Megaphone exhausts have enjoyed a high degree of popularity with motorcyclists for decades. Producing a deep tone and a free-flowing path for escaping exhaust gases, megaphones can be found on vintage race bike, cruiser type motorcycles and even some modern sport bikes. Although these cone shaped exhausts can be purchased through one of the many aftermarket manufacturers, fabricating a megaphone is well within the reach of the determined home mechanic. One of the most popular methods, rolling a cone, has been simplified by using a software program to determine the dimensions your megaphone should have. The second, more exotic method of hydroforming a cone using pressurised water is possible with the proper preparation.
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Things you need
- Mild steel sheet, minimum thickness of 0.9 mm (0.035 inches)
- Metal shears
- Welding equipment
- Pressure washer or high-pressure water pump
- Nut and bolt
Determine the size of your intended megaphone exhaust. Sketch out your intended design to help visualise the dimensions of your exhaust. Include the final length of the megaphone's cone and any inlet and outlet angles that you intend to build into the exhaust. Take into account the outer diameter of the exhaust pipe that connects to the headers. For example, the header pipe has an outer dimension of 3.7 cm (1.5 inches), so the inlet end of the cone must have an outside diameter of 3.7 cm (1.5 inches) to be welded on or have an inside diameter of 3.7 cm (1.5 inches) to be slipped over the header pipe.
Open the cone layout program and enter the specifications of your megaphone exhaust into the fields. Begin by entering the inlet diameter and the angle of the inlet. An upward swept megaphone could feasibly have anywhere between a 1-degree to a 45-degree angle. Enter the outlet diameter and angle next. Finally, enter the total length of your exhaust cone.
Save your megaphone exhaust model and print it out. The software will create a template for you to copy onto your steel sheet. Depending on the size of your megaphone exhaust, more than one page may be printed.
Trace the template onto your sheet metal and cut out the form using a hand grinder or metal shears.
Roll the metal form until the edges meet. Weld the seam carefully, being sure that there are no gaps or holes along the length of the cone. If desired, grind down the weld seam and paint the cone before mounting.
Mount the megaphone to your motorcycle either by slipping the inlet over the header pipe or by welding it into place, depending on your application.
Rolling the megaphone
Determine the size of your intended megaphone exhaust. Sketch out your intended design to help visualise the dimensions of your exhaust. Include the final length of the megaphone's cone and any inlet and outlet angles that you intend on building into the exhaust. Take into the account the outer diameter of the exhaust pipe that connects to the headers. For example, the header pipe has an outer diameter of 3.7 cm (1.5 inches), so the inlet end of the cone must have an outer diameter of 3.7 cm (1.5 inches) to be welded on or have an inside diameter of 3.7 cm (1.5 inches) to the slipped over the header pipe.
Flatten your design by creating a paper template. Imagine that you have made your megaphone and smashed it flat. For example, a cylinder-shaped exhaust would have a rectangular shape forming the length of the outlet end of the cylinder with the shorter inlet end being half the length of the cylinder. Be sure to include about 10 cm (4 inches) of excess material at both the inlet and outlet ends of the template; these will be discarded later.
Trace the template onto two metal sheets and cut out the forms using a hand grinder or metal shears.
Slightly bend in the outer edges of the forms and weld the forms together. Weld a nut onto the form's excess outlet end. Drill a hole between the centre of the nut and through one side of the sheet metal. Insert a bolt into the nut.
Connect the inlet end of the form to a high-pressure water pump or pressure washer and being filling with water at low pressure. Take note of any leaks in the weld seams by marking them with a marker. Remove the form from the water source and allow to drain. Weld any leaking areas and reconnect to the water source.
Fill the form starting at low pressure and remove the bolt from the outlet's excess end to remove any trapped air. Replace the bolt as water begins to pour out of the hole.
Increase to 4,137 kPa (600 psi). The form should begin to bulge as water pressure increases. Watch for leaks or buckling in the form. Remove buckling by tapping the form with a hammer at the affected area. Repeat Step 5 to remove leaks.
Increase the water pressure to 5,516 kPa (800 psi) and remove any buckles with a hammer. Once the form has taken shape, disconnect the form from the water source and allow it to drain. Cut off the excess ends with a grinder or hacksaw and tap out any remaining wrinkles or buckles.
Grind down the weld seams and paint the cone before mounting. Mount the megaphone to your motorcycle either by slipping the inlet over the header pipe or by welding it into place, depending on your application.
Hydroforming a megaphone
Tips and warnings
- Take your time and don't rush. Good things come to those who wait.
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