Power Bicycle

The Ultimate guide to Motorized Bicycles

Expansion Chambers

Expansion Chambers are designed to produce more torque (acceleration) and extra power (top speed).

Changing the length of the header pipe (moving the chamber up or down on the engine pipe) you can change the RPM at which the engines develops peak power.

Expansion Chamber Fitting Instructions

Expansion chamber for bicycle motor

The expansion chamber from Octane Zombie has several advantages over other power pipes.

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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.

Bicycle engine power pipe

position the expansion

It is recommended to follow the bicycle frame closely. Positioning the pipe too low may cause scraping when traversing obstacles.

Mounting a power pipe

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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.


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

The resonator (“bean can” end) of the power pipe is designed to attach a modified end cap (“stinger”) from a stock muffler. This stylish feature looks great and allows you to provide sound suppression similar to a standard muffler. Standard expansion chambers are available without the stinger, however, our premium package is fitted an OCTANE Zombie stinger which reduces the noise output without affecting performance of the pipe. .

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.

Expansion Chamber in position

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Bicycle engine power pipe

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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.

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