Possible loss of speed after new impellar?

[kachina]

Just changed out my impellar from and A to a B+ custom cut, took the boat out for the first time and it has great bottom end but doenst seem as though i have much better top end like i thought it would get. The boat is a 22.5 Kachina Labala with a 454 60 over and a mild cam. It does seem like is much more afficiant and has a sick roost but not to impressed with the top end, shine some light on my problem please.....

[sanger rat]

What RPM's where you running? And what are you running now?

[LVjetboy]

You're running a 22' and changed from an A to an A/B or B+. You got a mild 454 and you're happy with bottom but not impressed with top end.
Why is that? Did you measure top end with GPS or radar and saw a 1 or less mph difference? Or just seat-of-pants and it didn't feel much faster? 1 mph is hard to measure let alone feel. Did you expect more yet dissapointed? I'd quess if you expected more...you were misled.
jer

[Squirtin Thunder]

This is why I stepped up to a AA. More forward thrust !!!

[CMZRACING]

So Whats The Simple Rule Of Thumb For Choosing Impellar Sizes?

[kachina]

Thanks for all the replys, Im measuring everything by feel from how it was before i changed out the A impellar, it is defenitly a different feeling power band and could have something to do with the last idiot that built. This pump theywere able to get a .025 gap and the leading edge of the impellar was cut .200 thousands. I was told to cut a .125 to achieve an AB but we kinda went over that :wink: The boat was running at 4800 and is not running at about 5000. I though i would loose bottom end and increase top end and i may have, i guess i was expecting a dramatic difference. I have 2 34gal tanks that are 3/4 full and would make a diffferce in accelaration. The rule of thumb i would guess is boat size and horse power for the correct impellar size. The boat was running about 58mph and not sure what its running yet couldnt really open it up completly because we were at Castaic and the dam cops were wondering about, but i was expecting more accelleration. Bottom end is great cant complain but it seems that its working in the opposite direction from what i was told, and was told i would loose bottom end and gain top end. Im heading up to clear Lake this weekend for a week :rollside: and will be able to get a good idea on how it is really going to work

[Cas]

With that much gas on board doing a speed run is definitely going to cut down on the top end. Take out 40 of those gallons and I'd be willing to bet you will easily pick up 5 mph.
and get yourself a gps so you can get some more accurate readings.

[cook1]

The trick is to always match your impeller size to your peek hp or peek torque depending on how long you want the engine to last and what kind of fuel economy you are looking for. If your motor is falling off of peek torque before 5000 rpm then it is pretty hard to get good rpm increases/speed by runin a smaller impeller. If your motor just has a mild cam then it probably has peek torque at 4000-4500 and best hp at 4800 - 5300 (just a guess) so running the boat close to 5000 may give you slightly better top end but your impeller will be cut down and your out of the hole and cruise will suffer. Just my two cents.

[kachina]

Great thanks for the info, and yes i agree with the gas weight as well to ill give it the test next week and see what we got. The boat is definetly a change from how it was with the A impellar (better) even with the weight and size of the boat great improvement on bottom and pulls good, so you guys who have the same sep up dont be scared :p