I have never run anything larger than a 5-3. This was the prop for the TD engines. I think they had a longer stroke than the AME or VA. Start with a 5-3 and you can always cut it down. I think you should prop to around 24,000 with a 5-3. the 4.125-3 should get you closer to 29,000-30,000 with a good engine. The Cyclone and fora I think are doing much better. I do not have one of those though.