GreigA-inAus:

If you are talking about the APC 12.25x3.75 prop I can almost guarantee the K&B Sportster 45 wont like it.

To get reasonable flight performance that prop has to spin around 16K rpm, the Sportster intake is too resricted to pull it that fast. With a 12x6 it will fly well, a 12x7 or 12x8 might be even better.

That 12.25x3.75 is intended for a "3D" plane, the K&B Sportster is not your bet engine choice for that application.

The current K&B 48 would do very well. though.

Bill.