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Next you need to pick the pipe diameters. A straight header, if you don’t use the above tapered design, has a diameter with an area equal the exhaust port to around 1.2 times that size. The large diameter is around 2 to 3.5 times header diameter. Notice that the pipes above use a factor of nearly 3.5. The stinger diameter is between .6 to .75 times the header diameter. The stinger length is of secondary importance. A classic pipe diffuser has a horn coefficient of 1.2 to 2. I find that lower, not more than 1.4, is better. There are several on line pipe design guides that vary from simple to elaborate. he simplest one is on Martin Hepperle’s excellent site at https://www.mh-aerotools.de/airfoils/ javapipe_en.htm Note that the exhaust temperature is in degrees Kelvin. That’s the temperature in ⁰C + 273. Fig. 10 is a more elaborate designer: PROPWASH 6 April 2020 Fig. 5 - Highly Developed Tuned Pipe Fig. 6 & 7 show simpler classic and modern pipe designs for the same conditions and tuned lengths. The classic design, Fig. 5, has a two-stage diffuser while the modern design, Fig. 7, has a multi-stage diffuser. Both have low horn coefficients of around 1.2. Fig. 6 - Classic Pipe Design Fig. 7 - High Performance Design To design an exhaust system you need to start with the exhaust port area. Fig. 9 shows the best current design of the ducting leading up to the pipe. This has been developed for high power kart and motorcycle engines. The duct area actually contracts between the port and a point 1.5 times the bore down the duct. Nitro exhausts don’t need divided windows since we don’t worry about ring expansion into the window. However, the scavenging flow created by the three exit angles out of the cylinder may be important. Fig. 8 shows the pattern for an advanced nitro exhaust duct. Fig. 8 - Nitro Engine Exhaust Duct Fig. 9 - Exhaust Duct Geometry Fig. 10 =- Frits Overmere’s Pipe Design Diagram A speed of sound calculator is at https://www.engineeringtoolbox.com/speed-sound-d_519.html Again note that the temperature is in degrees Kelvin (⁰C + 273) or Rankine (⁰F + 492). A much more exotic designer is this spreadsheet: excel : http://users.telenet.be/jannemie/Jan...oke%201.2.xlsm manual : http://users.telenet.be/jannemie/Jan...oke%201.2.docx All the above design systems are based on years of experience and get a pipe that is close to ideal. Simulation programs are a good way to verify and refine pipe designs. Dyno testing, of course is the best way to prove a design. In the next part we will look at intake and transfer designs. I believe nearly all model engines could use improvement in that area.