Would that be a round head Magnum .36, or the older version square head Magnum .36 Pro?

Ward,current version 36XLS
Randy,thanks.

Hi Paul,

Just got back from flying this very engine and pipe setup.  17 1/2" is way too long for this engine with the side to rear header and a standard 40 size pipe.  The correct length measured in a straight line from the glow plug post to the first baffle of the pipe is 15.5" to 15.75".  I have been running mine at 15.625" for the past 4 years with excellent results.  Randy Smith has all the parts (venturi, needle/spraybar assembly, header and pipe)  Thunderbolt 4 stroke plugs work best for me.  I run the engines completely stock except for replacing the RC carb with venturi, needle/spaybar.

Here is what I wrote years ago after a lot of questions and it still current. 
The Magnum 36 is really not a mystery.  I have one with over 100 hours of running time and the only things I've done to it is change the plug and replace a rear bearing.  It is inexpensive and easily obtainable.  It does not require any modification.  I have never had the head or backplate off.  It is light weight (8.25 oz.) and extremely consistent in it's delivery of USABLE POWER. I have flown them on airplanes weighing 41 to 62oz.   (that's a very big weight range) and have never found a need for more power.  I have flown them on airplanes with wing area of 540 to 675 sq. inch, again a big range and still no lack of power.
Here's what I know about how they work best.  I use Sig 5 or 10% Nitro airplane fuel or I mix my own which is identical to Sig (20% total oil, half Klotz and half Castor)
The engine likes to run on the hard side (read more RPM) 10,600 to 11,300 at launch.
Don't look for a 4-2-4 type of run.  They like to run at a soft 2 cycle and don't seem to care if you run at a strong 2 cycle.
Pipe is standard Smith/Werwage 40 size pipe (not grunt pipe)  Set the length to 15.625” as a starting point.  Remember this is a side to rear header and also a different engine.  Ultimately, the pipe will be between 15.5 and 15.75”
PROPS.........they are the key to the Stunt Kingdom! Line length also helps.  I have used a variety of carbon props and am continuing to experiment.
Here's a good starting place. Bolly 12/4 narrow 2 blade, cut to 11.25".  I have had success with props between 10.5 and 11.25 in diameter but this is something for you to decide what you like most. Hope you have a pitch gauge or access to one or someone who can work with you on changing pitch.  If you are like most people you will find your comfort zone in lap times, not speed.  These are adjusted to your liking first with pitch and diameter of the prop and secondly by line length.  Most important is pitch.  That's how you get to the USABLE POWER.  The engine has a fairly flat power curve; once you get there it becomes very happy.  My comfort zone for lap times is a bit faster than most, somewhere between 5.1 and 5.2 seconds (anything slower is just not in sync with my timing).  No matter what lap times you are comfortable with you can get to the usable power with a prop and line length adjustment.
  Venturi size .250 to .270   Supertiger or Randy Smith(Nelson) needle and spray bar assembly. No muffler pressure. Lines 62 to 69' (center of plane to handle)    That's it! No real mystery. 

No Bill  Not  the  Classic Phil setup...
the setup I told him will work very well, in a very fast 4 cycle with 2 across the tops, the Magnum will run this way too, if you don;t use a short pipe. The pipe lenght  will vary in and out with RPMs,   I have setup this exact setup with a 2 and 3 blade prop  it works very well from 17 to 17.5 inches
This is not the Phil setup that i described

and  Phil  as you stated above running the motor in a solid 2 cycle at a faster RPM will result in a shorter pipe lenght, as you slow a motor down the pipe goes out, speed it up and the  pipe comes back in shorter.
I was not  trying to tell him the setup I described was  your  setup, your setup works well  and  if he wants to run it he should use all of your  numbers, his question  never  was your setup, he just asked me if the pipe  would work. I answered  that.
Your commnet about  replacing the  rear  bearing is  a good suggestion that I have made many times  and  I recomend  anyone using this motor to do just that before they kill the one in it, they are not costly to replace but if you let the stock one self destruct it can be  hard on the other motor parts

Regards
Randy

Sorry, Randy!  I saw Magnum 36, pipe, and thought of Phil immediately.  Didn't know what his *actual set up* is.................. now we do!

Bill <><

Hi Randy,

Thank you for validating my setup of the Magnum 36.  It definitely is consistent and trouble free.  For the record I do not recommend the replacement of any part of the engine unless it fails.  The bearing failure I mentioned happened after 100 hours of running over several years.  Ultimately the bearing failure did not cause any additional damage to the engine.  If and when bearing failure does occur care should be taken to replace the bearing with one that is not sealed (similar to or exactly) what comes in the engine.  I lost a new airplane in a flame-out and was able to pinpoint the problem to the use of a sealed rear bearing. 

Speaking of new planes I have a new design that is bigger and have changed the nose construction to easily accept different engines in the 40-size range.  I would like to do some serious testing of your 40 Lite.  It would require a second cowl with no side exhaust opening (for side to rear header) to insure proper airflow over the engine, header and pipe.  I am at the stage in construction where I can make a second cowl to accommodate the PA engine.  If you are willing I would like to get an engine for testing completely set-up by you and I will install it and run it exactly as you recommend including pipe, prop, plug, fuel etc.  Please let me know.

Phil

Hi Phil

I at the  VSC  and  will be  back next week, This is my first chance  to answer my email  and  this forum, I am ssure that  will be  no  problem, I will talk with you about it when I return.
I think you will be very happy with the PA 40 Ultra lite setup, They are several here at VSC  and  this motor is fully tested and ran for going on 4 years now, it is very responsive to turning the power up or down.

Be back tuesday  I will   email or  call  then

Regards  Randy