Originally Posted by partyhardryan16
Hey Guys. I just moved to Fort Collins, Colorado about 5400 ft up, from New York basically at sea level...I definitly noticed a difference in power. Is this normal? Also, they only sell 91 octane up here...
It's completely expected a normally aspirated engine will produce less hp at higher altitude.
The general rule of thumb is you lose 1% hp for ever 333 ft altitude gain. You also lose 1% hp for each 11F temperature increase, and lose more from humidity increase.
To pick a worst-case example, the hp difference between a low humidity 50F day in New York City vs a 90F humid day in Ft. Collins could be:
3.6% hp loss from increased temperature
3% hp loss from increased humidity
16.2% hp loss from higher altitude
22.8% total hp loss
IOW a bike producing 150 rear wheel hp on a cold, dry day at sea-level would only produce 115.8 hp on a hot, humid day at high altitude.
The effect is called "density altitude". There is little you can do to regain the lost hp except for forced induction.
Rejetting a carb will only minimize the loss to the above amount. It will not regain sea-level power or even approach that.
Most fuel injected engines have a barometric pressure sensor and will automatically reduce injected fuel to match the diminished oxygen level at high altitudes. Increasing engine compression ratio won't significantly help.
The engine must run at a certain air/fuel ratio to produce best power. There is simply less oxygen at higher altitudes, so the fuel must be reduced to maintain the same a/f ratio. Without this change the engine runs overly rich, and the loss is even greater.
Even with fueling changes, less fuel is burned per unit time, which means less power produced.