Engine wont rev past 3500 while on water, WHY??

Hi,

Im from Australia and we are limited to resources concerned with Jet boating.
I ski from my 86' Mdl, Hallett Hustler. It has a 308 Holden V8 (GM or Chev, which ever you prefer) and i belive it has a Jacuzzi Jet (it says Jacuzzi on the streering arm)!!!
My question is this.
How do I get it to rev past 3500rpm?? Will a super charger work?? is it somthing more complicated?? Can i "tune" it to go harder? do I need bigger or smaller jets in the carburettor?? different prop (Turbine)??

Im confused about what I should do and this is the only forum of its type with people that obvioulsy spend hours around their boats..

Look forward to hearing your replies.
Stuart

 

The impellors are generally similar, only blade length is different in a mixed flow type pump.

Is the motor capable of reving past 3500 with no load? I assume it hasn't some sort of 'miss' for other mechanical problem?

If motor is fine, it is easy to remove impellor and machine it it get more rpm. Your 308 won't be making a hell of a lot of power at that rpm, so I'd imagine performance to be pretty average?

 

At 3500 rpm you are already well onto the downhill slope of the torque curve of this dinosaur V-8, but if you insist on reaching 4200 rpm, the easiest way is to remove approx. 12 mm length from the impeller's length on a lathe, without changing the outside diameter. The American Turbine or Berkeley Jet website has a spec. sheet for modifications of a standard impeller.

Going further than 4200 rpm I wouldn't recommend, unless you consider the much shorter life expectancy of this engine acceptable.

 

This is going to turn into an 'old school' vs 'new school' arguement...

The slower an engine revs the more load per stroke is placed on internal components for the same power. Like riding a bike, put it in a low gear and you'll do less revolutions but put alot of stress on your legs, put in a high gear and your legs will cycle faster put with less stress on your ankles, knees, muscles etc. The thing is with labouring the engine at lower rpm you will need higher throttle percentage and therefore burn more fuel than if you let the engine find it's powerband and run it at the correct rpm with less throttle percentage. This is something we have done plenty.

Where did you get the hp / torque curve to say that at 3500rpm it's already on the downhill of it's power curve. Remember jets suck up torque, so pay more attention to the torque portion of any dyno graph. Jets are basically a dyno water brake, and so load can be easily and accurately calculated.

Do you know the current trim of your impellor? Or the hp rating of the motor? Limited data input will yeild limited data output... GIGO...

 

The line of reasoning: 1986 small block GM V-8 pushrod engine, lots of reciprocal mass. Although I cannot reproduce the power curve for this specific engine, look at a "modern" descendant which is still 1th generation:

http://www.gm.com/experience/techno...ecialized/industrial/2009_5700_Industrial.pdf

Maximum torque at 1400 rpm, max. HP at 4000 rpm. I know that the same engine when painted black and labeled Mercruiser has higher HP/rpm ratings, but the hp/torque curve becomes a well guarded secret then.

Shortening the impeller reduces it's wet area so there is less drag. That allows the engine to spin faster and the Jacuzzi to reach a higher jet velocity.

 

A jet pump is a dyno that ultimately uses horsepower to push you around. So, shoot for peak HORSEPOWER if you want ultimate performance.
The Jacuzzi is a mixed flow pump, which uses the diameter of the output side of the impeller to create pressure.
Changing "wetted" area won't do much, but a half inch is a bunch off the diameter. do a search on Berkeley A vs B vs C impellers, and there are sites to learn how to machine an A to get a B impeller.

 

at 3500rpm you are at the max for that small v-8. Being a jacuzzi(YJ i assume) is an AXIAL flow pump and not a berkeley(MIXED flow) you cant expect too much as far as performance. I would go a camshaft and carb change to get a few more ponies to improve some but if you want to get more performance, get a different boat with a berkeley style pump. Jet boats need TORQUE not HP as they are loaded all the time unless you are cavitating. There is a kit for a YJ jacuzzi pump called an "energizer kit" that converts it to a mixed flow berkeley style pump that would help but i would get you motor in the 400hp range to make it worth the investment.
hope that helps, ted

 

i agree