I am rather new to the mudmotor scene as a matter of fact i dont own one yet. This will be rectified soon. I have been thinking about and researching how these motors work and thought i would submit this question and reasoning to this forum since this is about engines and mods.
first
why is it that 2 stroke motors have been used in outboards for so long? its because of weight and sustained RPM. RPM is transfered from the power head to the prop via a shaft and to a gear box in the lower unit there it is reduced and sent to the prop.
second
4 stroke engines are mainly used to develope torque to turn higher gears that is why 2 stroke engines are not used in cars they are not designed to develope torque but RPM

Now where am i going with this? I have read alot about guys doing engine upgrades and carb kits and so forth that run into the thousands of dollars why not just build a bigger pulley for the engine onthe belt driven SD motors as this will translate into higher PROP RPMs this will not be a problem for the engine as it has plenty of torque to turn the prop.
if my thinking is incorrect please explain why.

Thanks

Your thinking is somewhat correct, but with the design of the MM prop and the conditions it is designed for (mud) a higher ratio can actually reduce performance if the motor doesn't have the torque to spin up properly. This is where the mods come in, it helps increase both the torque and hp which still enables you to turn the prop in the thick stuff (torque) along with the ability to spin it up in the clear water (hp). The belt driven motors have the convenience to swap out pulleys in order to achieve the different ratios which helps with matching the motors performance to the hull and load weight.

that makes sense since i dont really have a lot of mud i didn't think of the thick mud or grass issue. where do you get the pulleys to make the fine tune adjustments to gear ratio if you don't have the mud and thick stuff to deal with out in west texas most of the stuff we deal with is reeds, brush and rocks very little mud like say in louisiana. Part of the reason i was thinking about this was i see the amounts of money that are spent on upgrades and no one that i have seen has said anything also about boring out or resleeving the cylinders to get a bigger piston maybe im missing something. i have read about guys bending connecting rods from over reving these motors. my thoughts are bore it out .030 over or resleeve with a thinner walled sleeve and go up in size for the piston and this should increase your torque with out haveing to overspeed the engine and risk bending or breaking the rods. If you couple that with the carb mods to increase fuel and air then you would have one hell of a machine. But agian its a question of money how fast you want to go depends on how much you want to spend.

mharms909

If your area has hard bottom - then jet or tunnel hull is what you want...

You need mud to need a mud motor.... Or at the least a lot of water born vegitation to fowl up the more conventional water cooled designs....

It depends on the motor what is available to tune your motor/RPM to the rig. Changing gear ratios can adjust the prop speed at the same RPM. If you want to adjust the RPM, you can also try a larger prop or add pitch to one.

I know MB offers many sizes of gears both top and bottom and I bet the other belt drive motors do too. I played with mine going from 1.31 to 1.26 and 1.21. At 1.21 I killed my hole shot and now run 1.26. I had also tried adding 2" of pitch to my prop and it ran really well on my rig (until I broke it on a rock), a buddy tried that prop on his rig and said it was a dog. It takes some experimenting to find what is the best combination. You can gain some information here from other people that have similar rigs and have tried different combinations.

These are the gear sizes MB sells and ratios.

38 top and 50 bottom pulley (1:1.316 gear ratio)
38 top and 49 bottom pulley (1:1.289 ratio)
38 top and 48 bottom pulley (1:1.263 ratio)
38 top and 47 bottom pulley (1:1.236 ratio)
38 top and 46 bottom pulley (1:1.210 ratio)
40 top and 46 bottom pulley (1:1.15 ratio)