Rpm v/s horsepower

[Mike Roberts]

I have a 1998 speedster and was wondering about horsepower and rpm's. And new to the jet boat scene, I hear of all kind of performance items for pwc and boats. If 7000 rpm is all you can get out of these engines does it matter if you have 110 hp or 600hp,, not that you will get 600 but for an extreme example,! Isn't it true the machines are limited by the size of the jet pump, take for example a hopped up outboard, u can change the diameter of the prop but on jet boats your kinda limited by the size of the jet pump , true u might get to 7000 rpm quicker but once at 7000 rpm your not gaining anything, I am guessing that not far past 7000 rpm u get into not enough water volume thru the pump and will start cavitation,! Any thoughts on this , I might just be crazy. I don't know , lol.

 

[Dr Honda]

Ummmmmm..........


Yes and no.


First for the Yes: If you don't change your top RPM's... and everything else remains the same... you could have 1000 hp, and you aren't going to have any better top speed.


Now for the no: If you are doing engine mod's, looking for more power... then it would only stand to reason, that you would mod the pump to take advantage of those mods. So... if you have more HP, and don't want to spin the engine faster... then you will want to install an impeller with more pitch. And to reference to your outboard reference... you would still prop the engine for a given RPM/load.


Also... depending on what you are doing (assuming that you got 600 hp stuffed into a ski) would you really expect a pump that was designed for 100 to properly process 600 hp worth of water?? Of coarse not.



Finally: Yes, your statements above are basically correct... but they are very one sided. It's the same with mod'ing any vehicle. You can't expect the rear-end, and transmission of your car to hold 600 hp worth of mods.... and in your out-board engine... you can't expect the stock lower gears to hold up to 10k RPM, and 2x or 3x the HP. When doing extreme mod's... you have to take care of the entire package.

Just an FYI... if you buy a pipe from someone like "Factory Pipe"... then they give you a new rev-limiter in the kit, so you can get more speed from the new found power.