The A/B is actually an A cut down . Expect to lose some hole shot . Take it to a shop and have them cut it down and detail it . Money well spent when water enters and exits the impellor correctly along with the rest of the pump. 

-beerjet-

Click below as well 
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Is it because Im not making enough power for the b impeller? I was always told a impeller is like gear for a car, a A impeller is only going to go so fast and if you motor will pull a B impeller it will out run a A?

An 'A' or 'AA' cut impeller is much more aggressive and as such takes much more power to turn at higher RPMs. The more aggressive the impeller the more capacity it has for volume/pressure into the bowl of the pump. Add to that a decent amount of power (seems you already have that) and you'll be stylin.

What RPMs are you turning at 72-74? How much power was it producing between 5-6k RPMs? What brand impeller, what brand pump, how long is the boat? Have you had the tolerances in the pump checked?
To go from 72-74 to the 80's is gonna take a bit more than just impeller change. You can expect at best a 3 mph increase with just an impeller change. Your WOT RPM will also drop, and that drop varies with the impeller manufacturers version of the cut. Some are more aggressive than others.

You're probably gonna want to contact a Jet shop like www.jetboatperformance.com for advice/parts. If you can wait a little while call Dave at Aggressor Jets, he's just getting over multiple heart attacks. But he should have a batch of impellers inbound pretty soon.

5400 RPM's was as fast as it would spin?

we have a 78 18ft southwind /460 running eldelbrock alum heads w/a=b cut alum impellar
running close to 500 hp at about 5000-5500 rpm w/jc berk pump with out ride plate or droop
 running 78 gps mph=

i have pictures butt i am brain dead on how to post them

At 5400 on an A cut berk, I'd estimate you around 550 tops. My 454 was spinning 5400 with the Berk aluminum, I was in the 70's.  67 was my GPS number with three dudes averaging 215 and a cooler full of beer. Weight made a huge difference. The Aggressor dropped it to 5100-4900 depending on elevation (castaic vs Pyramid lake respectively) didn't get a GPS reading. Somebody sat on the GPS and broke the screen.

Based on RPM and impeller cut, sounds like you were conservatively lower mid 500's. Until you get a mag. bronze or stainless impeller don't mash on the gas out of the hole. You take the chance of spinning the center of the impeller right out of the vanes.

Sounds like your original setup was very similar to my old boat. Wasn't the Tahiti hull fairly similar to the Southwind?

You should be in for a good time with the new heads/cam. Looking forward to hearing what it does with it's current pump setup.

I'm really limited on what I can do to help you out parts wise. But give me a shot anyway if you'd like.

here is something to get you close .....

Where is 7,100 RPM's at on that chart? 

I recommend the Aggressor A cut. But we may not be seeing too many new ones for a while... With the newfound horsepower it should keep you at about the same RPM WOT that you were at before, give or take 100 RPMs. Have your bowl massaged by someone who knows their chit. You should be lookin at "just a few" more mph.

Tom and Josh at JBP are the current lords of all things jet pump. Take whatever advice you can muster out of them and run with it. They're also one of the only reliable parts sources around. On top of it, they're not hard to get a hold of and they're more than helpful. As far as my input is concerned I have really truly had hands on experience with ONE boat application. That was my old one, I have been privileged enough to hang out with the mastermind at Aggressor but once again have only been able to apply my concept and theory once. So I should be taken lightly.



I'd still like to get an apprentice position up there...

hope this is the right picture