I fished out of the depths of my hard disks some old XLS files where I had tried to assess what kind of performance one could expect from an accurately tuned Pantah. The starting basis was that “all Pantahs are created equal” that is to say, that the designers increased capacity, from the smallest 350 cc to the biggest 750 cc motor, keeping a number of factors similar, so that all motors, regardless of their capacity, are comparable in terms of fluid-dynamic and thermo-dynamic behaviour.
Of course, since there must be a certain standardisation of parts, some models share the same head, others the same crankshaft, but there’s a remarkable constancy in many parameters.
For example, the combustion chamber has the same shape and proportions, the compression ratio is virtually the same for all engines, and the bore to stroke ratio is respected to a great extent in all engines. These three data alone mean that the motors are all designed with the same fluid-dynamics behaviour in mind.
Power produced by a combustion engine is the product of combustion chamber pressure times revs times capacity. Supposing that pressure and revs are indicators of the “development” of an engine, I attempted to answer the following question “what power can be expected from a certain engine, if it is developed like the best engine of the family?”
Note that by “Development” I mean: “given a fixed capacity, reaching the maximum possible RPM and getting the highest possible pressure at that RPM”
So the first thing I did was to find out what maximum revs can be expected from each engine. Revs are ultimately limited by the reciprocating parts: piston, conrod and valvetrain. Supposing that the conrod and the desmo valvetrain are above any suspect, then the limit is given by the piston. A typical limit is set by mainstream literature at an average piston speed of 20 m/s.
This leads to a max RPM of 11800, 10400 and 9750, respectively for the 51 mm stroke crankshaft (350 and 400), the 58 mm one (500 and 600) and the 61.5 mm one (650 and 750). Setting the piston speed limit to a cautious 18 m/s gives the result of 10600, 9300 and 8800 which are very similar to the actual values of max power RPM obtained bu the standard production bikes. Setting it to 19 m/s gives 9800 RPM for the 58 mm crank, which coincides with the max power RPM value of the TT2 600. One good result.
Next thing, I took each motor’s max power, capacity and revs and I elaborated the mean pressure at max power of each engine. Among “standard” bikes, the best pressure is obtained by the 500, with 9.73 bars, somewhat lower than that of the TT2 600 (10.22 bars, and moreover obtained at a higher RPM). Among “special” bikes, the best mean pressure is that of the 750 racing tuned Montjuich, with a hefty 10.42 bar (assuming that the claimed value of 94 Hp at 11000 RPM is true, which I somewhat doubt). The 600, the 650 and the 750 all have similar mean pressures (around 9.15 bar) showing similar efficiencies, a bit lower than that of the 500. A bit worse are the two little engines, the 350 and the 400, stopping the gauge at less than 9 bar, but it must be considered that they reach this value at a much higher RPM than the other motors.
|Crank stroke mm||RPM @ 18 m/s avg. piston speed||RPM @ 20 m/s avg. piston speed|
Note that the best indicator of motor efficiency would be the mean pressure at max torque RPM, and not that at max power RPM, since when max power is obtained, the pressure has already begun to decrease. but we’re looking for a possible max power here so we will not investigate further what happens at max torque RPM. Moreover these data are not obtained from tests but from what the factory or the motorbike press tells, so their reliability is relative.
Now let’s combine things together: the best pressure with the maximum possible revs. This is playing with numbers, since it’s not all that easy to take the pressure from, say, a 600 cc revving at 9500 RPM and applying it to a 750 cc that revs at 8500 RPM but what I wanted to find was a “reasonable upper limit” a non plus ultra, a threshold to crave for.
So I did a series of calculations, using the three values of reference of average piston speed (18, 19 and 20 m/s) and various values of mean pressure.
The results are interesting and promising: a 600 cc motor able to develop a mean pressure of 10.22 bar, like a TT2, would output 61 Hp at 9300 RPM or 64 @ 9800 or 67.5 @ 10350 which is the maximum safe RPM for that motor.
Of course it would be either one or another figure, not all three together since the motor will be optimised for a certain RPM value, and pressure would be lower at different RPM.
|Motor type||Standard mean pressure at max power RPM in bars|
A 650 would set the dyno at 64 Hp @ 8800 RPM or 67.5 @ 9300 or 71 @ 9750. These values of power are not far from those of the 600 since the increase in capacity is in part negatively compensated by the necessary reduction in RPM to cope with the longer stroke. A 750 would show the following figures: 73.6 Hp @ 8800 RPM; 77.7 @ 9300 or 81.8 @ 9750. This can be reasonably taken as being the maximum power achievable by a Pantah engine without using rocket science or esoteric materials. It is clear from these figures how the specific power (Hp per litre of capacity) is reduced as the capacity increases.
Supposing a milder and more reachable mean pressure of 9.73 bar which is the value achieved by a standard 500, the values would be the following: 600 cc: 58 Hp @ 9300 RPM; 61 @ 9800 or 64 @ 10350; 650 cc: 61 Hp @ 8800 RPM; 64 @ 9300 or 67.5 @ 9750; 750 cc: 70 Hp @ 8800 RPM, 74 @ 9300 or 78 @ 9750.
|Capacity of motor cm3||Max power achievable@ revs, road tuning||Max power achievable@ revs, race tuning|
These are more reasonable figures for a road going mildly tuned bike, particularly the ones obtained at the intermediate RPM figure. Note that they are all around 100 hp/litre specific power, getting slightly lower as the capacity increases. Considering that it gets more and more difficult to achieve a high combustion chamber pressure the higher the RPM, and the greater the capacity of the engine, I would aim to these more reasonable intermediate values as achievable goals, unless one is seriously committed to winning classic races, of course.
So, now we have a fairly good idea of what kind of power we can reasonably expect from our motors. How to get this power in practice, is a wholly different matter of course. (LG)