Thr Irvine spins an APC 12-6 or 13-4 prop. That's more than a 46fx (12-4 if you're lucky). The Irvine has more torque, maybe you justdon't have it tuned right since it's new. I have one on a Magic Extra 300 and it pulls it vertical with ease. It's a little lighter than the US40 but still, you should see more than the 46fx. Also the Irvine is not any heavier than the 46fx. Irvine claims 400 grams which is 14.28 ounces. I assume no muffler. The 46FX is 16 ounces with the muffler. So it's equal in weight. A longer stroke gives more torque, especially at lower rpms. BTW I have the US40 with a 46FX and I use an APC 11.5-4 prop.

What prop are you running on the Irvine?

Joe