How-to: Calculate port duration
One of the first steps in cylinder mapping and tuning is calculating your port duration. In this how-to I will explain a step-by-step tutorial on how to calculate the port durations inside your current cylinder.
What exactly is port duration?
Port durations say something about what happens during a single crankshaft cycle. If we take one complete crankshaft revolution as a 360 degree cycle, then we can express the period in which a port is opened in a part of those 360 degrees.
Because the movement of the crankshaft-connecting rod-piston isn't as complex as you might think at first, we can easily translate distances inside the cylinder[mm] into durations[degree] by using a simple goniometric formula.
Knowing the duration of each type of port inside your cylinder gives you a rough indication of the rpm's that your cylinder can cope with. Most of the time it'll also give a rough indication as to how the cylinder will produce it's power in combination with different exhausts.
A rough indication, because if we want to be able to judge a cylinder better, it's always necessary to have a cylinder map. Port duration may give you some information, but it's the combination of this info with the port shapes and port angles that give you way more information about your cylinder (e.g. a exhaust port with a 196 degree duration may sound fast and cool, but the port might as well be just 10mm wide.... having a real small area through which the engine can breathe out it's exhaust gas).
Measuring VS calculating
There are different ways to find out your current port duration:
1). The degree wheel Having a degree wheel makes life easier. If you mount such a wheel on your engine, you'll be able to readout your port duration directly if you have your cylinder(ex. head)/piston/crankshaft mounted on your engine. This requires no further calculations.
2). Measuring and calculating If the necessary data is measured with a caliper gauge (with at least 0,1mm precision), then we can transfer this data into port duration. And that's exactly what we're going to do in this how-to.
Warning: never measure port dimensions from a portmap, always measure actual dimensions inside your cylinder, before writing them down on your portmap.
Getting your data
In order to be able to calculate your duration, you need to gather some data from your engine. Below you'll find all the data you need, and how to get it, step-by-step.
Engine stroke
The engine stroke is basically the distance your piston travels up and down the cylinder. The stroke can always be found in your engine specifications. However, if you don't know your engine type, or can't find those specs, you can measure the stroke by using a caliper gauge.
Below you will find the stroke of some commonly used 50cc engines:
stroke [mm]
Minarelli hor./vert.: 39,2
Minarelli AM6: 39,0
Piaggio (carb/injection): 39,3
Peugeot vert. (carb): 39,1
Peugeot hor. (carb): 39,8
Morini (carb/injection): 37,4
Kymco: 41,4
Honda (mb/mt): 41,4
Zundapp 41,8
Kreidler: 39,7
(have some changes or additional info for this list? please send me a PM)
Connecting rod length
The connecting rod length is the distance betwee the two centers of your big-end and small-end (figure 1). Measuring this can be quite difficult, due to the fact that you'll need to have your crankshaft disassembled. Fortunately this length is 80mm for nearly all of the 50cc scooter engines used at this moment. If this length is unknown to you, can't measure it, and can't find it anywhere in your engine specs, you can safely say that this is 2x the length of your stroke.
Figure 1
Figure 2
Port height
The most important dimension we need is the moment at which a certain port starts opening. This dimension can be measure by measuring the length between the top of the barrel (cylinder deck) and the top of that particular port. In figure 2 this dimension is shown by the letter E.
Warning: bevels around the port may cause some confusion when measuring this distance. These bevels may confuse you into thinking the port is higher than it actually is. However, always measure from cylinder deck to the point where the port actually starts opening effectively!
Figure 3 shows the difference these bevels can cause. In this case, write down the left (longer) dimension for your calculations.
Figure 3
Deck clearance
Because the top of the barrel isn't always the piston's TDC, we need to correct the dimensions we measured above. This correction is called the deck clearance. Deck clearance is the distance between the cylinder deck and the piston crown at TDC. This might look very similar to your squish, but be aware. Some cylinder heads have part of the squishband inside the head, in this case deck clearance is lower than your squish!
In figure 2 the deck clearance is shown by the letter C.
The final formula
Now that we have al the necessary data we can start using them in our formula, or we can fill them in in a program like MS Excel, in order to do the calculations for us.
The formula we need is:
D= 2* ( 180 - cos^-1 ( (T^2 + R^2 - L^2) / (2*R*T) )
R=stroke/2
L=connecting rod length
C=deck clearance
E=distance from port roof to deck
T=R+L+C-E
Example
To show a short example of the calculation, we'll calculate the exhaust duration on a minarelli engine with 0,60 squish and the exhaust port roof at 20mm under cylinder deck.
R=39.2/2=19.6
L=80
C=0.6
E=20.0
T=19.6+80+0.6-20=80.2
D= 2* (180-cos^-1 ((80.2^2 + 19.6^2 -80^2)/(2*19.6*80.2)) = 195.21 degrees.
Needless to say, letting software like MS Excel do this formula for you saves quite a lot of time
.
Blowdown
The blowdown period is the moment in which some of the exhaust port(s) are already opened, but all of the transfer ports still remain closed.
Calculating this number can be done by:
( Highest exh. port duration - highest transfer timing ) / 2
e.g. (196-135)/2=30.5
.
