Is anybody using the CMB21 RSV07 engine? If so what is their opinion on it, I hear that it is a bit peaky and difficult to get good power out of it compared to the green head and 21RS engines.
CMB21 RSV07
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 Teslaboats |
CMB21 RSV07 |
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Posts: 23 (09/18/08 04:53:41) |
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BoBoKwok |
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Posts: 705 (09/18/08 05:08:37) |
I don't have experience on 21RS.
My RSV07 is as good as the green Vavola. I don't find it peaky and difficult to get good power. |
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PavelV |
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Posts: 1 (09/25/08 08:36:10) |
Hi, I have RSV07 engine, but it is problematical motor - as for me. This year I prepare engine for wc, but I use this engine only on first race. on wc i used
older version. I can't set high rpm, in curve dropt "power". I don't know reason - (maybe only I) .
Pavel |
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BoBoKwok |
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Posts: 706 (09/25/08 20:52:44) |
CMB has never been an easy motor.
It is much better nowaday but I used to correct manufacturing defect by myself. Someone told me that the spray bar of his carb on RSV07 is not drilled well. You may check it. |
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Teslaboats |
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Posts: 25 (09/26/08 04:56:59) |
Pavel, You are not the only one. After having tried various props Mocom 2023, 1924 even a small Radioactive X45 prop and gear ratios 15/30 and 15/31 I have not had good results. The engine seems to bog a lot, worst is left turns were motor slows down and runs erratic but right turns it speeds up, adjusting the pipe does not seem to improve this either. Out of the box the head clearance was 0.2mm. Running on 25% nitro this resulted in many blown plugs and some fine pitting on the head and piston even when running sickly rich. Adding shims to give a head clearance of 0.6 has made it run better. I wonder if raising the transfer ports would improve things as the blowdown is already at 62º, but surely CMB cannot have got this wrong. Any recommendations for props and gear ratios would be welcome. -Tes |
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BoBoKwok |
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Posts: 708 (09/26/08 06:03:34) |
I run 0.2mm squish clearance on 35% nitro withour problem.17/33 or 17/34 on 1924 or 2026 I don't think there is a std marine or car motor with 62 degree blowdown from the factory. My measurement is 54 only. Your motor must be ultra modified.:P |
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Teslaboats |
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Posts: 26 (09/26/08 13:58:39) |
Bobo, I checked the engine again today and it is 62º blowdown. For reference the timing on my engine is :
Exhaust duration = 184 º Transfer duration = 123 º Boost port duration = 127 º Induction duration = 219 º
The engine has not been modified it is factory standard, it still has all the chrome in the ports. Your engine with a blowdown of 54 sounds more like the 21RSV, not the 21RSV07 does it have the larger red water cooling jacket? Maybe the timing varies on these engines which is why some seem to run better.
Last Edited By: Teslaboats
09/27/08 12:17:54.
Edited 1 times.
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 Dave Marles |
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Posts: 858 (09/26/08 15:30:38) |
Thats 59 degrees exhaust to transfer blowdown not 62. Which engine are you measuring?. The older redhead with the ugly shaped exhaust ports or the latest
motor with normal shaped main exhaust and 2 smaller sub exhaust ports?
I use the latest RSV piston liner in my greenhead engine (with suitable case mods)and I like that piston liner very much. I measure the latest one at 184 ex, 124 transfer , 60 (2 x 30) blowdown. The older redhead piston liner was useless in my experience. Bobo is measuring squish with digital calipers. I run smaller head volume than standard with 0.35 to 0.4mm squish but I check it with squashed solder method. The head on the later RSV motors has a narrower squish band than on the older motors which would normally give less torque but better rpm, all other things being equal.
Dave
Tuning software
Last Edited By: Dave Marles
09/27/08 04:28:06.
Edited 2 times.
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BoBoKwok |
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Posts: 709 (09/27/08 07:50:38) |
I think my data is for RSV07.
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Dave Marles |
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Posts: 859 (09/27/08 10:00:11) |
Bobo, You should take blowdown as exhaust less transfer because boost area is very small and in terms of blowdown effect it is relatively insignificant. Plus
we are then on the same page. :-)
Dave
Tuning software |
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Teslaboats |
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Posts: 27 (09/27/08 12:32:29) |
Dave, The liner I have is the normal exhaust shape, wider at the top with two small additional ports at the sides. I made a typing error and the transfer was measured at 123º. I guess there could be some difference in the measuring but from what you say I think I have the same liner as you. I will put it back in the boat and give it some more testing time. |
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Dave Marles |
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Posts: 860 (09/28/08 22:37:24) |
You can have more torque in the corners if you machine the transfers to give 127 degrees duration. Reducing the head volume to 0.19cc will help too.
Dave
Tuning software |
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PavelV |
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Posts: 2 (09/29/08 08:31:37) |
If I reduce volume to ,2 I have problems with glow plugs - damaged after 5 - 10 minutes
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Dave Marles |
EDITED RE. RESTRICTOR TYPO | ||
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Posts: 861 (09/29/08 10:16:13) |
.21's will break plugs but if it happens too easily then check squish is in the region 0.3 to 0.4. no bigger. Next, reduce waterflow and run richer. I
use a restrictor in the waterline of 1.5mm bore x 20mm long in all air temperatures. Restrictor is between waterpickup and cylinder head. Running water
through pipe /manifold first is not an option. If it still blows plugs then lengthen pipe 1 or 2 mm.
Dave
Tuning software
Last Edited By: Dave Marles
09/30/08 01:34:36.
Edited 1 times.
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PavelV |
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Posts: 3 (09/29/08 20:16:07) |
Thanks, I try it
Pavel |
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Dave Marles |
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Posts: 862 (09/30/08 01:36:46) |
Please note I edited the size of the water restrictor from 0.2mm to 1.5mm.
Dave
Tuning software |
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Andras |
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Posts: 9 (10/02/08 09:29:42) |
Dave Marles wrote:Dave, can you give me any explanation why it's not an option? I do it on my 3.5 (AXE not CMB, but I guess it's all the same) and never thought this could be a problem. I use crankcase cooling first, then it goes to manifold and head is the last, still I think that my engines run too cold (30% nitro and not using them rich at all) Thanks in advance! Andras |
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Dave Marles |
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Posts: 863 (10/02/08 12:07:37) |
The reason I say head first (backplate first is also OK ) is because I think you need a consistent cooling water temperature and the temperature after passing through the exhaust system will vary, for example, if the pipe length is altered or needle setting is altered. In my experience most people run too much watercooling on .21 and .45. Its better to run very little cooling water and to cool the motor by running richer. This way there is more fuel passing through the engine and hopefully more power and especially power through and out of the corners.
Dave
Tuning software |
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Andras |
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Posts: 10 (10/03/08 03:23:37) |
OK, that's a fair explanation, thanks a lot, Dave!
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uros |
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Posts: 1 (10/17/08 02:03:30) |
Hello,
I have this engine RSV07 Valvola too. I could not make any good result on races for a whole yeare. Engine is very quiete with low RPM and without speed and trque (crap). I had the same problems as all the others here. For this engine you must have gold CMB glow plugs all the others are not good!!! This are the only plug wich my engine don't brake after 1 run and I use 0,4mm between piston and chamber.
Uros_S.
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BoBoKwok |
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Posts: 711 (10/17/08 07:39:00) |
if you find not enough torque to rev the usual prop, just use a bigger gear ratio(1 more teeth in plastic gear)
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