The basics being that although a port timing measurement in degrees gives you an idea how long a port is open for.....it doesnâ€™t tell you what size the port is, or how much gas it will flow.
Further more, the relationship between inlet/transfer/exhaust ports is obviously linked, and thus you want all ports at the optimum size, so that they are all flowing together in the correct rev range to give the engine characteristics you desire.
e.g. - itâ€™s no use having an inlet that flows best at one rev range, transfers at something different, exhaust at something different again, and a pipe/carb set up that is mismatched again. The whole engine would be â€˜at oddsâ€™.
So you can use various calcs to help estimate where you need to be when designing a tune for your engine, and given various calcs/options for operating power/rpm/port durations etc you can make sure that all your engine components are matched together, so that they operate in harmony across the rev range you desire.
I hope we are all sitting nicely So I shall begin
The basic' s for finding the various calculations to get to Time area/displacement really follow the simple rules below.
The hardest part really is getting an accurate measurement of the port area.
Hope the following below explains to people what is being discussed here
Port Open Time formula
which converts the engines port timing in degrees of crankshaft rotation into real time.
Using this simple formula
T = ( 60 / N ) x ( D / 360 ) or T = D / ( N x 6)
we can find out how long a port is open at any given RPM and from that we can determine its time area.
T is time, in seconds
N is crankshaft speed, in RPM
D is port open duration, in degrees
Example: Exhaust port timing of 176 degrees at 6700 rpm
T = 176/6700*6 = 176/40200 = 0.00438 seconds
Port mean time
After finding Port Open time, we must determine port mean time.
The mean time is how long the port stays open from mean port angle to the same point on the upstroke
One way is to set up a degree wheel at TDC, then rotate the crank until the piston begins to uncover the port. Next, continue turning the engine to BDC and then back to where the piston just covers the port. This is the port's angle of duration.
Now, divide this figure by 4, then add the angle where the port just began to open. The piston should uncover 71 percent of the port at this angle.
This point is the mean port angle used to determine the mean time area.
It can simply be calculated in this example as port open time x 0.71 for Exhaust
Transfer use x 0.75
Inlet use x 0.65
Port mean time area
The area of the port perpendicular to the flow is determined as the corrected area. This is determined using the port map and correcting on graph paper for the angle of ports.
This area is then multiplied by the mean time to give the Port mean time area.
e.g. Area = 10 cmÂ² & Time = 0.003 sec
Mean time area = 0.030 sec - cmÂ²
Time â€“area /per displacement
Time-area (sec â€“ cmÂ²) divided by displacement (cmÂ³)
e.g. Time area = 0.030 sec - cmÂ² /displacement = 180cc
Time area/displacement = .00017
Intake port: 0.00014 to 0.00016 sec cm2 / cm3
Transfer port: 0.00006 to 0.00010 sec cm2 /cm3
Exhaust port: 0.00014 to 0.00015 sec cm2 / cm3
Rotary intake: 0.00018 to 0.00019 sec cm2 /cm3
Increases in transfer port areas tend to reduce peak power but add to the power curve at low and midrange.
Increases in exhaust port time area tend to reduce power at lower rpms but increase power in the upper rpm range.
There endeth the Lesson for today Smiffy (RinB)
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