there are two MAS 8x4 props. there's an older version with very wide blades, and a newer (but still out for the last several years....) version with thinner blades. I'm not talking about a new GF3-version (I'm not sure if MA redid the 8x4 for the GF3 series- the 8x3, for example, has not been redone). If you have one of the wide blade versions, it won't rev up as much, at least 1k down from the narrow blade version. the 8x3MA is kind of a "magic" prop for relatively heavy, draggy planes like SPADs and other combat planes. On a light, clean plane like a SIg Wonder or a Lanier Shrike, the 7x4 or 7x5 might be better, but I think the 8x3 is a good starting point for a SPAD. The 15FP should turn that prop about 17k. Some will exceed 18k. It depends on the engine. Things to do to improve performance: If you have the longer muffler with a center baffle section, remove that section or enlarge/remove the baffle hole. Make sure there is no casting flash at the joint of the muffler header to the main body portion, and also none in the stinger hole. Loosen the jam nut that holds the NVA to the carb (usually a 7mm hex) and back the spray bar out until the tip is just peeking into the side of the carb. This will really reduce the intake restriction. Finally, you can try removing the head gasket. It's pretty thick, like .017" or so. Some engines don't run better with the head gasket out, but most do, at least on 10-15% nitro. When resintalling the head, be sure to carefully tighten the head bolts so the head is on square to the cylinder. The 15LA seems to have more power than the FP, and other than removing the muffler baffle, it should turn 17.5k to 18k out of the box with no mods, on 15%. But a 15FP should be perfectly adequate for a SPAD or other combat plane up to about 3 pounds weight, over that, the performance will suffer. If you can keep it to 2.5 pounds, even better, a few ounces make a big difference on the .15 planes.