Cylinder Porting: Basic Principles By Eric Gorr

Eric Gorr

Engine Builder
Jun 29, 1999
The process of cylinder porting is a funny paradox. The people in the market to buy it are looking for information and the people in the market of selling it are hiding information on porting. So much myth and misinformation is associated with this complex machining and metal finishing process. Yet the tooling is easily available and the design of the ports is actually quite straightforward with resources like computer design programs. This article is an overview of how porting is performed and how it can benefit your performance demands.

Two-Stroke Principles
Although a two-stroke engine has fewer moving parts than a four-stroke engine, a two-stroke is a complex engine with different phases taking place in the crankcase and in the cylinder bore at the same time. This is necessary because a two-stroke engine completes a power cycle in only 360 degrees of crankshaft rotation, compared to a four-stroke engine, which requires 720 degrees of crankshaft rotation to complete one power cycle. Two-stroke engines aren't as efficient as four-stroke engines, meaning that they don't retain as much air as they draw in through the intake. Some of the air is lost out the exhaust pipe. If a two-stroke engine could retain the same percentage of air, they would be twice as powerful as a four-stroke engine because they produce twice as many power strokes in the same number of crankshaft revolutions. The following is an explanation of the basic operation of the two-stroke engine.

  • 1. Starting with the piston at top dead center (TDC 0 degrees) ignition has occurred and the gasses in the combustion chamber are expanding and pushing down the piston. This pressurizes the crankcase causing the reed valve to close. At about 90 degrees after TDC the exhaust port opens ending the power stroke. A pressure wave of hot expanding gasses flows down the exhaust pipe. The blow-down phase has started and will end when the transfer ports open. The pressure in the cylinder must blow-down to below the pressure in the crankcase in order for the unburned mixture gasses to flow out the transfer ports during the scavenging phase.
  • 2.Now the transfer ports are uncovered at about 120 degrees after TDC. The scavenging phase has begun. Meaning that the unburned mixture gasses are flowing out of the transfers and merging together to form a loop. The gasses travel up the backside of the cylinder and loops around in the cylinder head to scavenge out the burnt mixture gasses from the previous power stroke. It is critical that the burnt gasses are scavenged from the combustion chamber, to make room for as much unburned gasses as possible. That is the key to making more power in a two-stroke engine. The more unburned gasses you can squeeze into the combustion chamber, the more the engine will produce. Now the loop of unburned mixture gasses have traveled into the exhaust pipe's header section. Most of the gasses aren't lost because a compression pressure wave has reflected from the baffle cone of the exhaust pipe, to pack the unburned gasses back into the cylinder before the piston closes off the exhaust port.
  • 3. Now the crankshaft has rotated past bottom dead center (BDC 180 degrees) and the piston is on the upstroke. The compression wave reflected from the exhaust pipe is packing the unburned gasses back in through the exhaust port as the piston closes off the port the start the compression phase. In the crankcase the pressure is below atmospheric producing a vacuum and a fresh charge of unburned mixture gasses is flowing through the reed valve into the crankcase.
  • 4. The unburned mixture gasses are compresses and just before the piston reaches TDC, the ignition system discharges a spark causing the gasses to ignite and start the process all over again.

What is Porting?

Porting is a metal finishing process performed to the passageways of a two-stroke cylinder and crankcases, that serves to match the surface texture, shapes and sizes of port ducts, and the timing and angle aspects of the port windows that interface with the cylinder bore. The port windows determine the opening and closing timing of the intake, exhaust, blowdown, and transfer phases that take place in the cylinder. These phases must be coordinated to work with other engine components such as the intake and exhaust system. The intake and exhaust systems are designed to take advantage of the finite amplitude waves that travel back and forth from the atmosphere. Porting coordinates the opening of the intake, exhaust, and transfer ports to maximize the tuning affect of the exhaust pipe and intake system. Generally speaking porting for more mid-range acceleration is intended for use with stock intake and exhaust systems.

Eric Gorr

Engine Builder
Jun 29, 1999
These are some common words and terms associated with porting.

Passageways cast and machined into the cylinder.

The tube shape that comprises the ports.

The part of the port that interfaces the cylinder bore.

Exhaust Port
The large port where the burnt gasses exit the cylinder. Exhaust

The center divider used on triangular shaped exhaust ports.

Sub-Exhaust Ports
The minor exhaust ports positioned on each side of the main exhaust port.

Triple Ports
One main bridgeless exhaust port with one sub exhaust port on each side.

Front Transfers
Transfer ports link the crankcase to the cylinder bore. The front set (2) of transfers is located closest to the exhaust port.

Rear Transfers
The rear set of transfers is located closest to the intake port.

Auxiliary Transfers
Some cylinders have a minor set of transfers located between the front and rear sets.

Transfer Port Area Ratio
The area of the crankcase side of the transfers divided by the area of the port window.

Boost Ports
The port or ports that are located opposite of the exhaust port and in-line with the intake port. These ports are usually by-pass ports for the intake or piston and sharply angled upwards to help direct the gas flow during scavenging.

A mathematical computation of the area of a port, divided by the displacement of the cylinder, and multiplied by the time that the port is open. The higher an engine revs the more time-area the port needs. The higher the piston speed the less time available for the gas to flow through the port.

The number of crankshaft angle rotational degrees that a port is open.

Opening Timing
The crank angle degree when the piston uncovers the port.

Crank Angle
Measured in units of degrees of crankshaft rotation. On a two-stroke engine there are a total of 360 degrees of crankshaft rotation in one power cycle.

Port Side angle
The side angle of a port measured at the window, from the centerline of the bore with the exhaust port being the starting point (0).

Port Roof angle
The angle of the top of the port at the window.

Port Height
The distance from the top of the cylinder to the opening point of the port.

Top Dead Center (TDC)
The top of the piston's stroke.

Bottom Dead Center (BDC)
The bottom of the piston's stroke.

Chordal Width
The effective width of a port, measured from the straightest point between sides.

Brake Mean Effective Pressure.

Loop Scavenging
Scavenging is the process of purging the combustion chamber of burnt gasses. Loop scavenging refers to the flow pattern generated by the transfer port duct shapes and port entry angles and area. The gasses are directed to merge together and travel up the intake side of the bore into the head and loop around towards the exhaust port.

This is the time-area of the exhaust port between the opening time of the exhaust and the transfers. When the exhaust port opens the pressure blows down, ideally to below the rising pressure of the gasses in the transfer ports. Blow-down is measured in degrees of crank rotation and time-area.

Effective Stroke
The distance from TDC to the exhaust port height. The longer the effective stroke the better the low-end power.

Primary Compression Ratio
The compression ratio of the crankcase.
Secondary Compression Ratio
The compression ratio of the cylinder head.

Compression Waves
Pressure waves that reflects from the end of the intake or exhaust system and return to the engine.

Expansion Waves
Pressure waves that travel from the engine and out to the atmosphere.
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