Scooterotica

and another thing .................. with all the additional inlet/transfer port volume on a modern 2S, is it LPC by design or by consequence?

If you want LPC wouldn't you be better to take material out of the transfers so at least you can shift all that extra gas?..................but I'm only guessing

so what about mpc then as a compromise ?

Avantone wrote:

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....

I understand what you mean.........i was merely saying that most of the gas to be pumped up the transfer ports comes NOT from the CRANKCASE but from under the piston.....therefore the CRANKCASE is not responsible for a pumping action......

Avantone wrote: If you want LPC wouldn't you be better to take material out of the transfers so at least you can shift all that extra gas?

..................this was also my point for LPC.........but tapered ports.......


We used to measure our primary compression in this way........

We placed the engine horizontal with the piston (NO HOLE IN) with the inlet port facing up level......filled up with oil from a measuring jug to the cylinder wall port, you should gently rock the engine to dispel any air trapped, note the amount then write this volume down as V1......bolt on the inlet with the gasket and lift up vertical.......continue to fill up to the inlet tract flange and record this extra amount as V2.........you can add the carb. and bellmouth if desired for future need on the carburation.....to find the primary compression ratio you need the true swept volume of the engine...

Swept volume = 3.142 x stroke x bore2 /4
Geometric primary compression ratio or GPCR = V1 / V1 - SV

anyone got a pic of an rs125 honda or an aprllia disc valve? These have to be the pinnacle of race two stroke technology. Now just by looking you can see how they make their power.

Tony............
do you happen to know the actual pcr of your motor...........or could you find out maybe...........................?

In the absence of our own dyno comparisons, i asked Mark if he could expand on a few details relating to how the crank in the thread came about.......this is the message i got from Mr Broadhurst:

OK, I asked Paul Baker what cranks they were using in Stuarts bike, they said a low primary one, I ask if he had problems and the upshot was I said I would see if we could make a balanced, tighter LPC crankshaft for them to test. SRP Racetec run an LPC crank too, and I built the one in the thread before I set off on the Euro. Interestingly, I spoke to a tuner who said they get 3-4 bhp more on a RB with a LPC crank! They can not get the power from an RB without lowering the primary compression!

in a HPC ratio motor.......much of the inertia from the inlet charge is cancelled out because of the higher pressure in crank /flask area.................

where as ,a lower primary comp ratio will let more charge in,and also has more space inside to accomodate it.............
a lower PCR therefore allows an engine to not only rev higher and more freely(due to less pumping pressures).........but inhances the effects of tuned intake pulses and ex pulses to the max effect....................

sean brady scooters wrote:in a HPC ratio motor.......much of the inertia from the inlet charge is cancelled out because of the higher pressure in crank /flask area.................

where as ,a lower primary comp ratio will let more charge in,and also has more space inside to accomodate it.............
a lower PCR therefore allows an engine to not only rev higher and more freely(due to less pumping pressures).........but inhances the effects of tuned intake pulses and ex pulses to the max effect....................

So it's good for extra power at high RPM? i.e racing?

As the exponents have been CE, Stuart/SRP and MX in general, can we assume it doesn't lend its self to bottom end grunt and 4-speed boxes?

Reading between the lines on MB's quote regarding "problems", is balance an issue with it shaking out of balance on the big end? (hence the mention of "tighter")

no mate...............
if bottom end grunt as you call it was worse................how then would such motors accelerate..............
the pins are tighter ........just to accomodate the extra power ...................derived from these..................