New CMB 21 Rotor Assy

[Marty Davis]

Stu sent me one of the new Rear Drum Rotor Assemblies & Carb for the 2008/2009 CMB RSV engine. As most who bought that engine know, it had terrific RPM but lacked torque.

Let me tell you that this new rear rotor assy (and carb) is an amazing improvement to the .21 CMB engines (also a bolt on for the MAC 21). We tested the assy on a stock 2008/2009 CMB 21 and it pulled props that I pull with my modified engines. It is something that I can't wait to test on my own engines. I have had great performance with the old Andy Brown designed slant rotor assy's on my MACS and my CMB's but after seeing the performance gains with the new rear rotor assemblies, I am positive that my performance with the modified engines will be substantially improved.

I can't recommend enough this new assy. I know it is pricy, but for those who really are performance driven, it is WELL worth the cost.

As I understand it, the newest CMB's come with the new assy and carb. That should be a really good engine in stock form and also in modified form.

I have no idea if Stu has these in stock, but you might want to touch base with him to see.

 

[BB Mods's]

I concur, The new VAC intake has given me back all the torque my MAC"s have been missing. Long gone are my days of sub 3.2" pitch 21 props, I'm having no issues with 3.6/3.7" heat props that were cut to find the limit of my engines.

If your a valvola/mac 21 owner, this is the only intake option you should be considering.

Big thanks to Stu and Marty for improving my 21 program 10 fold. B)

 

[Jack ODonnell]

Stu sent me one of the new Rear Drum Rotor Assemblies & Carb for the 2008/2009 CMB RSV engine. As most who bought that engine know, it had terrific RPM but lacked torque.

Let me tell you that this new rear rotor assy (and carb) is an amazing improvement to the .21 CMB engines (also a bolt on for the MAC 21). We tested the assy on a stock 2008/2009 CMB 21 and it pulled props that I pull with my modified engines. It is something that I can't wait to test on my own engines. I have had great performance with the old Andy Brown designed slant rotor assy's on my MACS and my CMB's but after seeing the performance gains with the new rear rotor assemblies, I am positive that my performance with the modified engines will be substantially improved.

I can't recommend enough this new assy. I know it is pricy, but for those who really are performance driven, it is WELL worth the cost.

Marty; Can you explain why the new CMB rear rotor has more torque than the original slant rotor. J.ODonnell

As I understand it, the newest CMB's come with the new assy and carb. That should be a really good engine in stock form and also in modified form.

I have no idea if Stu has these in stock, but you might want to touch base with him to see.

 

[Marty Davis]

Jack:

Did you miss posting your thoughts?

It cracks me up that you are listed as a "Beginning Boater" by your name. Maybe one of the Admins can change this. I realize that it has to do with the number of posts that you make, but a "Beginning Boater"

 

[BB Mods's]

He posted them in your quote

here is what he said

Marty; Can you explain why the new CMB rear rotor has more torque than the original slant rotor. J.ODonnell

 

[Marty Davis]

He posted them in your quote

here is what he said

Marty; Can you explain why the new CMB rear rotor has more torque than the original slant rotor. J.ODonnell

Jack:

I have no idea why, I just know that it works amazingly well. Do you?

 

[Duck899]

Case volume

 

[dave roach]

I beleave you are forcing more fuel up between the block and the liner and not as much in the crank case.

Dave Roach

 

[HendricX]

We in Europe know these facts for months .... read comments on Dave Marles forum : L I N K

And just for the fun .... how the new intake looks like :

 

[tnrcboatracer]

Yes, the torque is improved due to reduced case volume which increases pumping efficiency and port velocity. The intake tract core volume outside of the case on the carb side; the angled carb places the tract volume on the case side of the intake tract seal (drum OD). This volume is significant. Torque is a function of case volume and port velocity. One of the highest torque motors I have seen in hte old K&B 1.0. The transfer port depth was only anout .1". I would have sworn that it wouldn't flow any air, but ****, did it pull a prop........Very efficient fuel burn, also.

Edit: the port velocity may also be increased due to the inertia of the flow already directed toward the boost and xfer ports. THe source of the vacuum that induces the flow is the underside of the piston, which is inside of the sleeve. I wonder how efficiently it flows on hte top end?

 

[What up Dog]

I concur, The new VAC intake has given me back all the torque my MAC"s have been missing. Long gone are my days of sub 3.2" pitch 21 props, I'm having no issues with 3.6/3.7" heat props that were cut to find the limit of my engines.

If your a valvola/mac 21 owner, this is the only intake option you should be considering.

Big thanks to Stu and Marty for improving my 21 program 10 fold.

Tommy what 21 Program are you talking about? Is this somthing that's out or is this somthing you made?

The Dog

 

[BB Mods's]

Not that type of program

Stu and Marty have been helping me get my .21 powered boats "up to speed"

Big thanks again to Marty for this Cougar hydro platform, it's like taking the boat out of the formula and let's me spend my engine/pipe/prop testing time more effectively.

Brian, if you still have your 21 sgx, that mac needs this new vac intake for sure!

 

[Chris Wood]

would it have to do anything with the fuel track ( or path ) more direct or efficent patch than before?

 

[DaveMarles]

The difference in case volume between the EVO21 and the RSV07 is about 0.3cc which is going to make almost zero difference to the performance. You can increase the case volume on a .21 by more than 1cc and see little change in performance and you can pack every corner of the crankcase to reduce volume and see little difference and that difference will defintely not be an improvement in torque and throttling. Been there, done that. I think CMB believe the improvement is because its a more direct route to the transfer / boost ports. On the latest VAC crankcase they have altered the transfers so that they link more directly with the induction tract and tell me that this is a good improvement. The change in performance on this motor is fundamental. It has changed also-ran racers here (UK), who couldn't get a 21 motor to perform well, into experts . I heard it said that it's because it's the first time we had a 21 where the conrod is not passing directly over the end of the induction tract. Who knows?

I do know that we had our first race here last Sunday and I was able to throttle down for long periods if necessary and pick up was instant every time plus I did open water and 2 x 20 minute races on the same plug which is unhead of for me. (Odonnell 99T). By the way we do use nitro in Europe.

Dave

 

[Ian Inverarity]

I heard it said that it's because it's the first time we had a 21 where the conrod is not passing directly over the end of the induction tract. Who knows?

Dave

Dave,

Do you think this is the reason tho? Looking at a crank induction 21 the beam of the conrod partially obscures the induction tract for only the first part of the inlet duration, after that the big end of the rod is above the induction tract until the inlet duration is complete.

Ian

 

[DaveMarles]

I heard it said that it's because it's the first time we had a 21 where the conrod is not passing directly over the end of the induction tract. Who knows?

Dave

Dave,

Do you think this is the reason tho? Looking at a crank induction 21 the beam of the conrod partially obscures the induction tract for only the first part of the inlet duration, after that the big end of the rod is above the induction tract until the inlet duration is complete.

Ian

I don't know, its just an idea. Crankcase volume is not the reason, of that I'm sure.

 

[Ian Inverarity]

Dave,

What about mechanical factors, is there anything in the new drum or housing that may reduce the rotational drag of the drum?

Ian.

 

[Marty Davis]

I heard it said that it's because it's the first time we had a 21 where the conrod is not passing directly over the end of the induction tract. Who knows?

Dave

Dave,

Do you think this is the reason tho? Looking at a crank induction 21 the beam of the conrod partially obscures the induction tract for only the first part of the inlet duration, after that the big end of the rod is above the induction tract until the inlet duration is complete.

Ian

I don't know, its just an idea. Crankcase volume is not the reason, of that I'm sure.

Dave:

Are you running the Carb as it comes or are you doing anything different? I had a little trouble with the low speed needle setting, but I had another problem which might have contributed to the setting problem.

 

[DaveMarles]

I heard it said that it's because it's the first time we had a 21 where the conrod is not passing directly over the end of the induction tract. Who knows?

Dave

Dave,

Do you think this is the reason tho? Looking at a crank induction 21 the beam of the conrod partially obscures the induction tract for only the first part of the inlet duration, after that the big end of the rod is above the induction tract until the inlet duration is complete.

Ian

I don't know, its just an idea. Crankcase volume is not the reason, of that I'm sure.

Dave:

Are you running the Carb as it comes or are you doing anything different? I had a little trouble with the low speed needle setting, but I had another problem which might have contributed to the setting problem.

Marty, I'm running the stock carb bored out a little. I had no problem with the carb or low speed needle. I'm just converting mine to use the OS radio needle.

If the boat is not on the stand perfectly level the motor floods very easily because the carb is so low compared to the fuel tank.

Dave

 

[Jerry T]

is the carb the same as the older CMB 21 with the angled housing?