Before I Start here is a disclaimer - I am not a professional CAD or Mechanical Engineer to know much about this. Also, All information I have is from the internet and have NO practical knowledge. Here we go then. I am currently working on building a High Performance Free Flow Exhaust for my Bike (Yamaha FZ16 Single 153cc Engine). Attached is the CAD design I made for the Exhaust (I am not a professional CAD designer but do it as a Hobby - by the way this is my first CAD Drawing). I was looking in to Details about how to make a FFE. I got a formula to develop a FFE. Here is the link to Document which gives details on how to calculate the Diameter/Length of the pipe. http://www.bgideas.demon.co.uk/tmanual/Tm_Ch4.pdf [Broken] Using the above formula I get the Diameter, Length of the header pipe, secondary cone etc. When I calculate the formula I got Diameter of the Primary Pipe around 30mm. I took 35mm as I will have bend just after the Gases Come out of the Exhaust Port. (Bend After 60mm from the Exhaust Port - Please refer Images Attached) Figure : All Dimensions (Click the image to enlarge it ) Primary Pipe - Measurements (Please refer Images for this) From my mock up CAD design I have taken the Main Pipe (From Exhaust Port till secondary pipe) Diameter as 35mm and length of which is around 650mm (65cms). Secondary Pipe Diameter I have taken as 45mm at the Expanded area (cone moves from 35mm diameter to 45mm diameter in about 100mm length). For the next 200-250mm I have the same Diameter of 45mm till the muffler. I have designed a slip-on muffler which gets connected to the this pipe (you can see in the CAD drawing). Figure : Without Muffler (Click the image to enlarge it ) Figure : 3D View. (Click the image to enlarge it ) Muffler Design - Measurements ( Measurements are taken looking at the Stock Muffler on the bike - Have increased about 10% as of the stock Muffler) For the Muffler I have taken 350mm length, 100mm diameter and Exhaust output diameter without DB Killer will be 60mm (double the Exhaust Port). (ON Stock Muffler this is 20mm) With DB Killer Exhaust output diameter will be 30mm (same as Exhaust Port). (Design - UNDER PROCESS) Figure : Muffler - Without DB Killer (Click the image to enlarge it ) ALL THE ABOVE DIAMETER INFORMATION IS INTERNAL DIAMETER. Bike Information: Displacement: 153cc Engine: 153cc, 4-stroke Maximum Power: 14 Bhp @ 7500 rpm Maximum Torque: 14 Nm @ 6000 rpm Gears: 5 Manual Clutch: Constant Wet Multi Plate Bore: 58 Stroke: 57.9 No. of Cylinders: 1 Valve Per Cylinder: 2 Chassis Type: Diamond Cooling Type: Air Cooling Calculating Primary Pipe Length And Diameter. Exhaust pipe length and size. why we need this?. As the exhaust valve opens, a positive or pressure wave front is created which travels down the exhaust pipe at the speed of sound. As this pressure wave reaches the end of the pipe, it expands and a negative or suction pulse travels back up the pipe towards the engine. As the negative wave front in turn reaches the cylinder, it reverses again and moves back towards the end of the pipe. This fluctuating pressure pulse effect can be used to great advantage in tuning the engine. If the system is designed in such a way that the negative or suction pulses return to the cylinder at overlap T.D.C., then they will assist in clearing the combustion chamber of exhaust gases.In turn, this will cause a depression at the inlet valve, which will help draw in the inlet charge. The following formula can be used to calculate the ideal length for Primary Pipe Length: LengthOfPipe = (129540 x E.T) / (R.P.M. x 6) Where: L = Primary pipe length in mms measured from the exhaust valve head. E.T. = Exhaust valve duration in degrees from point of valve opening before B.D.C plus the full 180 degree stroke up to T.D.C. (Assumption*) (Reverse calculation for the stock exhaust to get this value) R.P.M. = The estimated revs, at which max. power will be achieved minus five hundred. Example: Estimated maximum power R.P.M. = 7500 E.T. = 195 ~ 210 (I have reverse engineered the ET values from the Stoke Exhaust length) R.P.M. will be 7500 - 500 = 7000 (Max Power @7500 and FZ16 Redlines at 9000rpm) Result Primary Pipe Length: L = 600mm ~ 650mm (based on the ET Values 195, 210 Respectively) Having calculated the primary pipe length, we must now calculate the diameter as follows : Divide "L" by 10 to bring it to cms. Call this "L2". (60) Take the cylinder capacity in ccs and double it. (Say 153 x 2 = 306) Divide by "L2" as previously calculated. (306 / 60 = 5.1) Divide by 3.4 (5.1 / 3.4 = 1.5) Find the square root (√1.5 = 1.225) Multiply by two and add 0.3 ((1.225 x 2) + 0.3 = 2.75) Multiply by 10 to bring it back to mms. (10 x 2.75 = 28) = 26~28mms. (Diameter based on ET Values) - I have taken this as 35mm as, we don't have negative pressure from other cylinders as this is a Single Cylinder Engine. NOTE : The Above Calculation is for Multiple Cylinder - for 4-2-1 or 4-1 Pipe configuration. Above length is the Primary Pipe Only *Assumption is the Mother of all f^%$-ups. QUESTION TIME: 1. I am using the Formula LengthOfPipe = (129540 x E.T) / (R.P.M. x 6) to get the information. This formula is mainly used for Engine with more than one Cylinder. Is there a specific Formula we can use to calculate Pipe Diameter / Length for a SINGLE Cylinder Engine. 2. Also Can someone please review if my calculations are all ok.