Real Lifan 200cc engines are known as 163FML-2 OHC. It has a stroke of 63.5 x 62.2mm which is relatively long and contributes to it's low RPM torque.
Some look almost the same but most of them are Jailing or Stannic short stroke engines, 63.0 x 43.0mm and 61x 49. 5mm. They may or may not have OHC and they are not up to Lifan Quality.
That's both a misstatement of fact AND an apples-to-oranges comparison:
1. The numbers you specify for the stroke of the "163FML-2 OHC" are the bore and the stroke, respectively, expressed in millimeters.
Because the RICE (reciprocating, internal-combustion engine) is commonly categorized by its "displacement" -- the volume swept by its piston (or pistons, in the case of a multi-piston RICE) -- for RICE having round bores (as opposed to elliptical, etc.), the most accurate formula for computing displacement is that for determining the volume of a cylinder, multiplied by the number of pistons having the same characteristic bore and stroke.
For instance, the bore (B) of the "163FML-2 OHC" in this example is 63.5 mm; the stroke (incidentally, the stroke is twice the "throw" or "torque arm" of the crankshaft) is 62.2 mm. In the common geometric formula, stroke is substituted for the height (or "length") of the cylinder:
Where "*" indicates multiplication, "/" indicates division, and "^2" means to square the preceding number:
1 (cylinder) * pi * (B/2)^2 * S = displacement
Because we're looking for cubic centimeters, we want to keep units consistent; therefore, 63.5 mm becomes 6.35 cm and 62.2 mm becomes 6.22 cm. Using 3.141592654 as the value of pi (a geometric constant) and performing the math in the order of operations, we see that:
1 * 3.141592654 * (6.35)/2)^2 * 6.22 =
1 * 3.141592654 * 3.175^2 * 6.22 =
1 * 3.141592654 * 10.08625 * 6.22 =
196.98 cc for the Lifan.
This is very important, given the fact that you disparage the engines against which you contrast the Lifan:
Again, for a single-cylinder RICE:
63.0 x 43.0mm =
1 * 3.141592654 * (6.3)/2)^2 * 4.3 =
1 * 3.141592654 * 3.15^2 * 4.3 =
1 * 3.141592654 * 9.9225 * 4.3 =
134.04 cc for the first "competitor"
and that
61x 49. 5mm =
1 * 3.141592654 * (6.1)/2)^2 * 4.95 =
1 * 3.141592654 * 3.05^2 * 4.95 =
1 * 3.141592654 * 9.3025 * 4.95 =
144.66 cc for the second "competitor."
Such a "comparison" smacks of either ignorance (which I hope I have to some small degree helped you to overcome) or dishonesty (which I hope isn't the case).
Even limiting the discussion to "naturally aspirated engines fueled by gasoline" is inadequate to account for radical differences in engine output, reliability and longevity among engines having identical bores and strokes and numbers of cylinders -- and those differences have much less to do with brand or corporate philosophy than they have to do with actual engineering decisions and unavoidable manufacturing compromises.
There is no such thing as a "no compromise" RICE. Even before emissions issues forced engineering and manufacturing changes, packaging considerations often limited output or otherwise hurt efficiency.
Generally, the piston area exposed to the working fluid (combustion gases) is directly proportional to the bore -- for round bores, according to the formula for the area of a circle. Consequently, a larger bore is advantageous to the production of torque and horsepower.
The torque-arm of the RICE is the "throw" of the crankshaft; by definition, the "throw" is exactly half the stroke. However, doubling the throw does not translate to doubling the torque: packaging considerations internal to the engine itself become important. Hereinafter, "AOR" shall be construed to mean "axis of rotation;" "CL" shall be construed to mean "centerline;" "XOR" shall be construed to mean "axes of rotation."
In most cases, the CL of the cylinder's bore intersects the CL of the crankshaft's AOR (its main journal rotational-axis CL), and it is on such an example the following is offered:
Unless otherwise specified, all measurements refer to distances measured in a plane perpendicular to the crankshaft's AOR CL, and coincident with the CL of the cylinder's bore. The following also assumes no wristpin offset and no offset for either of the bores of the connecting rod, and does not cover complex geometry as might be found in a radial engine having coplanar cylinder-bore centerlines, etc.:
The throw of a crank is the distance perpendicular to the CL of that crankshaft's AOR and the CL of the AOR of the associated (connecting-rod) journal.
The piston is ordinarily connected to the crankshaft via a "connecting rod," the effective length ("RL") of which has virtually nothing to do with its overall length; rather, its effective length is the distance between the CL of the XOR for its associated wristpin (which fastens the piston to one end) and crankpin (the journal at which the connecting rod is attached to the crankshaft).
The point at which the piston most efficiently transfers force to the connecting rod is when the angle between the wristpin, crankpin and main journal rotational-axis CL (vertex at the crankpin) is 180 degrees; in this case, it occurs (when the piston is) at TDC (top dead center).
The point at which the connecting rod most efficiently transfers force to the crankpin is when the angle between the wristpin, crankpin and main journal rotational-axis centerlines (vertex at the crankpin) is 90 degrees. The distance of the piston down the cylinder from TDC at which this alignment occurs is governed by the geometry determined by the ratio of RL (the effective length of the connecting rod) to "S" (the stroke).
The compression height of the piston is the distance between the crown of the piston (the top "main deck" of the piston, above which may extend a dome or other feature, and below -- i.e., into the face of -- which may extend valve reliefs, a dish or other feature or features) and the rotational-axis CL of the wristpin (or the CL of its associated bore).
Taller pistons produce greater side-loading of the cylinder and are heavier (therefore placing undue strain on the connecting rod). Shorter pistons reduce cylinder-wall loading as contrasted against taller pistons and, because they are lighter, reduce strain on the connecting rod. However, practical pistons have to be sufficiently tall to allow the use of an effective "ring stack" -- for production automobiles, this typically consists of a compression seal, a wiper and an oil ring.
The rings are separated from each other by "lands" and ride in "grooves" in the sides of the piston.
The ideal measurements governing the minimum distance from the crown to the top of the uppermost groove, ring separation distances and allowable wristpin-bore intrusion are determined by the metallurgical (mostly mechanical and thermal) characteristics of the alloy from which the piston is produced, the manufacturing method, the shape of the piston, post-ignition combustion-fluid characteristics, expected longevity, etc.
The "best time" for such alignments is determined exclusively or almost exclusively by the combustion characteristics of the fuel-oxidizer mixture being used, which is influenced (often strongly) by combustion-chamber shape and piston topography, the rate at which the piston approaches and then accelerates away from TDC, the "dwell time" of the piston at TDC, the ratio of the bore to the stroke, homogeneity of the combustion mixture, etc., etc.
Generally, for equal strokes, shorter connecting rods allow the aforementioned 90-degree angle to be met earlier, with the piston closer to TDC -- which can be advantageous, as combustion pressure could be at or near its peak at that moment; however, as the crank continues its rotation, the efficiency of the short rod diminishes rapidly. From a tuning perspective, the engine is nearest adiabatic efficiency when it is nearest detonation of the combustion fluid.
Conversely, longer connecting rods achieve greater efficiency by minimizing the departure angle of the connecting-rod from the cylinder's bore centerline (described by the angle between the main journal's AOR CL, the wristpin's AOR CL (vertex) and the crankpin's AOR CL).
Tuning inefficiency caveats -- from the impracticability of perpetually operating as near detonation as possible while simultaneously avoiding detonation, to variations in fuel and oxidizer qualities -- ordinarily indicate superior performance from an engine having longer rods. However, longer rods are heavier; their construction requires larger equipment, more energy and more material: they aren't always worth the expense.
Short rods have regained popularity at the top levels of relatively unrestricted motorsports (NHRA and IHRA Pro Stock, FIA F1. etc.); long rods remain popular in low-rpm, extreme-torque applications usually also associated with better-than-average fuel efficiency. In the end, each engine is designed, manufactured and tuned to meet specific demands; that different engines offer disparate performance is unsurprising.
It is disturbing that you seem to have contrasted a 200 cc - class enduro engine against motorcycle engines having 35 percent less displacement, presumably loaded identically and subjected to test conditions favorable to the enduro.
I live in Mississippi and I own a TMS 250; I am 70 inches tall and weigh 281 pounds without a helmet or any riding gear. Wearing my helmet and gear, toting a military duffel bag grossing about 50 pounds, and starting my trip with a full tank of gasoline, my TMS 250 had no problem whatsoever briskly accelerating me and my luggage to a top speed slightly greater than 80 mph. My TMS 250 also averages 82 mpg in mostly rural highway (65-or-so mph) and Interstate travel.
My TMS 250 cost me less than half what I would have paid for a new Virago 250, it looks better (imho), and it has without incident held up to some surprisingly rough treatment. If I could've afforded it, I would have bought a larger Japanese bike; heck, I even considered the Hyosung, but it was just simply out of my price range -- and the TMS 250 has taught me that I don't need to "go Mach 2 with my hair on fire" to enjoy motorcycling.
