keean

05-09-2003, 06:08 AM

Some people have said that modifing the crank case volume to be a certain size ratio is important...

Taking the helmholtz resonance equation:

n=speed-of-sound/(2*Pi) * sqrt (port-area / (port-length * crank-volume))

Now measurements from a cylinder give:

average-port-area = 0.000728 square-meters

(from port height of 8mm widths 20mm,18mm,15mm,18mm,20mm)

average-port-length = 0.044 meters (average of 32mm and 55mm for inside and outside curve lengths)

speed-of-sound = 340 m/s (at 20 degrees above ambient)

so calculating the resonant crank volume for say 10000 RPM = 166 Hz

0.0165/((166*2*Pi)/340)^2 = 0.00174 cubic meters = 1740 cc

Now, the volume inside the crank case (minus the volume taken up by the crank + 1/2 cylinder volume)) is approx. 350cc !!!

if we work backwards from the given crank volume, that gives a frequency of 372Hz = 22323 RPM...

It would seem the crank volume is too small!!

But, thinking about the port opening, the port is only open for 1/3 of the time (I think the tranfer port is at around 120 degrees ATDC) this would mean the tuning to get the pulse back before port closure would effectively be 7441 RPM. Applying this idea to the volume calculation:

for 10000RPM = equivalent 30000RPM = 500Hz

the volume required would be approx 190cc

From this it would seem that adding material into the crank volume is going to shift the resonance from 7500RPM up to 10000RPM ... Traditionally you want the resonance at peak toque.

Firstly, does anyone have any more accurate measurements of transfer port area, port length, crank volume...

Any comments?

Taking the helmholtz resonance equation:

n=speed-of-sound/(2*Pi) * sqrt (port-area / (port-length * crank-volume))

Now measurements from a cylinder give:

average-port-area = 0.000728 square-meters

(from port height of 8mm widths 20mm,18mm,15mm,18mm,20mm)

average-port-length = 0.044 meters (average of 32mm and 55mm for inside and outside curve lengths)

speed-of-sound = 340 m/s (at 20 degrees above ambient)

so calculating the resonant crank volume for say 10000 RPM = 166 Hz

0.0165/((166*2*Pi)/340)^2 = 0.00174 cubic meters = 1740 cc

Now, the volume inside the crank case (minus the volume taken up by the crank + 1/2 cylinder volume)) is approx. 350cc !!!

if we work backwards from the given crank volume, that gives a frequency of 372Hz = 22323 RPM...

It would seem the crank volume is too small!!

But, thinking about the port opening, the port is only open for 1/3 of the time (I think the tranfer port is at around 120 degrees ATDC) this would mean the tuning to get the pulse back before port closure would effectively be 7441 RPM. Applying this idea to the volume calculation:

for 10000RPM = equivalent 30000RPM = 500Hz

the volume required would be approx 190cc

From this it would seem that adding material into the crank volume is going to shift the resonance from 7500RPM up to 10000RPM ... Traditionally you want the resonance at peak toque.

Firstly, does anyone have any more accurate measurements of transfer port area, port length, crank volume...

Any comments?