shan2129
You are correct in thinking there is less oxygen at high altitudes but it isn't just the oxygen.
Actually, the percentage of oxygen remains pretty constant at all elevations.
The thing that changes as the elevation increases is the air pressure (and density).
As an example, at sea level the air pressure is roughly 10.13 X 10^4 N/m²
At an elevation of 1000 meters the pressure drops to 8.988 X 10^4 N/m²
At an elevation of 2000 meters the pressure drops to 7.950 X 10^4 N/m² (21 % less than sea level)
At 3000 m = 7.012 X 10^4 N/m² (30 % less than sea level)
At 4000 m = 6.166 X 10^4 N/m² (40 % less than sea level)
At 5000 m = 5.405 X 10^4 N/m². (47 % less than sea level)
As you know, a reciprocating engine is just a big air pump which gets its power from the amount of air it can take in plus the energy in the fuel. More air will burn more fuel making more power.
The amount of air the engine will try to breath in is affected by the residual amount of exhaust left in the cylinder as the intake valve opens. Less exhaust left in the cylinder equals more fresh air that will be drawn in.
This would seem to indicate that a less restrictive exhaust system would result in greater power gains and to a degree, it does at higher rpm's but at lower rpm's where most of the engines operate, the effect may be much less than many wish for.
At lower speeds the stock silencer and exhaust system on the Royal Enfields is less restrictive than some think so the gain from switching to a less restrictive silencer offers little gain.
As Rakesh mentioned, many find the "gain" can actually be a loss in low and mid range engine speeds.
The reasons are complex dealing with exhaust velocity, exhaust length, the speed of sound and several other factors that are beyond the scope of a net forum.
About the engine running lean when less restrictive silencers are installed:
The prime reason is the engine can breath in more air during its intake stroke.
Although the velocity of the air thru the carburetors venturi increases and as a consequence it tries to suck more fuel, the restrictive jets that control the fuel mixture will not allow enough fuel thru them to keep up with demand. The result is the mixture becomes lean which can burn valves, overheat the piston and cause backfiring during deceleration.
Increasing the size of the jet(s) can restore the proper fuel/air balance.
This is usually a problem on engines that use a carburetor however, fuel injected motorcycles often can automatically readjust their mixtures to maintain a more correct fuel/air mixture.
They do this by sensing the pressures inside the throttle body coupled with the throttle position. (In automotive applications, they also measure the mass flow of the air but motorcycles usually don't have this feature.)
The Royal Enfield fuel injection sensors and the computer map seem to do a fairly good job of minimizing the effects of installing a different silencer with less restriction but I do not recommend going to a totally open silencer. The effects may be greater than the stock fuel injection map can handle.
By the way, the Royal Enfield fuel injection system is designed to maintain the correct air/fuel ratio up to altitudes of 5440 meters. RE knows what the Indian riders need.