Chainsaw Repair

i should have labeled what the numbers are in each row in the photo.
it is just showing what numbers were entered into the calculation for the time area.

rows from top down:
60
5500 (RPM).......chosen rpm point
41.5 degrees (where mean port area needs to be measured).....previous post
360
0.87 cm2 (mean port area).........from trace
49 cc............ motor size

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......hope to be back on topic again.

here again is the example the WRONG way to do the graph/Time Area thing.

in this example, the baseline motor T-A is on the left most column.

as mentioned in a previous post, the T-A for the exhaust port was below the range in the graph. Need to get it between (0.00015 to 0.00016).

to try to get it into the range in this example, only the MEAN Port Area (highlighted in yellow was changed).

baseline motor MPA = 0.000023 cm2 (left column)

.......the area was doubled in the second column.
.......tripled in the 3rd column
...... etc.
.........and finally in the rightmost column, the MPA was large enough to be in the range.  (green highlighted).

the thing is, this is not possible.

to explain why, looking at the blue squares below each column.  squares represent the MPA's of each column as the area of the port increased.  they are scaled accurate to each other.

to get the MPA in the range..... a port that big would not work in a small diameter cylinder.......it would take up more than 1/2 the bore.  not realistic.

to make bigger ports........the height would have to obviously increase as well.

two things learned: realistic port size; angle degrees MUST be changed in formula as port height increases.

.............here is another wrong way example.

here, the rpm was kept at 14500 as in the previous example.
the MPA's kept the same as previous example.,
ONLY the angles were changed.

what is going on? ........the T-A is still too small?  how can this be?  are the MPA's big enough?  RPM too high?  can the angles be made any larger?

at this point hopefully you can see the answer here.

Thanks for the explanation. ..

macdizzy has a T-Area graph on his site with port ranges updated for modern motors.

will find out if this motor may fit into his recommendations.

link here (halfway down the page): http://macdizzy.com/cylinder_map5.htm

I ran into this with the 7900 port  T/A .   All were deficient despite being hogged out as far as possible.

short stroke/small bore issue in the GJ chart seems like a strong possibility.  with the stroke so short the T-A chart range values listed would be very difficult to achieve.

macdizzy chart recommended ranges go a few T-A ticks lower..........but even so, won't be able to carve something out of the cylinder that is large enough to get with the T-A range he recommends.

macdizzy does some work with Angle-Area as well.  looks like the angle-area can be workable alternative to get into the range, for my 49cc motor.

will post something up in a bit.