It’s very difficult to obtain a reliable “absolute” measure of power. A normal dyno bench will give you a “relative” measure, useful if you want to check the progress in the tuning of a certain motor. Too many parameters influence very strongly the measure of power and precision and constancy of the measure can be very expensive to obtain.
Even if you do everything correctly, the precision of the measure can be fouled by bad use or by environmental conditions such as very humid or very hot air or factors you never even imagined. Multiple measures are always required and the difference between apparently identical measures taken with the same procedure can be surprising.
there are some obvious technical limits to the power that can be obtained from an engine. Power is the product of the pressure in the cylinder (usually expressed by the “mean effective pressure” or MEP, which takes into account also friction factors) times the capacity of the motor times the number of cycles per unit of time (equal to half the RPM for a 4 stroke since it takes two revs to complete a cycle).
Pressure. For an atmospheric breathing engine, pressure is limited by how much air it can “suck” in, by how much the sucked mixture can be compressed without blowing off by itself, by how quickly and thoroughly the air-petrol mixture can be burnt, and by how thoroughly and quickly the spent gases can be spat out to make room for a new fresh charge.
For a modern 4 valve engine, a good value is around 14000 hPa (=14 kg/cm2 =200 psi) but on an obsolete two valve engine you will be lucky to see 10000 hPa (=10 kg/cm2 = 145 psi) at max power RPM. This is not the maximum instant pressure inside the cylinder, which is a lot higher, but the MEAN along the cycle. Note also that this is not the highest MEP you can get out of that engine: in fact the max MEP will be obtained at max torque RPM, but since max torque is obtained at a lower RPM, the corresponding power will be lower. Since we are focusing on max power we will not deepen the matter of max torque and max MEP. Supercharging can increase MEP to whatever value you like, at least as long as you can keep the cylinder firmly attached to the cases.
Capacity: We all know what it is: Bore squared times pi/4 times Stroke times Number of cylinders. Its limits are dimensional. Since bore appears in the formula at the square power, its effect on capacity is twice as important as that of the other parameters. The limit to bore is thermodynamic: in a very wide cylinder the flame front will have a long distance to travel before it can burn the droplets of petrol hidden at the sides. A big bore—short stroke cylinder will also make it very difficult to achieve a high compression ratio.
Revs: they are a measure of how quickly an engine can complete a cycle and repeat it over again. Revs are primarily limited by the life of the reciprocating parts. Giving for granted that the valve train will survive higher revs than the piston-rod assembly (and it’s not always SO granted) the upper rev limit is usually obtained as a first approximation by the “mean piston speed”. If you want to build a hot rod that will be rebuilt every 1000 turns of the crank, you can go up to a mean piston speed of 28 m/s (92 feet per second). If you want your engine to last a race, you can risk 24 m/s (79 fps), if you want your engine to last a season without worry, you’d better focus on 20 m/s (66 fps). On a road going bike that is supposed to last several years, 18 m/s is the highest safe figure. These figures are for 1980’s technology motors. You can increase them by 25% if you are using 21st century technology.
Mean piston speed is linked to RPM by the length of the stroke: the longer the stroke, the lower the RPM at which the motor will reach its safe limit. The formula is: Max RPM = (max piston speed / stroke) x 30000. so if your engine has a stroke of 70 mm the piston will reach the 20 m/s limit at 8570 RPM; if the stroke is 85 mm the limit will be reached at 7060 RPM.
Now we have all the elements to calculate your engine’s “maximum reasonably obtainable power“. Unless you are using the SI measure system, you will need some conversion factors:
Power (Hp) = Mean Pressure (kg/cm2) x Capacity (cm3) x RPM / 912’483
Power (Hp) = Mean Pressure (psi) x Capacity (cm3) x RPM / 12’978’554
Power (Hp) = Mean Pressure (psi) x Capacity (cu.in.) x RPM / 30’847
example #1: take a 1150 cc twin cylinder, two valve motor with a stroke of 86 mm; setting a mean piston speed limit of 20 m/s, it will rev safely to: 20/86 x 30000 = 6975 RPM.
If you can tune it to obtain a mean pressure of 10 kg/cm2 at this speed, which is a good result, you will get: 10 x 1150 x 6975 / 912483 = 88 Hp
example #2: take a modern motor with the same capacity as the one above. Suppose it’s a 4 cylinder, 4 valve with a stroke of 54 mm, it has a safe piston speed of 22 m/s and the mean pressure at max power RPM is 12 kg/cm2. then you will get:
max safe RPM: 22/54 x 30000 = 12220 RPM
Power (Hp): 12 x 1150 x 12220 / 912483 = 185 Hp
It should be clear now how the modern motors can be able to achieve such outstanding powers with apparent ease, while we must go to great efforts to squeeze a reasonable performance from our poor old two valvers (LG)
So what kind of power can we expect to get from our Pantahs?