More power in a jet

[BrendellaJet]

Will having more power in a jet give you a higher top speed? I would like to hold the RPM constant. I dont care about efficiency. Would a 600 hp motor turning 5k rpm make my boat faster than a 400 hp motor turning 5000 rpm? It seems that they would go the same speed at that RPM. If that is true, then is the only reason for adding more power to make it so the motor can get to 5k rpm more quickly resulting in better acceleration?

[Snorider]

Yes, you are correct. 5000rpm is still 5000rpm no matter what amount of horsepower you have.
The main benefit to increasing horsepower is it will allow you to either 1.) change propeller to increase speeds or 2.) Instead of spinning 5000rpm, the motor may now have the power to spin to 5500rpm or higher - assuming the internal components (crank, valvetrain) can handle it.
For example, you wouldnt want to run an A2 impeller with only 400hp unless you wanted good fuel economy, but you could do so with a 600hp motor - if speed was your goal.

[HBjet]

600HP at 5000rpm will be faster then 400HP at 5000rpm only if you make the correct impeller change.
HBjet

[hack job]

no it wont!
if you change the impellor it would, but i think he was talking about was keeping the same impellor. if you went to a bigger impellor then if you could still turn the impellir to 5000 it would be faster. what the hp will allow to do is get there faster but you might not even reconize it . most likly if you kept the same impellor then you would spin more rpm. wink

[BrendellaJet]

Okay, I hope im not opening a can of worms here... So one impeller is not better than any other, they are just trimmed to allow the motor to operate where it makes most power?
So then, with the 400hp vs 600hp, using the same impeller, the 600hp motor would likely turn more RPM-does that mean that the 400hp motor could use a b or c impeller to allow it to turn similar rpm as the 600hp motor with the original impeller?
[ May 11, 2003, 09:31 AM: Message edited by: BrendellaJet ]

[malcolm]

Uuuh, yeah!

[bp]

BrendellaJet:
Okay, I hope im not opening a can of worms here... So one impeller is not better than any other, they are just trimmed to allow the motor to operate where it makes most power?
So then, with the 400hp vs 600hp, using the same impeller, the 600hp motor would likely turn more RPM-does that mean that the 400hp motor could use a b or c impeller to allow it to turn similar rpm as the 600hp motor with the original impeller? some impellers are better than others.
impellers should be trimmed to allow the engine to run near where it makes the most power.
depends on where the peak hp is between the 400 and 600hp engine, and what hp the 600hp engine is making at the same rpm the other engine was making 400. you should have a good idea of where peak hp is, and what the hp profile looks like before deciding what to do with the pump.

[Brknrun]

I went from a BBC turning 5200 rpms runin about 63 mph to a blown SBC putting out 6700 rpm and runnin 75 mph...No change in the pump..So why then would one be restricted in rpm and one is higher..As far as I know I'm runin an A......

[HavasuBarney]

Which impeller works best on Bass Boats?
Just cut the crap Brendella and lets go fishin! :) Or should that be cut the bait! wink
I also say, go ahead and open them worms, we loves live bait! :D

[BrendellaJet]

Take the day off today Barney? dont you have work to do? And what on earth you doing on my jet boat thread?! :) Thanks for posting that intake pic for me- that gives me an idea of how to shape my delta pad.
Brknrun- its pretty obvious that your small block has more power and is able to turn that "A" better than the big block, hence the difference in speed. Do you know where your motor makes its best power? I wonder if you cut that impeller a little if you could get more speed yet- although that RPM is plenty stout already...
Thanks for the help guys- I think I understand now! My pump has a B in it now-Jack says that should work for my mild power plans. If not, then back it goes to MPD for some trimming-but I think she will work well the way she is.

[Bow Tie Omega]

Yeah, dont you have work to do??????? :D :D :D
(I am just asking for a sledge hammer to my deck)
Just teasin Barney

[Chris J]

The answer to the orginal questions is NO, unless the 600hp motor is ligher. If you can spin your pump at 5000 rpm with 400hp. A 600hp motor would only use 400hp at 5000rpm. This assumes that your HP curve of the motor stays above the demand curve of your impeller to at least 5000rpm. It's possible to load a motor down and prevent it from ever reaching peak hp.
Say you have a motor that make 600hp at 6500rpm, but only make 375hp at 5000rpm. The motor would be bogged down and never get to 5000rpm and actually be slower.
As stated above you would have to install a larger impeller to increase speed without increasing rpm. Although it my actually take a smaller impeller to allow motor to reach peak HP

[LVjetboy]

So I'm thinking, why is "opening a can of worms" such a bad thing?