Motorized Bicycle Expansion CHamber
The Ultimate guide to Motorized Bike Expansion Chamber
Expansion Chambers
An expansion chamber exhausts is designed to produce more torque (acceleration) and extra power (top speed).
Although the length, and width, of the expansion chamber is fundamental to performance, changing the length of the header pipe (moving the chamber up or down on the engine pipe) can change the RPM at which the engines develops peak power.
The Best Expansion CHamber for Motorized Bicycle
The best exhausts for motor bicycle’s have a separate header pipe to allow performance adjustment.
Unfortunately, most power pipes (The banana pipe for instance) are a single piece systems which do not allow adjustment.
The performance of these systems can be hit and miss depending on your engine porting, tuning, and cubic capacity.
Sometimes you obtain a boost in power, from a standard pipe, however, most pipes require a little tweaking to get the best from them. This usually involves fitting (or welding) a more restrictive end tube to the expansion chamber.
As a rule of thumb, the end pipe should be around 1/2 the diameter of the header pipe. A smaller exhaust tip increases back pressure which can increase power.
80cc Expansion CHamber
Standard mufflers of 80cc bicycle motors can be adjusted to provide the benefits of a tuned exhaust.
In most bicycle engine kits, you can remove the “stinger pipe” at the end of the muffler can be easily pulled out after removing the bolt in the center of the muffler end piece.
You will find that most engines receive a boost in power simply by cutting 1″ off the end of the stinger pipe tube before replacing into the muffler.
A little more than 1″ is usually a little better, however, there is a point at which the stinger becomes too short and power begins to drop.
Unless you have some replacement end pipes, I recommend cutting no more than 1″ from the stinger.
Motorized bike exhausts
Standard mufflers such as the Phantom 85 expansion chamber are usually fairly well tuned to the motor.
Unlike mufflers, expansion chambers do not have any internal sound deadening.
Expansions can improve gas flow and always have the potential to improve the performance of a 2-stroke engine.
The caveat here is that you may have to tune the pipe to obtain any benefit.
Expansion Chamber Fitting Instructions
Fit the engine header pipe leaving the exhaust nuts slightly loose.
Fit the lower part of the expansion chamber over the header pipe. Slide the lower section about 20 mm over the end of the header pipe.
The expansion chamber in this position will give good all round performance.
position the expansion chamber
It is recommended that the expansion chamber follows the bicycle frame closely. Positioning the power pipe too low may cause scraping when traversing obstacles.
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Later you can shorten the lower pipe position by sliding the chamber up the engine header pipe. Use the metal strap supplied to hold the expansion pipe in position.
The expansion chamber is designed in two sections for ease of installation and to allow the rider to adjust the engine RPM where peak power is developed.
When you are happy with the position of the pipe, use the contact cement to glue the rubber strip to the metal strap. You can then clamp the expansion chamber to the bike frame.
The chamber swings freely (a ridged mounting will cause fracturing of the pipe and mounts) and is held in place by the pipe connection and the strap. Once you get the angle of the pipe where you want it, tighten the Engine Pipe retaining nuts.
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The strap will need 6.5 mm holes drilled in the appropriate places for the retaining screw and the clamp screw (not supplied). You may also need to fit spacers to suit your particular installation (see images). Once the pipe is installed, you may find the motor runs off tune’. To correct this you will need to change some carburetor settings and maybe the Spark Plug, depending on the type and model of your motor.
Note: If leaking occurs around the join between the Engine Pipe and Expansion Chamber, use High Temperature Silicone Exhaust Sealant in the gap. (Available from Automotive spares & accessory stores.
The resonator
Adjustable length exhaust manifold:
The separate exhaust manifold allows you to control power delivery – If you fit the expansion chamber towards the end of the exhaust manifold (long pipe) this provides improved mid range torque & speed.
A short pipe pushes the power delivery into the higher rev band for further boost to speed with a reduced affect in mid range torque.
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Fitting flexibility
The lightweight design with mid pipe support mount allows you to adjust the position of the expansion chamber to reduce complexities in fitting. The exhaust manifold has enlarged stud diameters to provide the down pipe some fitting flexibility.
Minimal weight and support for the expansion chamber reduces strain on the muffler mounting studs and lowers stress the engine mounts.
The exhaust manifold is designed to provide the maximum area available between exhaust studs. The outlet area is increased by over 90% and port matched to the Z48-4 motor. 66cc motors (some vendors class these as 80cc) usually have an elliptical or square section. Gas flow can be further increased by matching the barrel and exhaust manifold profiles.
This maximizes the outlet header pipe design to improve gas flow from the engine for increased power and torque.
