The end of the Lambretta crank as we know it ???

[sean brady scooters]

LPC does of course work best with efficient expansion chambers/pipes...........
and on vespas ,modified cranks do of course increase flow..........

but,...........how then would you justify such increases in performance if it was a case that
lpc was detrimental...................

are vespas really compromised with this against lambos..................?

[ralph95]

Really!! So why will this type of crank provide more power at high rpm then? Less mass? I think some people have strange ideas on primary compression. In fact as Adam said above no one had even thought about it until that article. Now we are seeing cranks like this appearing because the tuners out there think that this is what is best to try to achieve this desired low primary. Can someone explain low primary compression theory because up till recently everyone was padding cranks!

not everybody red herring's spring to mind

[sean brady scooters]

i think that some folk are quite confused over this issue of primary comp ratios...................
for eg.................just cos you have say a full circle crank.........does not always mean that you have a high primary comp ................
you could in fact have a very low PCR with a full circle crank,due to very large transfers/intake passage from say a reed valve...................

[Diablo]

I think with vespas LPC can be detrimental. If you look at some of the reed manifolds available for them especially the MB of NK ones they must reduce primary compression by loads and I've seen plenty of reedvalved 210s not make as much hp as std carb jobs. Some of the quickest 210s I've seen are ones with the reed right above the crank and the webs just chamfere.
There must be a point where the advantages of LPC (whatever they are) are outweighed by reduced gas speed. I know from my own experience its very easy to get it wrong on a vespa so you produce something that makes massive top end power but has no low down grunt and is very peakey.
I'm not saying it doesn't work or is not useful but I also don't think its a coincidence that Charlie uses LPC on a race bike.

[sean brady scooters]

i must admit al..............best results i,ve had from reed valve conversions is from the smaller /but closer mallossi blocks.............

[drunkmunkey6969]

If you look at some of the reed manifolds available for them especially the MB of NK ones they must reduce primary compression by loads and I've seen plenty of reedvalved 210s not make as much hp as std carb jobs.

Good post, but in that particular instance, would the loss not be more to do with the length and volume of the inlet tract, and the loss of velocity, rather than LPC in relation to volume?

I could be wrong, just speculating.

[tony]

I'm loving this thread!

[sean brady scooters]

me too...............

[tony]

Whilst Charlie is probably (so im told) bordering on genius, and was certainly the first person to talk about LPC in a magazine.....i dont think its fair to say that nobody ever thought about it before? Cranks like this are not new in Lambrettas.......MB did a crank like this in 1990 in a LC Lambretta, its still running today! As for Low or high compression, everyone harps on about cranks and compression........but no one mentions the fact that various combinations of Lambretta pistons with different comp heights, con-rod lengths and packers alter comp ratios......then everyone opens up transfers, welds them, makes big ear transfers and that all lowers compression too!

Now if Mb did one in 1990 what made him produce the full circle/plugged high primary cranks? The reason maybe is that he got better power out of these type of cranks with the then current off shelf or self-designed pipes he made. And so gave up the idea till now..Maybe Charlie approached it in a different way Dan ,hence the ridiculous power he gets from a 205 cc piston port un welded motor

[tony]

On a serious note....(and no red herrings ralph ) Its very likely that with a fairly mild ported motor, with an off shelf (ie not motor designed) a crank like the mb should improve things everywhere in the range. This takes us back to the 'article' if you read it again. However.. for high end stuff I think there are better ways of balancing the compressions of the motor to achieve better power for say the track. As has been demonstrated of course.