What'd you put it on?
Where's your propshaft centerline compared to the running surface of the hull?
Do you have spacers,

My propshaft is even with the bottom of the hull. No spacers. I'm thinking about adding spacers but how thick?

The whole reason for that drive is to run it surfaced. I would think that you should go higher and run bigger diameter props with as many blades as you need to get on plane.
I plan on running one soon. Mine should be 3 inches above the bottom of the running surface.

I'd have to raise my engines to get the propshafts higher which I'm not going to do so I'm stuck with "what it is". I hear about "skiing" the drive and understand it so I'm wondering about sinking it deeper. Another reason for the -4 is reliability and depth adjustability, if you can't fully surface it.

I would not go lower , on my 42 lightning when I went from standard SCX,s to the -4,s I was 3.5 above the bottom and had blowout @ 100 or so the boat hooked R or L no spin out thank god she stayed up right put 2 inch spacers back down to 1.5 above where it was set up to be and worked great . I assume your using #6 props .

I have 6 props.
Slip is 16-18%.
A friend put on standard SCX's with the 1400 lowers and he's 2" below the hull with 9% slip and no loss of speed over the SC's he had.
My boat's faster and handles better with full tanks so I'm thinking that going deeper may give me less stern lift.
Does Brett have an SCX spacer program? Where the heck should I start?
Vid of the drive at speed shows that the water is hitting the drive at about the center of the bullet/propshaft.

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I have a pair of -4 2 inch spacer,s . If I remember right I had like 18% slip also .

I would try more blades on the props first to bring your slip down. TOO low of a slip # breaks drives. Why do you think that You need to change something? Just the slip # or is there another problem.

I don't have a slip problem mine is 8%. but what im getting is some serious chine walk at about 105. my lowers were blueprinted at Wilson. my propshaft is also even with the bottom, but mine is a side by side. according to Wilson I need to take some diameter out of the prop go from 17 to 16.75 or maybe even 16.5, because we are spinning them over 6000 rpm. the boat is at Reggies having a new dash put in, he sent the props out to Wilson. Just waiting to get everything back together to try.

It's faster and handles better with full tanks which hints that the stern is lifting too much. More blades would probably add to this. A 20 rake 37P would be nice to try but a spacer might do just as well for less $$$$