Ive got a brand new pkxl large cone race crank and want to build a reed valve engine for the s/f. My question can i have the driveside lip chamfered to avoid the expense of another new (full circle) crank and will it be as effective.
yes of course you can drive with a „special lip“ cranckshaft".
The full circle for me has been a product of marketing more then anything else. In fact we have to remember that the vespa cases are not born for this kind of cranck. Yes it rises the cranckcase compression, and so?
It is not a question of having the most cranckcase compression, but the right compression.
It is a question of mainly three factors. These factors give you the cranckcase pressure time: intake port duration, transfer ports duration and cranckcase compression.
To avoid having too much cranckcase compression with a full circle cranck you have to do more space in the cases. If not you will have too much cranckcase pressure time.
Depending on CPT(cranckcase pressure time) you will have more torque or more peak power…so now you understand that is not just a question of having the highest cranckcase compression as possible…
So yes to special lip, but balance it as well as possible.
thanks, Curare , knew i could rely on you for an indepth answer. Yes i fully appreciate the balancing out statement. i was just worried the uneven profile of a standard race crank even with the lip on it would cause unecessary turbulence.