went waay back in prop pitch and then oil ran hot, boat had no cruise speed, sucked all the way around. There is a supposed engine builder in PA that has inflated dyno sheets and outrageous correction factors, I hope you aren't another victim of his, Smitty

You haven't taken it above 4900rpm or that's all it will spin up to? That's way too low. I have N/A 540's and true 700hp. Max rpm's is 6500 on a hydraulic roller set up custom cam and worked aluminum heads so 700 is easily doable. Your definitely not spinning them fast enough for some reason? Gear ratio in the drives won't make that much difference in rpm so long as the right props are used. my guess is spin them faster to 6000 and see where that gets you. If you can't them you have other issues, rev limiter, lack of fuel, wrong props too tall, timing not advancing, etc?

Sounds like its starving for fuel. I have a 540 700 hp in a 29 fountain and it hulls ass.. find george goble on here, he has a 1996 26 scarrab that runs over 100 mph. goble 1

then @ 4900 you may be only at 600hp or less....what ignition are you runnung ?

I'm pretty sure the builder is not playing with the dyno numbers he is a family friend and has built motors for our race cars inculding a 1200+hp procharged small block ford!! But yes i def do have a prob! I'm running worked AFR cylinder heads msd ignition and distributor with a 6000 chip the motor made its peak power at just over 5562rpm. I found my dyno sheets and i was off a little on the numbers it made 689.4hp @5562 and 650ft/lbs @5562. The cam is a solid roller set up. Also a Quick fuel 1050 carb. Thanks for all your help guys!! I just hope i can get this boat to run faster soon!!

Did you change your pickup tube in he fuel tank 3/8 wont get it. I went to a 5/8 tube. You can have pressure but not enough volume. Are you running a bypass on your aro 1000. Also check your vent from your tank to the outside of the boat I had to take the cap off and cut the vent back to open it up. Take the dang mercury computer out and throw it far way.

Couple of questions.

Was that dyno run with all accessories through the same exhaust you have on the boat now or was it a bare block hooked to the dyno using the dynos waterpump & with dry auto exhaust with huge pipes and collectors?

This can make a huge difference in output to the prop.

What the fastest similar sized scarab running with similar power-? Scarabs are heavy and arent known to be water rockets.

a 700 in a 26 should rock balls. Something isnt right you're either starving for fuel or the prop is buried way too deep.

It should rock - my heavy 28 has 700hp and it runs 90 with a light load and 1/4 tank (25 gallons) on a mid 70 degree day and mid to high 80' all the time spinning a 4 blade 28.

Uncle Dave

Yhe key is why is it only turning 4900?

hmmmm

exhaust restriction (stock marine exhaust maybe? )
Timing got bumped back.
Bad tank of gas.
Propshaft too deep

dynos 689Hp when you take the car exhaust off, put the accesories back on, misjet it because of the wet and different exhaust, and bump a degree or two of timing out of it likely isnt 689 anymore but you are right that prop isnt turning what it should.

Uncle dave

The boat is overpropped. I bet the prop is deep in the water and getting a good bite. So with a low prop slip 5800 rpm and a 24 pitch prop, you will be going 80mph. You may even have to go to a 22 pitch. My old 30 Scarab went 72 at 5200 rpm with 23 pitch props. To do the same at 5800 rpm's would take a 20 pitch prop.

I know most of the accessaries where on the motor including the water pump when it was dynoed. I have changed the pickup tube to 1/2in including the 90degree fitting on top of the tube so its a full half inch to the pump. I;m going to ask the builder about the timing. And i do have a 23p 3 blade i'm going to try this week to see if i can get a baseline. So hears a stupid question the more and more i hear about the props doesnt necessarily have to do about how much horsepower you have. Just because i have close to 700hp doesnt mean i have to run a bigger prop. Thanks for the schooling this is my first boat so i'm still learning lol!!! Also i just received my fuel pressure sending unit so now i can see my fuel pressure from the dash, any clue what it should be at full throttle?

With that much power you will need to run a bigger blade, a 26 to 30 pitch. A small blade you will turn it so fast it will run at 7000 rpm I have scarab 22 with 600 hp and i run a labbed 30 bravo 1 at 82 mph on gps

Just because your supposed to have 700 hp doesn't neccessarily mean you would run a taller prop in every instance BUT if cam is right and motor makes a decent amount of tq you usually can turn a taller prop. A prop is alot like a dyno, ie-if 500 ft lbs of tq at 5000 rpm's can turn a 25 pitch 5000 and propel your boat 75 mph, if you made 550 ft lbs tq (a 10%) increase at the SAME rpm you should be able to turn a taller prop the same rpm and see a gain in speed or turn old prop a higher rpm and see a gain in speed.
Like I mentioned in my previous post though, I have a friend who has a 540 in a 27 fountain, boat ran 72 mph with his old 502 mph (warmed over some), new motor supposedly made over 700 hp BUT at quite a high rpm (I think it was 6800). Boat would not turn his 24 pitch prop the 502 had previously turned and even go 70 mph, he dropped back to a 23 pitch and motor was still out of its power band, last I heard he was running a labbed 22 pitch and oil was getting hot, boat was sucking fuel like crazy, boat lost cruising speed, I never heard if he ever threw out the cam for something that worked better but he fully thought boat would be running well into the 80's when he was done and he never got there. I hope you find it and it's something simple,what size afr's and what was done to them?
FWIW too my 540 dynoed at 650 hp at 5600 N/A with no accesories (afr cnc'315's with extra porting) and my 272 Baja has ran mid 70's with a 26 labbed 4 blade/1.50 ratio drive gears on a good day

6 lbs. is enough. don't go above 8 or it will overcome the needle and seat and overflow the bowls. I'm not saying there isn't something else holding you back but you need to get the rpm's up to where the engine makes power. Think of a prop being the same as a rear end gear in a car. You have to match the rpm's to the boats speed.