First off I would like to apologize to Stu for not making a post about the new 21 VAC engine.
The new 21 is completely new except the head button and the water jacket. The inside of the case now looks
like a shrunk down version of a 67/ 80 case.The tranfer ports are alittle narrower and the boost port is wider
than any of the other CMB 21 cases.It comes flowed for the new back plate.The exhaust port area is smaller
in volume than the last case, but not as small as the green head case.The lower part of the case is smaller
in volume also and combined with the new back plate assembly the torque of this engine is beyond any
21 I personaly have ever used.
The piston has a narrower and shallower dish, thus giving a slightly smaller total compression volume.
The sleeve ports are all different also. NO MORE TRI PORT EXHAUST.The width is the same as the
tri port but it is flat across the top of the port and the timing is 184.
The transfer ports are narrower to allow for the wider boost port. They are wider top to bottom to give them
the time area back and they have 130 degrees of timing.
The boost port is .408 wide verses .259 on the old sleeve.The time area is much larger and the timing is 129.
As you have seen the crankshaft is shaped differently and also has two bronze slugs to help with the balancing.
The motor is smoother than it's earlier versions.
The con-rod is larger and beefier at the bottom and has a V machined on the back of it.
The back plate assembly works so much better I could'nt believe the amount of torque gains that I got.
The timing on the rotor that I got is it opens 32 degrees abdc and closes at 61 degrees atdc.
One might think that it's alittle short on the opening side but you might want to try it in your boat stock
before you change it.
NOTE: Take set screw out and red lock tite it back in before you run it. I got lucky and noticed that it had backed
out before any damage had happened to the bushing or anything else.
The carb works great stock but the trick to it is open the low end needle out .I got it to work good if it just entered
the spray bar when it's about 3/4 closed. Stu wanted me to try it cut out to see if there was much of a differance so I did and I did'nt see much on the lowend but the top end was better.
I tested the engine bone stock with .008 deck with the CMB new 21 pipe in my rigger. I used 60% nitro and a Mc Coy
# 59 glow plug.
The first thing that we noticed was it left the beach so easy we quickly knew that we were going to need more prop.
We went from a H-10 with 3.75 to a 1450 cut to 48mm and pitched to 4.2 .
This was still too small of a prop so we started shortening the pipe. We cut off .500 and ran it with no header and then it started to really turn on the rpms. When all was said and done on the boat we were testing it in it was exiting the corners at 73 mph and was 76 mph in the straights.In my opinion that is pretty good for a bone stock 21.
It still has a good bump and compression after three gallons of fuel. The only thing that I have replaced is a set of bearing.
If you have any Question I will be glad to help any way I can.
Mark
