full circle cranks
HPC / Full circle cranks come into their own only when the transfers have been enlarged considerably otherwise the inlet gasses compress too much in the crankcase (due to the transfers restricting their flow into the combustion chamber )which produces extra heat as well as resisting the downward stroke. The extra cubic space that a HPC takes up also helps to compensate for the increased volume created by the larger transfers. Low Primary Comp is best left for the racetrack where the usable rev range is a lot higher up the scale and exhaust suction is the key to success.
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Avantone
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I think it's an argument built on myth and assumption, all kicked of by the (overly quoted) Charlie Edmonds article some time back in Scootering!
You don't need to fit a special crank to reduce primary compression, you simply need to add volume to the crankcase and there are far easier and cheaper ways of doing so if LPC is what's wanted.
LPC is an unavoidable consequence of having multiple, big transfers and inlet tracts with big reed cages. The solution to the "problem" is to use full circle cranks and expansion chambers that are capable of getting the charge moving up the transfers when there's severely reduced pumping effect due to this unavoidable LPC.
In terms of improving performance there are numerous alternative modifications that would give a measurable improvement, and simply reducing the PCR isn't going do anything (IMO).
It's also a bit of a myth that racers want rev monsters - mid range tractability particularly when you're stuck with a 4-speed box is more likely to get you through the gears and out of the corners.
Lets see a dyno graph showing full circle vs. LPC cranks.........
You don't need to fit a special crank to reduce primary compression, you simply need to add volume to the crankcase and there are far easier and cheaper ways of doing so if LPC is what's wanted.
LPC is an unavoidable consequence of having multiple, big transfers and inlet tracts with big reed cages. The solution to the "problem" is to use full circle cranks and expansion chambers that are capable of getting the charge moving up the transfers when there's severely reduced pumping effect due to this unavoidable LPC.
In terms of improving performance there are numerous alternative modifications that would give a measurable improvement, and simply reducing the PCR isn't going do anything (IMO).
It's also a bit of a myth that racers want rev monsters - mid range tractability particularly when you're stuck with a 4-speed box is more likely to get you through the gears and out of the corners.
Lets see a dyno graph showing full circle vs. LPC cranks.........
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GP Kevo
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- Joined: Wed Mar 11, 2009 7:48 am
- Main scooter: 1969 150 DL / GP225 TS1
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If I remember that interview with Charile Edmonds he was talking about how HPC cranks were decreasing crankcase volume and what he was doing was maximising crankcase volume. He also talked about maximising the pull out of the turns which is what every racer I've talked to was on about. Whatever he's doing, it's certainly effective.
Avantone, what's up with BSSO videos this year? Anything to look forward to? Your coverage of last year's racing was top class!
Avantone, what's up with BSSO videos this year? Anything to look forward to? Your coverage of last year's racing was top class!