I have an RV-6 with an O-360, carb and mags and the old style wheel fairings. I had Sensenich repitch my 72FM prop to 85" from 83". Because the 83" pitch turned out to be the wrong recommendation (mine's an early prop) they did it for only shipping cost. It was well worth it. I lost no perceptable performance at the beginning of the takeoff roll but can cruise a lot more efficiently. CS is still best but I think 85" works for me with the FP prop. At WOT, 8,000 ft I get 174 kts TAS @ 2725 RPM and @ 12,000 I can run WOT and be below redline at 2670 RPM. For you with better fairings 86" might work better unless you fly out of higher altitude strips. Scott McDaniels has commented on this but I can't find his post.
