Scooterotica

whats the latest thoughts on full circle cranks, a few tuners have turned their noses up lately as a waste of time and money.

i need to buy a 60mm mec crank to sort out my mugello 225 so was wondering if i should pay the extra or just buy a standard one at 60mm

ive got an alpha crank in the ts1 but i would nt know if it accounts for anything or not really

I just fitted a full circle in my parmakit which used to run a standard rotary crank and cant really say I notice any difference,a dyno may show it does but if you cant feel any difference,it aint worth it.

Careful Cezeta, you could open up the whole primary compression ratio debate again.

I'm looking at one of JB's 61mm items for my 240 and 250 engines. These are full circle, but I have heard discouraging things from other respected tuners about full circle cranks.

My thoughts are that full circle are best for engine developing power over a wide range of engine speeds, and LPC cranks are best for maximum power at high revs. However, Soosh's findings maybe entirely representative of reality.

cezeta wrote:whats the latest thoughts on full circle cranks, a few tuners have turned their noses up lately as a waste of time and money.

i need to buy a 60mm mec crank to sort out my mugello 225 so was wondering if i should pay the extra or just buy a standard one at 60mm

ive got an alpha crank in the ts1 but i would nt know if it accounts for anything or not really

Nick

I have had a full circle crank built for the Rapido. Hopefully getting back on Friday?

I have traded it against my 60mm Maz crank, so someone we know has one if required?

Of course it's been 'Mazzed'

Maz did you go for 61mm or 63mm

hello cezeta
harry barlow is building me a nice ts1 based engine.
i supplied a 60mm full circle MEC crank
he inspected it and said it was fine but would have used a normal race 60mm MEC as they are cheaper but no worries

mickdale wrote:hello cezeta
harry barlow is building me a nice ts1 based engine.
i supplied a 60mm full circle MEC crank
he inspected it and said it was fine but would have used a normal race 60mm MEC as they are cheaper but no worries

yes thats the feed back ive been getting too, kind of "its fine but why bother". i have a mec not hpc in there at the moment which is very smooth, im only changing as the standard 58 stroke/mugello 225 does not realy work. so im just doing a swap as advised by scooter centre to the 60mm

The only full circle crank I ever bought and ran was for my Malossi 210 big reed valve engine. It seemed to run smoother but that was probably because it was balanced and trued where the old crank wasn't.

What is the big debate about LPC vs HPC? Do the newer, fatter pipes pull the charge through so LPC and greatter crancase volume is what you want? I'm a little fuzzy about this subject.

GP Kevo wrote:What is the big debate about LPC vs HPC? Do the newer, fatter pipes pull the charge through so LPC and greater crankcase volume is what you want? I'm a little fuzzy about this subject.

I'll try an answer this one, but it does contain my usual dose of "what I know, what I think I know, what I've heard and mis-interpretted, and what I've guessed at", so be warned.

Whilst higher primary crankcase pressure is good at sucking-in the fresh charge, any pressure below the crown of the piston resists the downward movement of the piston. Therefore, a lower primary compression ratio has benefits, as forcing the piston down as quickly as possible is what we are all aiming to do. However, taking the primary CR too low will adversely affect the ability of the engine to draw fresh charge into the engine.

You mention the fatter pipes pulling the charge through more efficiently. Well it is easy to argue that all expansion pipes do this, and the choice of pipe will often depend on where this occurs in the rev range. Now before we get off track on that issue, let's just remind ourselves that with the correctly tuned length of expansion pipe, you can run a two-stroke with virtually zero primary CR. This is, as you imply, because the negative pressure created by the pipe is able to pull the charge right through the engine to the top-end, via the transfers, from the bottom end. The disadvantage of this knowledge is that the engine will have a very restrictive rev range. This is not a problem on certain engine applications, though motorcycle engines is not a good example.

This knowledge can be applied to certain motorcycle applications though. Racers are able to exploit LPC as they run their engines at a pretty constant high revs. They don't filter up to the traffic lights or queue at roundabouts, so they can compromise their slow speed running and reduce their primary CR by building better and better expansion pipes.

Meanwhile, people who do still have to operate in the real world (traffic lights, roundabouts, 30mph limits) may well benefit from higher primary compression ratios which will never allow the engine to produce 40bhp like their racing cousins, but should be easier to ride on the streets.

Like all tuning, there is compromise to be made, but I wonder if these things are really measureable.

Meanwhile, there are probably better explanations available in the "search" function of this forum.

I've tried HPC cranks and had no positive gain. I think that this is because Lambretta engines have a very high primary compression as standard. Warning with MEC 60mm cranks, you will need to relieve the crankcase or the big end eye on the rod for these to rotate in a lambretta crankcase. Pretty good crank though.