CL Stunt Pipe Length Chart
with data supplied by Randy Smith
(Pipe length is measured in inches from the glo-plug to the first baffle)

 

 

  PA AERO
  RO Jett*
40\51\61\65\75

OPS
OS
ST

OPS
OS
RO Jett*
YS

OS
YS
Exhaust Timing 130 degrees 135 degrees 140 degrees 145 degrees 150 degrees
Launch RPM          
  9000 18  1/2 18  3/4 19 5/8 20 1/4 20 7/8
 9500 17  3/4 18       19       19  5/8  20 3/8
10000 17 5/16 17  1/2 18 3/8 19       19 3/4
10500 16  3/4 17  1/4 17 7/8 18 5/16 19      
11000 16 5/16 16  3/4 17 5/16 17  7/8 18  5/8
11500  16         16 5/16 16  3/4 17  3/8 17 3/4
12000 15  3/4 16  1/8 16  3/8 16  3/4 17 1/2
12500 15  3/8 15  3/4 16 1/16 16  3/8 17      
13000 14  7/8 15  3/8 15 9/16 16        16 1/2

(*Note:  The RO Jett series of engines are not all timed the same.  Check which model you have.)

Starting points for the RO-Jett and PA AERO 40-65 engines:

Select a prop diameter (in inches) of around:
        11.5 for the 40
        12 for the 51
        12.5-13 for the 61
        13-13.5 for the 65

Select (or set) the propeller pitch to around 4 to 4.25 inches

Launch RPM for all engines should then be around 10,000 to 11,000 RPM

The pipe starting point should then be around 16.75 - 17.5 inches.

Move the pipe out (lengthen it) if:
        the engine run is too hot or it comes on too strong in maneuvers.
        the plane speeds up during the flight.

Move the pipe in (shorten it) if:
        the plane does not come on strong enough in maneuvers    
        the plane slows down during the flight.

In general:
        2-blade props, shorter props, and those with narrow blades will use a slightly shorter pipe setting.
        3-blade, longer props, and those with wider blades will use a slightly longer pipe setting.

When you find the proper pipe setting and run characteristics, you can also:
        add pitch to the prop if the plane is flying too slow
        decrease pitch to the prop if the plane is flying too fast.

Remember, all of the above are starting points, not absolutes.
We are trying to tune the pipe length to match the in air rpm of the engine.
Venturi size, nitro percentage, number of head shims, temperature, all play a part
        along with YOUR preferences and style of flying.
 

Sample Venturi/Prop/Pipe Length/Launch RPM Combos
For The RO-JETT and PA AERO 40 - 65 Engines

Engine
size

Venturi Size

Propeller Type

Pipe length
in inches

Ground Launch
RPM's
40 .152-.166  Bolly 11.75 x 4.25 cut to 11.5 inches 16.75

11000
51 .169-.182  Bolly 12 x 4.25  3-blade as-is    
 Bolly 12.2 x 4.2  2-blade as-is		 17 to 17.25
17 to 17.25

10,500-10,800
61 .189-.201  Bolly 13 x 4.5  2-blade as-is
 Bolly 12 x 4.25  3-blade  as-is
 Bolly 13 x 4.5  3-blade cut to 12.5
 Bolly 13 x 4.5  3-blade as-is 
 Bolly 13.5 x 4.5  3 blade as-is		 17 to 17.25
17 to 17.25
17 to 17.25
17.25 to 17.5
17.5 to 17.75

10,500-10,800
10,500-10,800
10,500-10,800
10,000-10,500
9,800-10,000
65 .189-.209  Bolly 13 x 4.5  3-blade as-is
 Bolly 13.7 x 4.5 3-blade (extended)		 17 to 17.50
17.5 to 17.75

10,000-10,800
9,800-10