This week I want to talk about two-strokes. To kick off this post I have some awesome news. The Two Stroke Dirt Bike Engine Building Handbook is off to the printers and will be available for pre-sale very soon! Getting the book off the ground has been no cake walk. It's been two years coming and we are so thankful our riders and fans have been patient with us! At the end of this post I'll give you instructions on how you can stay updated on the launch. With that said, let's get started. Today's post aims to provide an overview of the important aspects of the two-stroke cylinder and answers a couple commonly asked questions relating to cylinder modifications.
The ports found within a two-stroke cylinder in combination with the exhaust system have the greatest influence on power, torque, and the RPM at which maximum power is created out of the various engine subsystems found within a two-stroke engine. Typically when a new engine is designed the port characteristics are one of the first parameters to optimize. With this being the case they are also one of the first things anyone planning on altering an existing engine should consider improving or tailoring to their specific application. A two-stroke cylinder consists of exhaust, transfer, and occasionally inlet ports (true inlet ports are only found on piston or rotary valve controlled engines). The port heights, widths, areas, directions they flow, and relationships to one another all have a significant influence on how the engine will behave. The cutaway of the cylinder shown details the port arrangement and common nomenclature.
The inlet port/passage delivers air into the engine’s crankcase, most commonly through a reed valve, on a dirt bike engine. On older engines, a rotary valve or the piston may also be used to control the opening and closing of the inlet port. On modern machinery, the inlet simply connects the reed valve to the cylinder or crankcase. In this case, the primary restriction in the inlet port is the reed valve and as such the valve’s geometry and flow capabilities often dictate the inlet port's performance.
The transfer ports are responsible for moving fresh air and fuel up from the crankcase into the cylinder. This occurs as the piston travels downward after the cylinder has fired. Once the piston uncovers the tops of the transfer ports the blowdown phase is complete, at which point much of the exhaust gas has been expelled from the cylinder. As the transfer ports begin to open, the exhaust pipe sucks fresh mixture up through the transfer ports into the cylinder. To a lesser extent, the downward motion of the piston also aids in creating a pressure differential between the crankcase and cylinder. The shapes and flow capabilities of the transfer ports play a big part in how effectively the cylinder can be scavenged of exhaust gases and filled with fresh air and fuel. The transfer ports also help cool the piston.
The transfer ports are responsible for moving fresh air and fuel up from the crankcase into the cylinder. This occurs as the piston travels downward after the cylinder has fired. Once the piston uncovers the tops of the transfer ports the blowdown phase is complete, at which point much of the exhaust gas has been expelled from the cylinder. As the transfer ports begin to open, the exhaust pipe sucks fresh mixture up through the transfer ports into the cylinder. To a lesser extent, the downward motion of the piston also aids in creating a pressure differential between the crankcase and cylinder. The shapes and flow capabilities of the transfer ports play a big part in how effectively the cylinder can be scavenged of exhaust gases and filled with fresh air and fuel. The transfer ports also help cool the piston.
The exhaust ports dictate how much and how well exhaust gases depart the cylinder. Similar to the transfer ports, the duct shape, angle, length and volume have a large influence on how well gases can flow through the port. Typically, dirt bike engines commonly feature bridge port or triple port designs.
General insights into a cylinder’s performance can be made by characterizing attributes such as the timing of the exhaust and transfer ports, the port widths, and the directional flow angles, but a deeper analysis is required to truly optimize a cylinder. Today, tuners and designers rely on computer software which computes a port’s specific time area (STA). As defined in the EngMod 2T software suite, “STA provides an indication of the effective port window area that has to be open for a certain length of time to allow enough gas to flow through the port to achieve the target power at the target RPM for the given engine capacity”. STA values are used to quantify the exhaust, transfer, and inlet port geometry as well as the blowdown phase of the two-stroke cycle. The blowdown phase occurs between exhaust port opening and transfer port opening and is one of the most important parameters in predicting engine performance.
By manipulating STA values and subsequently the height, shape, and size of the exhaust, transfer, and intake ports, an engine’s power characteristics can drastically be altered. Port modifications can be made which allow more air to move through the cylinder, ultimately increasing the power of the engine. Conversely, ports can be filled or welded and reshaped which tame the engine and provide less peak power but a broader spread of power. Simple modifications to the ports can also be carried out which improves the air or exhaust gas flow through the port yielding better cylinder scavenging.
General insights into a cylinder’s performance can be made by characterizing attributes such as the timing of the exhaust and transfer ports, the port widths, and the directional flow angles, but a deeper analysis is required to truly optimize a cylinder. Today, tuners and designers rely on computer software which computes a port’s specific time area (STA). As defined in the EngMod 2T software suite, “STA provides an indication of the effective port window area that has to be open for a certain length of time to allow enough gas to flow through the port to achieve the target power at the target RPM for the given engine capacity”. STA values are used to quantify the exhaust, transfer, and inlet port geometry as well as the blowdown phase of the two-stroke cycle. The blowdown phase occurs between exhaust port opening and transfer port opening and is one of the most important parameters in predicting engine performance.
By manipulating STA values and subsequently the height, shape, and size of the exhaust, transfer, and intake ports, an engine’s power characteristics can drastically be altered. Port modifications can be made which allow more air to move through the cylinder, ultimately increasing the power of the engine. Conversely, ports can be filled or welded and reshaped which tame the engine and provide less peak power but a broader spread of power. Simple modifications to the ports can also be carried out which improves the air or exhaust gas flow through the port yielding better cylinder scavenging.
Can I modify my own cylinder?
Unless you have a deep passion for two-stroke tuning, are willing to spend money on software and porting equipment, and are comfortable throwing away botched cylinders, I would recommend having a reputable professional carry out any desired port modifications. Experienced tuners have developed a number of porting combinations that will work well for various makes/models and riding applications which will take the guesswork out of the situation and provide you with a good performing cylinder.
Who should consider two-stroke porting modifications?
For the sake of simplicity, I will lump porting modifications into two categories: major and minor.
- Major port modifications would include tasks such as significantly changing the port timings (by either removing or adding material), altering the shapes of the ports, or changing the directions the ports flow. Anyone drastically altering their engine, such as turning an MX engine into a road racing engine, should consider major porting modifications. Other examples of applications that may require or benefit from major port modifications include drag racing, hare scrambles, ice racing, or desert racing.
- Minor port modifications would include basic tasks such as removing casting flash, slightly altering the ports to achieve the stock port timing, and correcting areas that result in minor flow deficiencies. Just about everyone could benefit from these types of corrective actions; however, if the engine is already performing or producing adequate power, they often aren't considered.
 | I hope you enjoyed this writeup on key features affecting the performance of two-stroke cylinders. To stay officially updated on The Two Stroke Dirt Bike Engine Building Handbook we created an email sign up for our readers. Click this link to see the new cover, the Table of Contents, and some sneak peek pages right from the book. Thanks for reading and have a great rest of your week! -Paul |
