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Help on new impeller size

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Help on new impeller size

Postby LundNW » Thu Oct 18, 2018 7:40 pm

I have an SD309hd and i'm looking into replacing the aluminum "A" impeller for a 9.25" or AA stainless.
Its powered by 5.7L fuel injected 340hp marine power and my current WOT is 4750rpms.
The boat runs too fast on the water and lacks low speed planning capability. Plus when loaded, hole shots are not great to get up on step.
My understanding is that there is a 200rpm drop from an "A" to a "AA" and another 200rpm from an "AA" to a 9.25". So i would be dropping my rpms to about 4350 with the 9.25".
The larger 9.25" will move more water thus improve my over all low speed planning and should improve my hole shot to get on step with a load in the boat.
Question is, i'm i on the right track or is this wrong?
By installing a bigger impeller, will this throw the motors power curve out, as i suspect the motor peak power will be lost as i would loss about 400rpms off the top.
Thanks for any help.
NWjet 19.6 Freedom, 5.7L efi, SD309AT.
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Re: Help on new impeller size

Postby phatboy07 » Thu Oct 18, 2018 8:18 pm

Throw an "AA" impeller in and you will be all good. It will turn approx. 4200 rpm with your 5.7 when everything is in spec....i.e impeller and wear ring clearances. It will be a noticeable change in power curve in the lower end for holeshot and cruising speed. I suspect you might actually be out of spec on your current setup on the wear ring to impeller clearance as your rpm with an "A" impeller seems high. Conversely you could just check your clearances and fix that if it is an issue and test it out, you may be happy enough with the performance after fixing any issues (if there are any) and wont need to drop any more coin on a new impeller.
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Re: Help on new impeller size

Postby Ahusk » Thu Oct 18, 2018 8:46 pm

A different impellar can change planing rpm and may improve holeshot but it will do absolutely nothing to change planning speed. That is a function of your hull.
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