David,
I always wondered, why Vincent twins are so very different from singles. Even Ernst´s short
stroke engine, which reaches 8000 rpm, shows very little sweat over a race weekend. The normal Comet racer
also does not collect any blow by in its bottle over a season.
His Lightning is relatively dry for a twin, ie, over a race weekend, it collects about 0,1l of mud in the collector,
but no oil. If there are long straights on track, she shows some mist here and there.
My Norvin (no race use) is so different: On a weekend trip over some 500 miles of riding, it is visibly oily and
drops, whereever parked, after arrival.
Ok, a motorway trip is a much longer time period of running to let the engine collect blow-by and add up
internal pressure, compared to a 15 - 30min race. In this point, blow-by seems to be an additive effect,
which remains non-critical at short time intervals.
You may be right, stating that blow-by is more at low speeds, say up to 1500 rpm. This is about the limit, until
timed breathing is effectiv. After this, you will need sufficient atmospheric breathing (as Honda uses everywhere) But, that is not the point: If it is right, when an engine is less gas tight at cold temperatures, than
at operating temperatures, this must be seen per working stroke, which means, that every single blow by volume adds to a total in your crankcase, depending on revs. Even , if your engine is more gas tight at higher
revs, it will add to a higher total pressure in your crank case due to double revs.
My impression is, however, that bow-by is worse at high revs at high engine temperatures, which is easier to understand, when thinking about how a piston ring works. His tightening abilities are more difficult at high
speeds, the more in the aspect, that the tightening force is mainly done by gas pressure from behind the ring.
The only reason, why a twin engine does leak more than a single, is - to my impression - that, per cyl.-volume,
the relating crankcase volume on a twin is smaller, than on a single. This could easily be measured, like bakers measure bread volume: by pouring poppy seed into a glas cylinder, in which lays the bread to be
measured.
This could also be done on any engine, but it must be dead dry inside, to get out again 100% of the poppy
seed to measure its volume.
Good, that at present times, bikes with "patina" are valued higher, than gleaming dream bikes, which I
could never afford....