Scooterotica

.........as always things get bantered about, not only on this forum ......i have understood from the banter elsewhere that 1.2to1 PC was the standard Lambo figure and not 1.7 .....this figure is too high ........don`t you all think that 1.2 looks like 1.7 if read in a hurry ........for road racing a minimum geometric PC ratio of 1.4:1 is ideally required, 1.5:1 would be better which is more is difficult to achieve without more drastic measures.......because we are only dealing with points of a whole one the amounts removed from the crankcase are more easily achieved than first thought......with the tapering effect of the transfers is where a lot of cc can be achieved towards the LPC.....
....the true (or trapped) primary compression of the mixture starts not at TDC but when the back face of the piston closes the inlet port. The vast majority of the gas to be pumped up the transfer ports comes not from the crankcase but from under the piston as it descends, and the gas has to do a 180 turn to go up the best tapered transfers. Note that there is no pumping action from the crankcase. There is however a ramming effect from a correctly designed inlet tract which can greatly increase the gas pressure prior to primary compression starting..........by the way, i hope i am not included as a passer by, i did my apprenticeship with bikes in the early sixties and at Brands Hatch.........

PS
..........although the engine with the HPC ratio greatly increases the pumping losses and can decrease the power ....... a longer stroke unit that uses small transfer ports, even though pumping losses do increase there is an increase in power.........

I think youll find a Lammy crankcase PCR to be in the order of 1.63-1.65

156 D wrote:Note that there is no pumping action from the crankcase.

Isn't the "pump" defined by the piston moving to BDC with the inlet port closed - the pressure is higher below than above and so it's a pump....

156 D wrote:There is however a ramming effect from a correctly designed inlet tract which can greatly increase the gas pressure prior to primary compression starting

Yes, but "greatly" is overstating it unless it's forced induction

Where do your quoted numbers come from? - the ratios need to be put into a context with their application (and age).

Here we go again From old site


The problem is how do we measure the case volume here, this is where the problems start. Because in essence there are 2 crankcase volumes to take effect of the gas flow.

CVt Crankcase Volume at TDC: this is taken into account for Inlet Dynamics & the transfers volumes Vt but doesnt include the Inlet volume Vi.

CVb Crankcase Volume at BDC : This is taken into account for the Transfer Dynamics & includes the Inlet volume Vi but not the transfers volumes Vt.

So basically we are looking at the Inlet Dynamics and Transfer Dynamics
of crankcase compression.

So what we are looking at here for the Crankcase volume Cv is:

Cv = CVb+Sv+Vt (Sv is swept volume)
Or
Cv = CVt+Vi.

Now lets translate this into Compression Ratio using the new above Crankcase volume calcs.

PCr = Case volume at TDC CVt + Inlet volume Vi/ Case volume at TDC CVt+ Inlet volume Vi –Swept volume Sv

The best way to work out all these volumes is by using an old piston & puting a hole in the top & measure with a burette & liquid with the need for transfers & inlet to be accounted for.

There is some of the theory above
So what about practice, well as we are aware there are two schools of thought?
The use of high Primary compression ratios or low Primary compression ratio.

It use to be practice to reduce crankcase volume for high speed motors to produce high Primary compression ratio’s but now the opposite is being used.

The figure now being used is 1.5, which is deemed to be more then sufficient for high speed engines. This also being because the case volume changes the natural Helmholtz frequency of the Inlet and Transfer cycles and the CVt is used in the formula

The above formula as you can see takes into account the Inlet volume which when you look at the differences between a Piston Ported Barrel inlet track, & that of a TS,RB reed inlet track you may now see how this has a dramatic effect on Primary compression ratio.

Let’s now take 2 kits the Rapido Piston port & the TS1 reed valve (Not actual Figures)

Rapido 225 with standard crank no padding could give say PCr of say 1.55
You now put a full circle padded crank this could then increase PCr say to 1.6

Now let’s take TS1 with standard crank no padding could give say PCr of say 1.4
You now put a full circle padded crank this could then increase PCr say to 1.45

Why do think these sort of figure will come about i.e. TS1 having a smaller PCr then Rapido well think how much volume there is between the two inlets.
The TS1 Reed cage inlet has a much bigger volume.

What I’m trying to show here is, it again down to application what is fit for one engine may not be good for another

Hope I’ve explained myself here


Smiffy

DO WE HAVE TO DO THIS ALL OVER AGAIN

RinB wrote:DO WE HAVE TO DO THIS ALL OVER AGAIN

No not really. My point is that theory, dusty old tuning manuals, and assumption don't move things forward and settle the issue unless it's backed up with fact .................. there are now cranks and dynos but no back to back results (i.e LPC crank/exhaust vs. HPC crank/exhaust).

Tony

You are quite right which one is right I think both are right if done properly.

As for back to back test I dont think Charlie will let us look inside his engine

RinB wrote:DO WE HAVE TO DO THIS ALL OVER AGAIN

Yes....ON YOUR FEET SOLDIER!!!

RinB wrote:As for back to back test I dont think Charlie will let us look inside his engine

No....but at some point, either we will have the info from my TS1 or MB will have the info from thier exploits (both on and off track).....so its just a matter of time.

RinB wrote:Tony

You are quite right which one is right I think both are right if done properly.

As for back to back test I dont think Charlie will let us look inside his engine

Yep, that's why I keep banging on about the application......... my assumption (and I'll admit it's only that) is that LPC would have the potential for greater peak power, where as HPC would have a wider (though lower) spread of power.

I love the ideas and innovation, but there comes a point when it has to be backed up with cold fact...... as you say Charlie is doing the business, but you'd be foolish to believe anything a racer tells you

It would also be a disservice to credit a race performance against an engine only - rider talent, setup and prep. play equally large parts in the end result.