First you need to determine your current props diameter and pitch. Take a look at your prop and find a set of numbers phrased like this: 14.25 X 21
The first number is the diameter, the second number is the pitch. This diameter X pitch is a standard phrase for all props. Hopefully yours is has not been removed. The common place to find these numbers is on the hub. Once you determine the pitch, now you know what pitch you're working with to produce the rpms.
Second, look at your owners manual/service manual. In the specification section should be listed the max rpms at wide open throttle (WOT). It's normally listed as a range, such as 5500-6500. Once you determine this, now you know the max rpms you should get from your prop at WOT. This is an engine spec that is commonly overlooked because the engine performs what is perceived as normal even if its under-propped or over-propped. But eventually, you will experience engine problems. These engines are manufactured to run at WOT at specific rpms. If it doesn't, timing issues will arise and this leads to bad mojo.
Now that you have done all of the above it's time to determine the correct pitch prop to use. And here is when it becomes a little bit of guessing game. As an unwritten rule of thumb, every inch of pitch equals 150-200 rpms. If you decrease pitch, you increase rpms. And vice versa. For example, lets say you determine you have a 17" pitch prop. And with this prop you're turning 7000 rpms at WOT. Too much, it's under-propped. Let's assume your manual lists the max rpm at WOT to be 5000-6000 rpms. So you need to drop 1000 rpms to be at the top end of the range, which is ideally where you want to be. If you went with a 21" pitch, increasing the pitch by 4", you can approximate the max rpms you'll get to be at the top end of spec rpm range (4" x 200 rpms = 800 rpms). Question is, do you go with the 23" or the 21". That's where it becomes a guessing game. However, if you locate a reputable prop shop in your area and tell them what rpms your currently getting with your prop, then tell them where the rpms need to be, they will give you their best advice. After purchasing the prop and you determine you need to go up a pitch or down a pitch, most shops will accomodate. After all, a new SS prop ain't cheap. You need to be sure you're buying the correct one.
Good luck.
