SCX prop help
 

I just picked up a 2004 38 Fountain semi staggered big twin step setup with the -2 IMCO SCX uppers and lowers with #6 shafts, engines are 525efi with whipples 7psi, props are 32p 16.5 18 rake 5 blade Herings. At sea level with 4 ppl and full tanks I get 5300 rpm 102mph. In Tahoe where the boat lives 95 percent of the time with 4 ppl and full tanks I get 4700 rpm and 91mph and it doesn't like getting on plane, it gets up but takes a bit with the props ventilating at 4500rpm. I am looking for suggestions for what I need to go to for the rpms to get back where it needs to be.

Yikes that’s a lot of rpm loss. Any idea who built the motors? Maybe talk to them or I’d probably start by talking to whipple about the tune and see what can be changed to compensate for the elevation change.

Whipple said I can go to 8 psi on the stock tuning, the rpm is low even at sea level, I should be at 5600-5800.

With 4 ppl and full fuel you’re not far off of 5600. You could see what’s out there for 31” or 30” pitch props. What size pulley do you have on the motors? 4.75”?

4.25 upper, I am looking for some 30 pitch props right now, 7000 dollar gamble.

where's the limiters set, 5800 on a stock bottom end might be pushing it

Hmm. 4.25” should be more boost than 7psi. I have 4.5” on my whippled 525s and I want to say I’m over 8psi. Smaller pulley equals more boost. The way I see it your options are

1. stick with same diameter props but lower pitch and get more rpms, but have same planing and max speed due to lower pitch.
2. Go bigger diameter, but I think 16 7/8” is max on scx, and lower pitch to gain some rpms and help ventilation. And your slip #s are already decent.
3. mess with spacers along with 16.5” diameter props to help with ventilation.

the rabbit hole goes deep. Your trimming to your sweet spot for the WOT runs? How much testing did previous owner do with setup?

Trimmed the right way, my lower is 8.75 yours might be bigger explaining more boost for you. Rev limiter is at 6000 rpm but don't ever plan on running more than 5600 for any length of time. These drives just eat up hp real bad. IMCO told me 16.5 was the max diameter on the -2 SCX. Any lower and I will drag the bullet through the water so I don't want that. Sounds like I need to ad hp.

Yes they do eat HP

On this style of prop what is the rpm gain by dropping pitch? Is it the same thought of 100rpm per pitch change?

General rule is 200 rpms per inch of pitch change.
If you start changing number of blades, rake or diameter, that affects rpm as well.

Would the 18 degree rake be causing my chine walk at top speed? 15 degree would give less stern lift correct?

IGetWet,

The question that was asked was about the pulley on the engine. A smaller pulley on the engine would result in less boost, not more.

Thanks. Brad.
Model Machine and Precision LLC
(937)545-8991

The OP said he had a 4.25 UPPER.

Huh? There was never any question about the pulley on the blower or the engine... Either way it’s irrelevant to the OPs question.

IGetWet,

I see where we went sideways....

You asked about the pulleys on the motors, the reply specified the pulleys on the blowers, and I missed that curveball.

I'll step aside now.... :rolleyes:

Thanks. Brad.
Model Machine and Precision LLC
(937)545-8991