# Blog

## PCM Engines – H6 (450) vs. XR7 (550)

In the 2017 Centurion line there are a few different motor options to choose from and we get a lot of questions about which one to pick. In particular there are 2 different engines available in the Centurion Ri Series that are each excellent options depending on what you are looking to achieve – the PCM H6DI and XR7. Here we will dig into the details so you can determine which one would be right for you.

At face value the motor specs are pretty self explanatory:
PCM H6 = 450 HP and 465 ft. lbs. of torque
PCM XR7 (Supercharged) = 550 HP and 570 ft. lbs. of torque
As you can see the XR7 delivers significantly more horsepower and torque, however there is a bit more than meets the eye when it comes to these engines and their performance.
While horsepower is important when looking at the performance of a boat engine, the most important number is torque, at least when it comes to surf boats and the load they experience when ballasted. Even more important than the torque specs of the engine is the actual toque numbers that occur at the propeller. The way this is calculated is by taking the engine torque specs and applying the gear reduction amount. As an example the H6 engine has 465 lbs. of torque and a gear reduction of 1.72:1 which creates a total power output of 799 ft. lbs. of torque at the propeller (465 x 1.72 = 799).
Here are the torque specs w/ the gearing applied to the two engines:
H6 – 465 ft. lbs. w/ 1.72:1 gearing = 799 total output
XR7 – 570 ft. lbs. w/ 1.42:1 gearing = 806 total output
As you can see the total power output of these two engines is within 1% of one another. What this means is that for surfing, wakeboarding and doing any activity that involves low end power the engines will have nearly identical performance.
So why would Centurion design the engines to perform this way and why would you upgrade to the XR7 if there is no performance increase???
When looking at the way a boat performs there are 2 important things to look at – low end power and top end speed. The most important factor when talking about a surf boat is low end power because without it you can’t surf. The 1.72:1 gear ratio on the H6 allows it to supply plenty of low end power even at higher elevations and when fully loaded with ballast and people, however by running an increased gear ratio it does reduce the top end speed of the boat. This is the reason why there are two engine options and also why the XR7 runs a higher gear ratio.
The XR7 engine provides a LOT of torque and because of this the XR7 will deliver all of the low end power you need to surf, wakeboard and do any other activity you want even with the higher gear ratio. If the low end power desires are met then the next step when optimizing a surf boat is to increase the top end speed and this is the main benefit of the XR7 engine.
While both engines will perform great for surfing, wakeboarding, etc. the XR7 will deliver a higher top end speed (roughly 4-7 mph) compared to the H6. The absolute top end will change based on the propeller, elevation, boat load, etc. but on average the H6 in a Ri237 will deliver 34-37 mph and the XR7 will deliver 38-43 mph. In addition to this top end increase the XR7 will also offer reduced cruising RPMs when going the same speed and slightly reduced surfing RPMs.
The bottom line takeaway is both the H6 and XR7 offer fantastic performance and work great in any of the Ri series boats. If you are a person that spends most of the time surfing and rarely takes their boat above 30 mph then the H6 is a fantastic choice. On the other hand, if you want higher top end speed then the XR7 is the motor of choice.
Let us know if you have questions on them and if you need any additional help choosing the best options on your boat. Now get out there and spend more time on the water.
Cheers!

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