Chainsaw Repair

Ok cool.  Look forward to seeing the difference in measuring.   Thks

here is a link to the macdizzy site showing how to get the 'effective port width'   http://macdizzy.com/cylinder_map2.htm

the whole port mapping article can be seen on the macdizzy site home page.

here is something that is necessary to complete the mapping process.

put some notes on it to hopefully make it clearer to some folks who may someday give it a go.
 
in the next few days or so will give 4 or 5 examples of some things to watch out for when using the graph or doing the math.

there are lots of thoughts regarding the value or validity of this method.  maybe we can clear some of this up.

-joe

EDIT: for those not familiar with adding some steps.  to make it easier to read....... and hold down ctrl and hit + key.  maybe click on it?
1.  First find the area of the graph that you need.  This is an exhaust port, so the answer is in the yellow somewhere.  The range of Time Areas for exhausts is from 0.00015 to 0.00016.
2.  Choose an RPM point.  Follow this rpm line up from the bottom of the graph into the yellow area.  PLEASE
 NOTE: the rpm point is NOT the maximum rpm the motor will turn.  More on this later.
3.  Where the RPM point falls within the exhaust high and low limits........is where to look on the graph for your MEAN PORT Area size.  The total area of the port in the cylinder, is not the same thing and will give erroneous results if used.
The green dot on the graph is the goal MEAN Port Area size.

The answer I showed for the motor was: Time Area = 0.0000223

This is below the range given on the graph.  Need 0.00015 to 0.00016

So, the next step(s) is to run some more numbers, until the result is in the range.  I will get back to this in a bit.

Think one the best ways to learn things is by making mistakes.  The following are the wrong ways to go about this Time Area thing.....but with explanations to show why they do not work.

The following photos show the numbers that were used. 

On your example the 41.5 angle degrees represents port height?

it is a "height", but it represents the crank angle where Mean Port Area is measured.  in other words, the area of the port that is exposed above the piston edge.

this angle can be solved by one of 3 ways:

 the jennings/macdizzy way by drawing (without using a degree wheel).
to get the degrees you have to do the tracing steps, measure widths with caliper, then draw out the
after tracing the ports, measuring the trace that is wrapped around the piston to get the correct width.  then draw the stroke/connecting rod length circle and connect some lines.  then measure half the angle made on the drawing from where the port just starts to open ATDC to BDC with a protractor.  see the macdizzy site for more detail on this.  it is a simple way that takes little time and works well, imo. 

second way using just a degree wheel:  measure half the angle made from where the port just starts to open ATDC to BDC.  ie. from 96 to 180.
next, 180 - 96 = 84   then divide 84 by 2 to get 42 degrees.
 
third way is the way i use: 
after making the port map tracing.  i measure the distance from the squish band to the exhaust port roof.  subtract my after machining goal squish distance.  then convert this distance in millimeters to degrees using a program i made.  i don't use a wheel for much of anything.  no point in making a map then wheeling everything too.  whatever works.