Porting - The basics


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. 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.


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 Bridge
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.

Tools of the Trade

There are two main types of tools used in porting, measuring and grinding. Here is an overview of how these tools are used.

The basic measuring tools include a dial caliper, an inside divider, and an assortment of angle gauges. The caliper is used to measure the port height, the divider is used to measure the chordal width of the port, and the angle gauges are used to measure the roof and side angles of the ports. Calipers and dividers are available from places like Sears or industrial supply stores. Angle gauges are fashioned from cardboard and specific to individual cylinders.

The most common grinding tools are electric powered. They consist of a motor, speed control, flexible drive shaft, tool handle, and tool bits. The power of these motors ranges from 1/5th to 1/4th HP with a maximum rpm of 15,000. Popular manufacturers include Foredom, Dremel, and Dumor. Each company sells a full compliment of accessories for all sorts of hobbyist activities. The most popular source for cylinder porting tools and accessories is CC Specialty in Tennessee (1-800-762-6995).

The tool handles and bits are the secret to porting. There are two types of tool handles; straight and right angle. The straight tool handles are used for machining the port ducts. The right angle tool handles are used to gain access to the port windows from the cylinder bore. Over the years I've tested hundreds of different tool bits and arrived at some simple materials and patterns for finishing the different surfaces of a cylinder. The materials of a cylinder range from aluminum as the base casting material, to a cast iron or steel liner, or nickel composite plated cylinder bores. Here are the basic tool bits used for porting; tungsten carbide works best for aluminum, steel, and cast iron, stones are best for grinding through nickel composite. The tungsten carbide tool bits are available in hundreds of different patterns and shapes. The diamond pattern is the best performing and the shape of the bit should match the corresponding shape of the port. Stones, or mounted points as they are termed in industrial supply catalogs, are available in different shapes and grits. The grits are graded by the color of the stones. Gray being the most course and red being the most fine. The finer the grit the faster it wears but the smoother the finish.

Making Ports Bigger

Generally speaking, if you're trying to raise the peak rpm of the powerband with an aftermarket exhaust system of clutching on a snowmobile, the ports will probably need to be machined in this manner; widen the transfer ports for more time-area and raise the exhaust port for more duration. Most OEM cylinders have exhaust ports that are cast to the maximum safe limit of chordal width. Often times widening the exhaust port will cause accelerated piston and ring wear. In some cases the port will be widened so far that it breaks through into the water jacket. The internal casting on some cylinders is so thin that it prevents tuners from widening the exhaust port. Transfer ports should be widened with respect to the piston ring centering pins. The ports should have a safe margin of 2mm for the centering pin. The height of the transfer ports is based on the time-area of the exhaust port above the transfer port opening height. That is called Blow-down. The exhaust port has to evacuate the cylinder bore of burnt gasses before the transfers open, otherwise backflow will occur into the crankcase. That can cause a variety of dangerous problems like blown crank seals, chipped or burnt reeds, or in extreme circumstances a fire that can extend out of the carb. The angles of the transfers are important too. Generally speaking when the side angles direct the gasses to the intake side of the cylinder, or the roof angles are a steep angle 15-25 degrees), the porting will be better for trail-riding. When the side angles direct the gasses to the center of the cylinder and the roof angles are nearly flat (0-5 degrees), the porting will be better suited to drag or lake racing.

Making Ports Smaller

Ports are purposely made smaller for several reasons. One or more of the ports could have been designed too big, or a well-meaning tuner may have been overzealous, or a customer may have asked for more that he could handle. There are performance gains to be had from smaller ports, for high altitude compensation or for more punch for trail and snowcross riding. Simply using a thinner base gasket or by tunring-down the cylinder base on a lathe. Cometic Gasket Co. in Mentor Ohio makes graded gaskets from .25 to 1.5mm and even custom base plates for stroker engines. (www.cometic.com) Another method is by welding the perimeter of the port, although that entails replating the bore. Transfer and intake ports can be made smaller with the use of epoxy. Brand name products like DURO Master Mend or Weld-Stick are chemical resistant, easy to mold to fit, and can withstand temperatures of 400F. Master Mend is a liquid product and Weld-Stick is a semi-dry putty material. The epoxy can be applied to the roof of the ports to retard the timing and reduce the duration. It can be applied to the sides of the transfers to reduce the time-area, and it can be applied to the transfer ducts to boost the primary compression ratio (crankcase volume).

Porting for Big Bores

WISECO offers big bore piston kits for most popular snowmobiles. (www.wiseco.com) The average increase in displacement is 50cc per cylinder. This requires that the cylinder be over-bored 4-8mm. Because the ports enter the cylinder bore at angles, when the bore size is increased all the ports drop in height. The steeper the port angles the greater the port height will drop. Lower port heights mean retarded timing and reduced duration. The exhaust port gets narrower and the transfers get wider. A larger displacement cylinder will require more port-time-area. Normally the exhaust port needs to be raised higher than stock to compensate for the compression ratio. If you're adding a set of performance pipes at the same time as the big bore, you'll need to compensate the port timing to get the best gains from the pipes. It's a complicated thing. Sometimes tuners use thicker base gaskets to compensate for big bores, but ideally the port-time-area needs to be calculated before any serious porting changes are made. If you are strictly trail riding at high altitude, you can just have the cylinders bored and replated because the porting will inherently change to suit that type of application.

Big Bore Specs Link (coming soon!)

Computer Design Software

The best-kept secret in high performance tuning is the use of computer design software. These products became popular about six years ago when Tom Turner adapted the SAE programming code and added in his own empirical data from his career as a motorcycle drag racer and tuner. Tom's products are named TSR software (www.tsrsoftware.com) and available in MS-DOS format for PCs. The programs cover individual engine components. The most popular program for porting is PORTTIME 2000 and it sells for $200. It features target specs for all sorts of vehicles including snowmobiles. Basically a tuner types in engine spec dimensions like the bore and stroke plus all the individual port measurements. The program runs mathematical calculations in order to provide a simulation of how the porting changes will affect the engine's powerband. TSR's programs will get a tuner 90% to the engines potential. The next level of programs includes 1D and 3D gas dynamics simulators. Dynomation is a 1D simulator that sells for about $500 (www.audietech.com). It enables tuners to combine all the engine components together to simulate dyno runs on the computer so they can save time machining metal and swapping parts on actual dyno runs. Dr. Gordon Blair programmed the 3D simulator named Virtual Two-Stroke, the most well know two-stroke engine researcher. His program is marketed through Optimum (www.optimum-power.com) through lease programs. Priced at $12,000 a year, it's intended for use by engine manufacturers.