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The reason is simple. There are more "GOOD" blocks around for the 56 compared to "PERFECT" blocks for the 49.7.
It is easier and less costly to build a competitive T850 motor using a 56 powerhead. The 2013 rules make any 56 motor very good even if you have average parts. In order to have a 49.7 perform equal or better, it needs almost perfect parts.
Regardless of what block you use. Not all motors will run the same even if they are built by the same engine builder. They can be close and many will be the same, BUT, there is always the one that is a little better. When re raced SST60 in the USA many of the larger money racers spent 1000's every year just on powerheads in order to get that special legal 10 out of 10 powerhead. Sam Haraway who build lots of them will tell you how difficult it is to get the 10 motor and once you have it you race it as long as you can. They are difficult to replace.
Because the motors vary so will the prop selection. Just because one prop works on one combination does not mean it will be the best prop for every similar combo. You have to take into account the characteristics of each motor, the total weight of the outfit [boat, motor and driver] and the driving style of each driver.
The prop you use is the prop that provides the best handling and acceleration . It may or may not be the prop with the most top end. Normally it is not.
Rostertails are pretty, but ,they indicate the prop is not hooking up. Props that hook up in the calm water may not hook up it the rough water. If your bow is flat in the rough water and you have are rostertail chances are your motor is too high, you need more prop diameter or more stern weight. It may look wilder, but the boat running on its tail with the nose high does not catch in all the rough water. It flys over it. Jake's T750 is a good example. The motor angle should be 5th pin hole max to get the boat in that high position. If your running more trim then you need to add set back and move weight back. It takes power from the motor to lift the transom. You want to use the power to go forward not up. Balance will do that.
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