New CMB .21

[Marty Davis]

I had the opportunity to test one of the newest CMB .21 engines this past weekend on the dyno. WHAT AN AMAZING engine. I ran the engine STOCK except that I balanced the drum. The ONLY thing that was not stock. Here is a dyno run printout.

If you will look at the printout you will see several things that are of interest.

1. The BMEP is the highest that I have seen for several years.

2. "n" the measure of launchability and course acceleration ability is over 1.4 which is considered to be EXCELLENT.

3. The max torque is 99 which will pull about any prop.

As I indicated, the engine was run stock except for the drum balance. There is a considerable amount of performance left in this engine with a change in head clearance and MAYBE a change in compression ratio. I did not document the volume of the head button and piston dish, so I don't know the CR.

I was not able to pull off the image of a better run with a different pipe, but I will post as soon as I can get that.

The timings are suspiciously similar to what I normally run, so maybe Stu was able to pass on my suggestions - who knows. I don't know of anyone that runs the same timings as I do, so I suspect that they may have listened.

As you can see, the Blow Down Time is 29.5 degrees (my desired time). Intakes are low for torque and exhaust low also for torque. You can make RPM with pipe rather than higher exhaust duration.

I know that the engines are terribly expensive right now, but you also can see that you will get a GREAT engine with this series.

Hope you enjoy this information..... Thanks to Norm Doerr and Rod Hendricks for their assistance with this dyno session. Kind of different not having the old crew of Ackerman, Kramer, Coleman and Sparks running the dyno with me.

Marty Davis

 

[John Knight]

Marty,

Which engine, the RS or Valvola? Pipe brand and length for the test will help too.

 

[Marty Davis]

The newest and just released CMB engine. I am not at the shop so I can't give you the letters on the side of the case. I can tell you that the older engine was very lucky to get 1.9 hp and after modification would only get to about 2.2 hp. This engine is a BRUTE.

The pipe is shown on the screen capture as is the length (actually there are 2 lengths shown. The correct length is 8 1/2".

Marty Davis

 

[carlnoki]

Marty;

is the motor like a Nova Rossi style or a picco 67 style. (to just let us know which one you have)

thanks

 

[Mike Larson]

Marty! Said it before, you're the man..... I for one appreciate just seeing your data on the dynamics of two stroke engines....

Being in my infancy compared to most, i'll throw some thanks your way for showing yet another aspect in my quest and many others to learn....

The dyno and the info retained from it is a useful tool indeed...... the new cmb does look to be a serious little 'plant......

Possibly future printouts comparing different manufactures, and modifications could follow?......

another piece o' the puzzle.... throwin it out to ya, thanks, mike

 

[Ron Jefferson]

Marty,

Did you use this pipe you talked about earlier?

"I was asked to help test some new pipes this past weekend. It was 80 degrees here in St Pete (why did I put up with the snow for so many years) I know that it has been warmer than usual most places, but 80 degrees !!!!

All I can say is wait till you can get these. We tested the 40 and 20 pipes and I can say WITHOUT RESERVATION that both are better than ANY pipe I have ever run on either sized engine. The 40 pipe was tested with a stock CMB 45 and the 20 pipe with a modified MAC 21. Both pipes had more RPM and a TON MORE mid range and launchability than my pipes. The 40 was especially impressive. We were able to launch and run one sized bigger prop on a Sport 40 with MORE RPM and a LOT more speed. On the 20, I was able to turn more rpm and come out of the turns harder, also more speed. It was obvious that the pipe was much better. As many of you know, I tested as many pipes as I could get my hands on for my 20 over the years. It has always been by far my favorite class and this pipe puts all the other to shame. Yes, it is a quiet pipe. It is also loud enough (a somewhat different sound) to be able to hear the engine above others running.

I also saw the pipe for 67-90's run and it also appeared to be impressive. Didn't really see enough to make the same strong statement as I have on the 20 and 40 pipes, but it was designed using the same formulas, so I would bet that it is very good.

Can't say more until the builder decides to put out the infomation, but it is cool that someone has done his homework and we have better pipes than at anytime before".

 

[Terry Keeley]

Great info Marty, thanks!

Is it the CMB .21 Valvola RS-7 Hydro/Offshore "Direct Drive" second from the top here:

http://www.hobbysupplies.com.au/CMB%20Engines.HTM

 

[Saugling]

Has anyone a picture of this new 2008 "light weight drum rotor"???

I am just curious what the difference to a 2007 stock drum is.

 

[John Knight]

The newest and just released CMB engine. I am not at the shop so I can't give you the letters on the side of the case. I can tell you that the older engine was very lucky to get 1.9 hp and after modification would only get to about 2.2 hp. This engine is a BRUTE.
The pipe is shown on the screen capture as is the length (actually there are 2 lengths shown. The correct length is 8 1/2".

Marty Davis

Marty,

Is the pipe the standard manufactured one or the shortened version in such high demand?

 

[tnrcboatracer]

From my discussion with Marty, the pipe in the dyno pull shown in his post was an aluminum OPS 3280.

 

[Marty Davis]

Great info Marty, thanks!
Is it the CMB .21 Valvola RS-7 Hydro/Offshore "Direct Drive" second from the top here:

http://www.hobbysupplies.com.au/CMB%20Engines.HTM

Terry:

YES, that is the engine.

CMB .21 Valvola RS-7 Hydro/Offshore "Direct Drive".

Marty Davis

 

[Marty Davis]

The newest and just released CMB engine. I am not at the shop so I can't give you the letters on the side of the case. I can tell you that the older engine was very lucky to get 1.9 hp and after modification would only get to about 2.2 hp. This engine is a BRUTE.
The pipe is shown on the screen capture as is the length (actually there are 2 lengths shown. The correct length is 8 1/2".

Marty Davis

Marty,

Is the pipe the standard manufactured one or the shortened version in such high demand?

The run shown is the standard OPS 3280 pipe with stinger at .282". That was one of the best pipes tested, but not the best.

Marty Davis

 

[Don Ferrette]

"As I indicated, the engine was run stock except for the drum balance."

How far out of balance was the drum and was it a big deal to correct it? :huh:

"That was one of the best pipes tested, but not the best."

And the best one so far was ...................

 

[John Knight]

Great info Marty, thanks!
Is it the CMB .21 Valvola RS-7 Hydro/Offshore "Direct Drive" second from the top here:

<a href="http://www.hobbysupplies.com.au/CMB%20Engines.HTM" target="_blank">http://www.hobbysupplies.com.au/CMB%20Engines.HTM</a>

Terry:

YES, that is the engine.

CMB .21 Valvola RS-7 Hydro/Offshore "Direct Drive".

Marty Davis

Interesting to note the promo details for the engine are 2.9HP @ 36K rpm. The dyno show peak HP at about 29K rpm, unless I misread the report. That is a 7K difference. Any reason for that much of a difference?

I agree with Don F's questions. Would love to see the responses.

 

[Marty Davis]

Great info Marty, thanks!
Is it the CMB .21 Valvola RS-7 Hydro/Offshore "Direct Drive" second from the top here:

<a href="http://www.hobbysupplies.com.au/CMB Engines.HTM" target="_blank">http://www.hobbysupplies.com.au/CMB Engines.HTM</a>

Terry:

YES, that is the engine.

CMB .21 Valvola RS-7 Hydro/Offshore "Direct Drive".

Marty Davis

Interesting to note the promo details for the engine are 2.9HP @ 36K rpm. The dyno show peak HP at about 29K rpm, unless I misread the report. That is a 7K difference. Any reason for that much of a difference?

I agree with Don F's questions. Would love to see the responses.

I would suspect that MOST manufacturers just plug in something for HP and RPM. I doubt that many have access to a dyno. Just a guess, but those figures are not realistic at all. I have NEVER tested a .21 engine that made peak HP over 30K except for one Nova Rossi that we called "Dremel Tool" that I ran in a mono.

Pipe: The best pipe that I ran was an old Irwin pipe that I used on a Nova Rossi. I won 2 US#1's with it and have had it in a box for YEARS. I had my friend weld on a header so that I could use it for my rear rotor engines and this was the first test that I made with it after so many years. The second best pipe was the Andy Brown Parabolic (non-muffled). The muffled parabolic pipe (long muffler section) was way down from the nitro version. Anyone have some AB Parabolics Nitro Version

We are just starting to do testing on some new Carbon Pipes that I designed and one that Rod Geraghty designed. Too early to have reliable results on them.

Now, I will do some mods to the new CMB and see how smart/dumb I am. I am hopeful of 3hp which will be far above the best that we have ever seen on a .21. I think it is possible, but we will see......

I would suggest that if you buy one of the new CMB's that you stay on the rich side of the needle. If not, you will get a lot of detonation and ruin the engine. It makes so much HP that you will need to feed it a lot of fuel to cool it. DO NOT RUN IT IN A LEAN CONDITION. You will also make much more HP if you run it on the rich side of the needle. For your information, when we ran the engine the least bit lean, the HP dropped to below 2 hp quickly.

Marty Davis

 

[Don Ferrette]

Thanks for the info Marty, it sounds promising. What about the balance on the drum rotor, was it a big deal to correct?

 

[Marty Davis]

Thanks for the info Marty, it sounds promising. What about the balance on the drum rotor, was it a big deal to correct?

CMB had done some balancing from the previous model, but I wanted that to be perfect. I took off a lot of metal to get it correct.

Marty

 

[DaveMarles]

Marty, I wouldn't want to be argumentative (as if) but that looks like a very long pipe length to be testing at. I don't understand why you are testing at such a low rpm for a .21. I have on water rpm of just over 34,000 rpm plus and Mauro from CMB runs higher rpm than me although I've not tacho'ed his motor. If I tried to race at 26 to 28,000 rpm I'd be going backwards compared to others. Your dyno graph shows the motor completely without power over 32,000 and only .75 bhp at 30,000. Thats not normal.

Dave

 

[Rokieboater]

So who is carrying these motors so I can get one?

Robert

 

[gratte-tout]

I think it's marked, but I just want to be sure, which % of nitro did you use for the test? It's marked 65%, but it is the right value?

It's quite amazing the ton of nitro you run in USA, here we are at maximum 30%... The price is not the same I also think!

Did you add some head shims for the test?