This is the Magnum 28 XLAII RNV engine. 

 

The Magnum 28 and the OS 25FX are the most popular B class combat engines with OS outnumbering the Magnum fliers by quite a bit.

 

The Magnum does have some things going for it.  First it is $30 cheaper and cheaper parts.  I like the needle valve and I like fitting an air filter that works.  The OS is a smoother and easier starting engine.  Both develop similar power.

 

I set up the engines for combat flying, prefering a Master Airscrew 9x4 prop. .

 

I also run a DuraTrax air filter designed for RC cars.  It fits perfectly over the carb and is held on with a small zip tie.  I feel this goes a long way from keeping dirt out of the engine on the crashes and landings.  They can be purchased from Tower Hobbies.  Click here to view Tower web page.  Never been able to get one to fit on an OS 25 FX.

 

The rear needle valve is vertical.  When shipped from factory the RNV assembly is designed to stick out to the left side of engine.  Pictures on the manuals show RNV mounted vertically or horizontally.  The horizontal mount is prone to break because it sticks out so far.  I have never had the vertically mounted unit to break. To mount vertically you will need to remove the phillips head screw and the tube connection to the carb and swap positions.  Then mount RNV vertically to the backplate.  Be advised that you'll likely need to remove some material from  the left rear corner of your engine mount.  The vertical mount adds some depth to the engine. 

 

Mufflers for the OS, GMS and Magnum interchange.  So I use the cheapest.  Shown is the GMS muffler.  I also add the Dubro exhaust deflector.  This does cost you a couple hundred rpm.Although a combat plane is dirty, abused and tough,  deflecting the exhaust oil can only help keep the oil from seeping under and into the tape, radio, etc.

 

I Feb. 2008 I experimented with different mufflers.  The stock magnum muffler, the GMS .25 muffler, the Macs one peice muffler.  Using a Master Airscrew 9x5 prop here are the results:

 

Muffler                               RPM

Magnum                           12,600

Macs One Peice             13,100

GMS .25                          13,500

Kentucky Mousse Can   14,300

 

Looking at the magnum orginal muffler, it always struck me as small.  The GMS muffler is almost 2" longer and the Macs the longest of all.  I think the muffler chokes the engine.  The mousse can made the engine run like a scalded dog.  The mousse can is too strong of tuning effect or too short.  With the 9x5 prop it wanted to overheat and blow plugs.  This is generally a sign the pipe is too short or the rpms too low.  I switched to a 9x4 prop and got a full run.  I now  run the Magnum with a 9x4 mousse can.  Get the Macs header in marine style.  It angles down and you can just tuck the can under the wing. 

 

I also use a weak link in my throttle servo connection.  I slid a short peice of fuel tube over the arm.  I pin the tube to the arm with a short peice of 2-56 rod through one of the rod holes.  Then I attach the clevis to the tube not the arm.  You'll need the extra throw the length provides as the movement is loose.  This arrangement is intended to provide break away before the throttle servo strips.  I have most of the problems with the throttle servo stripping.  The elevator and ailerons have weak points in the surface or linkange.  But the throttle servo everything is strong. The control rod connects to the metal arm on the carb.  The gears strip.