Jim, This is a pretty simple choice. If you are racing the AMA 424 event you may only run the APC 9/6. This is the universally approved prop. If you are flying for fun, start with the APC 9/6 and then experiment. A strong TT Pro 40 on the latest, and greatest quickie 500 air frame with the 9/6 APC should do about 115mph. Your Viper is a 20 year old design, so it might be a little slower. Don't expect top performance from a new TT Pro. Most of them need a lot of running. There has been a lot of information on this Forum, and others, about legal changes that can be made to the TT Pro for AMA 424 racing. Probably the most important is to replace the bearings, which are known to disintegrate. In fact most of the approved changes are not meant to improve performance, but rather to improve reliability. Good luck, Greg