Chainsaw Repair

Interesting thread that I had not seen and need to read more carefully.  I'm still confused as to the definition of crankcase volume and exactly what volume this includes.  Does it include the volume under the piston?  Everything under the piston at BDC?  Is transfer volume included?

Then there is the question of what volume is this being compared to - it makes sense to me to compare it to displacement, but ratios will of course be different if a different denominator is used.

just volume at TDC divided by volume at BDC.  some don't include the transfer port volumes, some do.

the volume under the piston is included at BDC.  the volume under the piston is included at TDC as well.

without transfer ports volume included:
case volume @ TDC = 200 cc
case volume @ BDC = 100 cc

so, the case volume ratio = 200/100 = 2.00

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with transfer ports volume included:

say, motor has only 2 trans ports, each trans port measured out at 5cc each. so, 10cc total trans volume.

case volume @ TDC = 200 cc + 10 cc = 210 cc
case volume @ BDC = 100 cc + 10 cc = 110 cc

then, case volume ratio becomes = 210/110 = 1.91

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the difference in ratios is 0.09

rounding it off = 0.10

up to you, to decide whether to include them or not.


.....the folks who got the ball rolling on this case volume thing as referenced by the article linked in the thread, calculated the ratio differently.