2 Stroke Cylinder Mapping - What To Do With The Data

2 Stroke Cylinder Mapping- Part 5

Primer - Part 1 - Part 2 - Part 3 - Part 4 - Part 6

What To Do With The Data.

What is known about this test engine.
This engine has a bore and stroke of 72.25 mm x 72 mm. This calculates to an engine displacement of 295 cm^3. It's connecting rod is 125.3 mm - center to center. The exhaust port opens at 89° ATDC for an open duration of 182°. The transfer ports open at 117° ATDC for a duration of 126°, leaving a 28° blowdown period. The cylinder head volume has been cut to 25 cc's, the head gasket is 0.5 mm thick and it is 72 mm in diameter. The deck height is -.61 mm. This makes the engine have a trapped volume of 162.76 cm^3 (figured from the top of the exhaust port) for a compression ratio of 8.17:1 or an uncorrected compression ratio (UCCR) of 14:1. This engine produces 180 PSI static compression. The minimum octane gasoline run through it should be about 100.

Time Area
It should be clear to anyone working on the ports of a 2 stroke cylinder that as the speed of the crankshaft increases, the time a port is open becomes shorter. The fast moving piston opens and closes the ports by sliding by them at incredible speed. That causes problems to a certain extent. Big ports will pass a lot of mixture at high RPM, but it may make the power delivery unacceptable at moderate RPM's - where the engine spends most of its time. Raising ports to make them open longer is one way to make them larger but doing that usually has trade off's as well - it may raise the speed at which good power delivery is available so high that the engine becomes unusable, too peaky or unpleasant to drive. Fortunately ports can be made wider, which will not increase their duration but still give increased flow due to increased area. Doing this is called increasing the time area because the port is able to pass more mixture (or exhaust) in the same amount of time due to an increase in the port width. The port tuner finds a combination of time and area to match to needs of the engine and its intended purpose.

The volume of gasses that can flow through a port tunnel is limited by the tunnel size and/or the port opening. Therefore if the port is opened up in the cylinder it may not pass any additional gasses unless its tunnel is increased in size as well. If you push air down a hallway which has to exit through a door at its end which is the same size as the hall, simply installing a larger door will not let more air pass through the hall - even if the end of the hall gets bell mouthed to accept its additional size. Even if the door gets attached at an angle so that it has an increased area - the restriction is still the diameter of the hall. It is necessary to widen the hall and install a wider door to get more air through.
Sec-cm^2/cm^3
Sec-cm^2/cm^3 is the way to say that we divided the cylinder volume (in cm^3) into the mean area (in cm^2) of the port we're questioning. We then multiply that number by the time in seconds the port is open. We can figure out how much time that is by looking at the Port Open Time formula which converts the engines port timing in degrees of crankshaft rotation into real time.
T = ( 60 / N ) x ( Z / 360 ) or T = Z / ( N x 6)

T is time, in seconds

N is crankshaft speed, in RPM

Z is port open duration, in degrees

Using this simple formula we can find out how long a port is open at any given RPM and from that we can determine its time area. Using an 8000 RPM limit as an example I can tell that the exhaust port of this test engine is open for 0.00379 seconds (60 [seconds in a minute] divided by 8000 RPM [crankshaft speed] multiplied by 182 [duration in degrees of crankshaft rotation the port is open] divided by 360 degrees [a full circle] - or 60 / 8000 * 182 / 360). Since the displacement of this cylinder is 295 cm^3 and the mean exhaust port area is 10.78 cm^2 we divide the displacement into the mean port area and arrive at 0.0365. Multiply that by the time the port is open - I get 0.000138 sec-cm^2/cm^3.

Additional units of measure

One of the other popular units of measure you'll see is s-sq mm (sec-square mm). This figure represents the port duration in degrees of crankshaft rotation divided by the time the port is open in seconds, multiplied by the mean port area (in square mm).

Another one is s-mm^2/cc*10^3 - also referred to as TA value or time area per unit of displacement. This formula takes the mean port area in (cm^2) and divides it by the displacement of the engine (in cm^3) then multiply that by the time the port is open in seconds - it looks an awful lot like sec-cm^2/cm^3 - time area.

Angle Area

The angle area is a number which can be helpful to determine proper port timing as well. It might be thought of as a sort of short cut to quickly determine port areas. Because angle area does not consider the speed of the crankshaft it is necessary to use both the time/area and angle/area calculations to help determine the final area. Using the example above - start with the mean area of the exhaust port already divided into the displacement of the cylinder (10.78 cm^2 / 295 cm^3 = 0.0365) and multiply that number by the 182 degrees of the ports open duration (0.0365 * 182 = 6.643). The angle area for this port would then be 6.643 deg-cm^2/cm^3. This number is useful to compare to a chart which has some established values outlined for particular types and sizes of engines.

The example below is very basic. For an engine turning at 8000 RPM the "most useful" range for the exhaust port angle/area is from about 5.8 to 9.2 - looking at the chart I can see that the 6.643 deg-cm^2/cm^3 falls on the low side of the range.

Update - 5/4/99 The area needed to obtain the target RPM varies within the range of engine parameters - there is overlap. This chart provides no exact answer - it is only a guide. Look at the exhaust and transfer port angle / area numbers independently. The red and blue lines represent extremes. As an engine spins faster, more time area / angle area is needed for the port to do its job. In 1973 Jennings decided on transfer port time area boundaries of .00008 to .00010 sec-cm^2/cm^3. The exhaust port time area boundaries were .00014 to .00015 sec-cm^2/cm^3. For a piston port engine the intake time area was .00014 to .00016 sec-cm^2/cm^3 and a rotary intake valve time area is.00018 to .00019 sec-cm^2/cm^3.

I made this chart to reflect a more modern engine design. Its peak values are spread quite a bit farther apart than they were 25 years ago. The range is quite different today because we are dealing with engine designs that take advantage of as many design improvements as possible. Today we have exhaust valves, boost bottles, super efficient exhaust systems, huge crankcase volumes, reed valves, high output digital ignitions, liquid cooling and other wonderful advances in technology like Nikasil and other super slippery bores. All of these things stretch the limits that were established back when 2 stroke engines were being uncovered and discovered - they all added power, reliability, fuel economy or drivability to the machines that carry them.

The angle / areas that Jennings listed looked something like this:

Angle Area - deg-cm^2/cm^3

RPM Exhaust Transfer Piston Port Intake Rotary Valve Intake

4000
3.5 to 3.7 1.8 to 2.5 3.5 to 3.9 4.5 to 4.8

5000
4.2 to 4.5 2.4 to 3.0 4.2 to 4.8 5.5 to 5.8

6000
5.1 to 5.4 2.8 to 3.6 5.1 to 5.7 6.5 to 6.9

7000
5.9 to 6.2 3.4 to 4.2 5.9 to 6.7 7.6 to 8.0

8000
6.7 to 7.2 3.9 to 4.9 6.7 to7.7 8.7 to 9.1

9000
7.6 to 8.1 4.4 to 5.4 7.6 to 8.6 9.8 to 10.3

10000
8.4 to 9.0 4.8 to 6.0 8.4 to 9.6 10.8 to 11.4

11000
9.3 to 9.8 5.4 to 6.7 9.3 to 10.6 11.9 to 12.6

12000
10.1 to 10.7 5.7 to 7.2 10.1 to 11.5 12.9 to 13.7

13000
10.8 to 11.6 6.2 to 7.8 10.8 to 12.4 14.0 to 14.9

I noticed the time area of the exhaust port on the test engine was less than the minimum and its angle area shows up here as having too low of a value as well - this would back up what I suspected early on when transferring the port dimensions to paper. The blow down duration is too short for good power at high RPM. With an efficient pipe working on the engine it is very possible that the reflected positive (stuffing) wave that returns to the exhaust port outlet will arrive too soon and perhaps stuff some exhaust back into the transfer ports - not just prevent the escape of fresh mixture out the exhaust.

To proceed to Part 6 - A New Angle - Click here.

Primer - Part 1 - Part 2 - Part 3 - Part 4 - Part 6

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