piston info required to use in port timing calculator

[Avantone]

when looking from the base of the cylinder up the bore with the piston in and at tdc the two smaller transfers are obscured by the piston skirt, is it worth trimming back the skirt ?

No, the transfer windows have long closed at TDC so what you do with the skirt doesn't come into play at this point in the cycle.

However, it's worth aligning the piston in the correct plain with the transfer windows just opening (being uncovered as the piston moves from TDC to BDC) to then see if the skirt obstructs the transfer cut outs in the base of the cylinder.

[tea5ive]

:wtf: trim skirt where for what?

near the transfer windows, the smaller of the two on each side of the cylinder and from bottom of piston at the inlet side, and in this aplication around 2.5mm to be trimed to get an inlet duration of around 150 ish degrees

[tea5ive]

yes, will be doing this as i have done with other cylinders when maching to casings etc.

when looking from the base of the cylinder up the bore with the piston in and at tdc the two smaller transfers are obscured by the piston skirt, is it worth trimming back the skirt ?

No, the transfer windows have long closed at TDC so what you do with the skirt doesn't come into play at this point in the cycle.

However, it's worth aligning the piston in the correct plain with the transfer windows just opening (being uncovered as the piston moves from TDC to BDC) to then see if the skirt obstructs the transfer cut outs in the base of the cylinder.

[tea5ive]

what about port widths ?, die grinder waiting

[Avantone]

what about port widths ?, die grinder waiting

I'd address the inlet duration first an then see what TA you get against what you need (see posts on timed area).

It's also worth assessing the cross sectional area of the inlet tract from carb to cylinder wall to see if there's a steady transition, or whether it fluctuates and/or restricts.

[tea5ive]

thanks for the help tony and eden,


can you tell me how and where to measure the timed area ? . i think the next step is to get the top end on a casing with a crank and see how things line up. cheers simon

[jonny snatchsniffer]

cutting the skirt of the piston is the easy way to get more inlet duration but actually enlarging the port will give better results, the exhaust pot widening will depend on your rings