cut the impeller or not?

**75 TX** · 09-11-2010, 11:12 AM

My engine makes max power @ 6000rpm. I had been planning on running nitrous part time so I had Jack set the pump up to run best on the juice. I've since decided against using nitrous, which brings me to my question....what would be faster, spinning a B+ to 5600rpm or spinning a B to 6000?
Boat's already @ 90mph with the B+ spinning 5600, and I believe there's more in it with some more tuning. And since I'm not gonna use the nitrous I'm wondering if there's any mph to be gained by cutting the impeller.
Thanks

**GT Jets** · 09-11-2010, 11:19 AM

Gonna take a shot in the dark here....

It totally depends on the amount of HP you are generating at the stated 5600 RPM.

Speed is determined by nozzle velocity and volume. The volume will equate to torque and the velocity will equate to pressure, which requires more power. If you look at the curve, there is almost always a point on the curve where you can do the same "work" at a different RPM, if the lines cross there are o gains to be realized. Personally I think I would try a bowl extension before cutting the impeller to see if things move in the right direction.

In short, if the torque falls off at the 5600 RPM point, it may simply never get there, cavitation will increase too....

I will be watching this to see how far off I actually am, but think I am pretty damn close..

**HKROB** · 09-11-2010, 11:22 AM

How much more power does your engine make at 6000rpm Vs the 5600rpm.

**widowmaker** · 09-11-2010, 01:19 PM

You'll be surprised how much speed you'll increase by cutting the impeller. not that much.

I just did an impeller change on my 21 daytona. Peak power was at 6000rpm

It ran 95-98 mph in perfect air at 6000rpm with an MPD B- cut SS impeller. Holeshot was non existent.

I swapped it out for a AA SS unprepped impeller and last weekend in horrible air it ran back to back 85 mph at 5200.

Note the air quality is probably worth about 200 rpm.

So I lost about 600 rpm and gained alot of holeshot. But most importantly the speed didn't drop that much considering the air quality I'm going to guess 8 -10 mph for air quality.

I would say run the impeller a little on the large side and you'll be happy.

**Gearhead** · 09-11-2010, 08:45 PM

Originally Posted by widowmaker

"not that much."

" I'm going to guess 8 -10 mph ..."

I guess it is all relative to what you are looking for and what is much. To me... that 8-10 MPH is half a lifetime

Give me all or give me none

Back to the post. As has been mentioned, take a look at your dyno sheets and see what the difference in power is from 5600 to 6000. If you are leaving 40 HP on the table, then I would most definately make the impeller cut change. If it is 5 to 10 different, then you won't see as much of a change.... especially if your just driving around on the lake. But...... if your racing .... or racing on the lake.... I believe the B would be the best move for MPH and most definately acceleration.

**Sleeper CP** · 09-11-2010, 09:18 PM

Originally Posted by 75 TX

My engine makes max power @ 6000rpm. I had been planning on running nitrous part time so I had Jack set the pump up to run best on the juice. I've since decided against using nitrous, which brings me to my question....what would be faster, spinning a B+ to 5600rpm or spinning a B to 6000?
Boat's already @ 90mph with the B+ spinning 5600, and I believe there's more in it with some more tuning. And since I'm not gonna use the nitrous I'm wondering if there's any mph to be gained by cutting the impeller.
Thanks

I doubt that Jack would cut it to run any more rpm than 5,800..... Call him on Monday.

FYI: Read this

http://www.performanceboats.com//showthread.php?t=67767

S CP

**MudPumper** · 09-11-2010, 09:19 PM

Originally Posted by Gearhead

I guess it is all relative to what you are looking for and what is much. To me... that 8-10 MPH is half a lifetime

Give me all or give me none

Back to the post. As has been mentioned, take a look at your dyno sheets and see what the difference in power is from 5600 to 6000. If you are leaving 40 HP on the table, then I would most definately make the impeller cut change. If it is 5 to 10 different, then you won't see as much of a change.... especially if your just driving around on the lake. But...... if your racing .... or racing on the lake.... I believe the B would be the best move for MPH and most definately acceleration.

Pretty sure he is saying the boat dropped 8-10 mph mostly because of air quality (Hot and Humid with high DA) not so much because of the change in RPM or impeller cut.

**MudPumper** · 09-11-2010, 09:22 PM

Originally Posted by Sleeper CP

I doubt that Jack would cut it to run any more rpm than 5,800..... Call him on Monday.

FYI: Read this

http://www.performanceboats.com//showthread.php?t=67767

S CP

Agreed....He likes a cut to be about 200 below peak HP.

**75 TX** · 09-11-2010, 10:00 PM

Originally Posted by Sleeper CP

I doubt that Jack would cut it to run any more rpm than 5,800..... Call him on Monday.
S CP

Yeah, I actually went by Jack's this afternoon and spoke with him about the impeller and several ather topics. I'm not gonna have it cut.
I was lucky enough to borrow a loader with some blocker in it as well as a 2.8* backcut shoe. Gonna try out those two (1 at a time

), and see if I can free up the ass end a bit & score some more top end that way. Already made my unofficial goal so anything else the boat will do is gravy.
Thanks to all that responded.

**Gearhead** · 09-12-2010, 11:47 AM

Originally Posted by MudPumper

Pretty sure he is saying the boat dropped 8-10 mph mostly because of air quality (Hot and Humid with high DA) not so much because of the change in RPM or impeller cut.

Man... that would be some BAD air! What part of the country does the D/A change that much within a few days?

Thanks!

Gear

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