CMB 21 VAC

[ids987]

I was asked to test the CMB 21 stock which I did. After a little run time it had outstanding power through the corners and it would throttle down to very low rpm on the water and pull away every time without struggling. ABC set is the same as last years motor and induction was 149/55. I later modded mine to my usual specs and bored the carb to 9.5mm and still the same outstanding torque and throttling. Unlike any 21 engine I've ever come across. This is going to make running a 21 motor a walk in the park. I've converted 2 of my greenhead motors to the new drum and carb assembly and the same kind of improvement was there. I figure it has to be something to with the fact that the conrod is not obstructing the induction port but that's only a theory

Dave

Dave,

Do you think that the improvement in torque, could be partly due to improved pumping - smaller case volume etc ?

Ian

Ian, It doesn't have a smaller case volume.

Hi Dave,

Sorry, case volume may not be the right way to describe what I meant. With the old style drum - open end facng the crankcase, I was including the space inside the drum as part of the case volume. As the new drum has the closed end facing the crankcase, the internal volume of the drum is a separate entity. Presumably this means that there is less trapped volume when crancase compression is underway ?

Ian

 

[Martin Hamilton]

Dave are you refering to the case volume being the same without rear cover and valve in place, As i have to agree with Ian on total case volume being lesswith the vac drum valve compared with the old Valvola reverse drum valve. The old valve assembly had a total volume of around 2.5cc this is out to the face of the valve which includes the beveld part of the valve. A typical drum assenbly of the vac design and size is only around .8cc when the valve is closed as appossed to the 2.5cc volume. By the time you factor in the extra amount you have to remove to form the channel up into the case (useing the old case fitted with vac rear) the volume is still considerably lower than the old motor. I think this is one area besides any timing changes (if their are any) where you will see an increase in torque and much better throtling and improved pick up. Regards Martin.

 

[DaveMarles]

Ian, By case volume I include the volume inside the drum on the old version and I actually took a few quick measurements on a 2008 redhead and the new motor and there is a difference of ca 0.3cc and that means nothing in terms of performance.

Martin, I didnt say it had the same volume. Your calculations are way out by the way.

Do I think altering the case volume has caused the change in performance? No. I see this as a fundamental change in the operating characteristics of a 3.5 engine and changing case volumes does not have that effect. I have simulated worse case situations such as quite long periods of low throttle running and the motor still picks up when throttle is opened and it also pulls away from the shore with the biggest prop I use with ease every time I try it. This is not normal for me! My racing involves a lot of throttling and hard cornering and this kind of improvement is just what I need and I would expect its just what a rigger needs too.

Anyway this is just my take on this motor. I've also converted 2 of my old Valvolas to the new drum /carb assembly and the change in characteristics has been the same for them.

Dave

 

[Martin Hamilton]

Dave my calculation was done useing a syringe and filling the old Valvola rear cover and valve with this assembly removed from motor and sitting vertically so the valve was filled to its face and it took 2.5cc with no leaks. The drum motor i used was very simular to the vac in size and held around .8cc with the valve closed. So i did not actually calculate this but filled it and those are the figures i got. Regards Martin.

 

[DaveMarles]

Dave my calculation was done useing a syringe and filling the old Valvola rear cover and valve with this assembly removed from motor and sitting vertically so the valve was filled to its face and it took 2.5cc with no leaks. The drum motor i used was very simular to the vac in size and held around .8cc with the valve closed. So i did not actually calculate this but filled it and those are the figures i got. Regards Martin.

Martin , I have no time for a p**ing contest. I have the motors here and so I'm not guessing.

I can say that on the old greenhead Valvola a plug in the drum reducing the crankcase volume by approx 1.1cc made little difference to the performance and so I doubt a 0.3cc change in volume is going to either. As Mark Grim reports a similar improvement to me I guess there is something there and its not related to the crankcase volume. I put forward a theory but thats all it is. Someone has a better idea, then I'm all ears.

Dave

 

[Martin Hamilton]

Dave your thoughs on the fuel charge entering at the top of the housing with a more direct route to the cylinder. Do you see any simular advantages with the Zimmerman valve motors over the HR in this area. Martin.