basmntdweller

My Feedback: (9)

I have an old OS 61 VF rear exhaust pattern engine in an old MK Joker pattern plane. I had always used 11-7 props back in the 80's when flying a 60 size plane. I put an 11-7 on this engine and was only getting 11300rpm. I tried a 12-8 and still only got about 11200. I was expecting quite a bit more out of this engine. I just replaced the bearings in the engine after noticing a little roughness but it didn't show any gains on the tach. Is this all I can expect out of this engine or am I propping it wrong?
I tried to fly it with the 11-7 but was very unimpressed. These older pattern planes used to haul a**. This one can barely get out of it's own way.
Thanks,,,basmntdweller

Bax

My Feedback: (11)

The O.S. Max .61 VF rear-exaust, front-intake engine should turn nearly 13K on the ground with an 11x7 prop and 10%-15% nitro fuel. Of course different brands of props that have the same diameter and pitch can give widely-different RPM readings.

The main consideration would be the type of exhaust system you're using. Optimum performance would come by using a tuned pipe that was correctly-adjusted. This engine was intended for pattern use at the time, and it was also intended to be used with a pipe.

JWN

My Feedback: (42)

In actual use, the OS .61 VF would turn an 11X7 at, or slightly above, 15K on the ground.

Make sure you are using a cold plug such as a Rossi 8. After that, start playing with the pipe length. 13" from the plug to the first baffle in the pipe would be a good starting length. Then, start cutting 1/4" inch at at time until you reach max revs.

John

basmntdweller

My Feedback: (9)

14-15K is more like what I was expecting from the enigne. What I didn't expect was for it to turn both an 11-7 and a 12-8 essentially the same RPM. Can I expect tuning the pipe to improve the performance from 11300 up to 14 or 15K?

PS, I tried it with only the header, no pipe and it lost about 300 RPM on the ground.

Thanks,,,basmntdweller

ChuckAuger

My Feedback: (12)

If you are only getting 300 rpm gain from the pipe, I'd have to say it doesn't sound like it's set properly. On the other hand, I don't think you are gonna get 2.5~3.5K more by setting the pipe.

I'd say the engine sounds a little sick, and the pipe sounds like it's not tuned.

P.S. I was gonna guess 14.5K on the 11 X 7

JWN

My Feedback: (42)

With just an open header, I would expect extremely poor performance. The exhaust duration is far too long to run the engine without a pipe.

Would working on the pipe length improve the performance? That depends on what it's set to now. What fuel and plug are you running? 15% should be about max for this engine with 5-10% being more in-line with what I remember guys running. Plug selection is also very important. Hot plugs will cause pre-ignition and a substantial power loss. The fact that you are seeing similar revs with the two props you mention leads me to believe the pipe is simply too long, but be sure you have the right fuel and plug first as they are the easiest things to change at this point.

John

basmntdweller

My Feedback: (9)

I'll try new fuel and a new cold plug. I don't know what is in it at the time so I'll have to check that out(probably an OS#8). I'll probably have to plan for a night at the field and spend it just trying to get this thing tuned in.
Thanks,,,basmntdweller

ewspears

My Feedback: (12)

Just got finished testing one of my 61VF's yesterday. This one had sleeve removed from 7D carb, Had the Exhaust port raised .020", running 15% Omega, Macs #1160 Muffled Tuned Pipe set at 16.25" (Glow Plug to end of rear converging cone)
It will turn a standard APC or Rev Up 11 X 7 at 15500 RPM.

A stock one with sleeve in carb will turn same props just over 15K with same pipe stretched out 1/2"

Azcat59

Some good answers above. I flew this fine engine for several years in pattern competition in an MK Arrow. As I recall, I was using an OPS pipe, Macs header, and a Rev-Up 11 x 7 1/4. My pipe setting was 12 1/4" from the glow plug to the high point of the pipe (where it broke from a taper to the straight cylinder). I used 10% Cool Power or Red Max, and I turned around 14,500 on the ground, sometimes a little more. I would burn a K&B 1L plug out every several flights, but the OPS plug would hang in there longer. I remember trying to shorten the pipe another 1/8" and the needle got extremely sensitive so the 12 1/4 was at maximum for my altitude and set-up.

I also flew the MK Joker with a long stroke OS RF, and I believe you would be better served flying that larger airplane with a larger prop (12 x 8 for instance) which will require a longer pipe, and lower rpms. (And less noise!)

Clair Sieverling

Flyboy Dave

My Feedback: (21)

....if you are looking for rpm's....put an 11-6 MAS on
the thing....make sure the prop, spinner, and back-plate
are balanced to the "enth".

It'll spin 16-17K on 15%....on the pipe. Try it....