Impeller Pitch, How Impellers Work & Choosing The Right Impeller
To some, choosing the right impeller can seem like a job for a Jet Ski shop. Anything that has to do with the backend of the Jet ski or boat can be a scary thing. Despite popular belief, choosing the correct impeller for a particular PWC, Jet Ski Or Jetboat and Engine is actually a fairly simple thing to do, once you understand what a Impeller really is. First think of a impeller as a gear, whether it be on a motorcycle or even a bicycle. The lower the gear, quicker hole shot, more power, but revolutions are much faster so at a point you can only turn that gear so fast until you over rev the engine. Now think of your motorcycle stuck in only 5th or 6th gear, very slow out of the hole, little torque, but eventually it will catch up with itself so you can cruise to a good speed. The same basic concept applies to impellers, how to Find the impeller size that will give the best overall performance. Sound difficult? It's really not. Your engine has a recommended wide open throttle rpm range. In most cases you purchase a PWC with a stock impeller on it that hits for example 6900rpm. Let's say this motor has a 15-20 pitch impeller on it and at wide open throttle under a normal PWC load, the engine turns 6900 rpms. Well it's in the range so it's the correct impeller, right? Well yes and no. For the health and longevity of the engine it's okay, but for overall performance, you want to run impeller near the top of the range. Well how do you change the size to get that? Remember this rule of thumb: For impeller degree pitch, rpm's will change approximately 400 rpm's. As you drop in pitch, rpm's increase, and as you go up in pitch, rpm's decrease. So if we want to raise our rpm's we should choose a lower pitch impeller. Following the rule, going down to a 14/19 pitch should raise our rpm's to about 7300. That would help the hole shot and lose top end. As you follow the rule of thumb, remember that sometimes rules are meant to be broken. This is not an iron clad rule, sometimes varying Degree of pitch will only change rpm's by 200. Switching impeller brands or manufacturer, it can sometimes throw off the rule too. At this point the most important thing to understand is that less pitch means more rpm's, more pitch means less rpms. For your Jet Ski or jet boat to perform at its peak, you need to run at the upper end of your engine's recommended rpm range. That's it. Once you're there and still want more speed, that's when choosing the right impeller brand comes into play. With the aftermarket performance impellers, They are designed for a certain engine combination. You want to go with their recommended impeller pitch for a stock engine or modified.