Poor guy, just gets on here and 6 days later his engine blows up. What are the odds?
Measure Twice, the numbers can be even larger than you think. My 125 KTM here puts out nearly 40hp at the crank as do most 125 motorcross bikes, the 250s put out around 50hp. This little 125 can easily run down the highway at 140kph (87mph).
Traditionally it was figured that 2 strokes could make double the hp of 4 strokes mainly they fired twice as often. This was the basis of 250 4 strokes competing in the 125 motocross class, and 450s in the 250 class. Research and development has pushed the advantage to the heavier more complicated 4 strokes like this Husaberg 450 (my son riding, not this old fat boy).
It all depends on the state of tune. Race engines work at higher pressures and RPM than plebeian work engines so race engines often make 1hp per 3-10cc. Work engines have long life and low stress in mind and may take 40cc to make 1hp. That said, my plebian minivan makes 1hp with every 12cc with no superchargers or other power adder tricks. The times they are a-changin'.
The formula for making horsepower is simple: PLAN = power
Pressure x Length of stroke x Area of piston x Number of power strokes per minute.
From this you must subtract the parasitic losses to get actual output.
(This is in Gordon Jennings "2 Stoke Tuner's Manual" by the way)
So, to increase power you must:
1) increase the cylinder pressure average through the stroke
2) increase the displacement (length x area)
3) increase the firings per minute (RPM or number of cylinders or 2 stroke over 4 stroke)
For us, what does the mean?
1) Add pressure? Compression, air in, tuned exhaust, better burn, spark, port timing
2) Displacement? bored or stroked, it has to be larger, 49cc vs 66cc, 38mm stroke vs 40mm
3) More firings? RPM! Balance the crank, port timing to rev, pipe to make her scream.
And finally parasitic losses? Thinner oils and greases, better alignments, smoother surfaces, more accurate gear and sprocket tooth cuts, lighter rubbing seals or even better just shields, less pumping restrictions and losses, less electrical load.
The PLAN formula gives an idea of what gains for any modification are possible.
5mm stroke? 13% more displacement, 13% power improvement possible
Get the picture?
There is no way a 0.020" (.5mm) over-bore is going to make 20% more power.
So why do these engines only make 1.5 to 3 hp instead of the 12 or so that a 50cc motocross does?
Have a look inside one of those engines. Between the ports is only a narrow 2-3 mm band.
Almost the entire cylinder diameter of this 125 is a window to allow mixture and exhaust to flow.
Plus the intake and exhaust tracts are tuned to pump flow through those ports,
PLUS the whole thing fires at 10,000 - 15,000 rpm or more.
The little China Girl cannot do that.
Steve