I would like to get some answers to the fuel ratio of my bike.

I'm thinking of replacing the stock exhaust system with one that is louder than the one I have now and also changing the air intake and air filter with a hypercharger.

Will I have to change the fuel ratio to compensate these changes?

  • You should not need too if the sensors are doing their job although I am not familiar with the ECU of bikes
    – method
    Feb 9, 2017 at 9:02

2 Answers 2



Bikes come from the factory running as lean as is acceptable, for emissions and fuel economy reasons. Adding a higher flowing air filter and/or exhaust will push your fuel ratio even more lean, which is not something you want.

A slip-on exhaust won't change much, but if you're getting a full system it can upset the air fuel ratio. In that case, you'll want to tune the bike with a fuel controller or an ECU flash. Same goes for the air filter if it flows more air than stock and your bike isn't able to compensate for it.

If you're just getting a slip-on and the air filter flows the same as stock, it will be fine.


"It’s true that modern bikes run on the lean side of the air/fuel mixture spectrum to help meet emissions." - http://www.motorcyclistonline.com/does-your-new-motorcycle-run-too-lean

"When you upgrade your bike with a new exhaust system, air box or hi-flow air filter it may upset the air to fuel ratio creating a mixture that is too rich or too lean." - http://www.weeksmotorcycle.com/Carb-jetting.html

"Which is why we have products that allow you to retune the fuel and ignition systems to improve performance and accommodate changes such as a different exhaust or air filter." - http://www.motorcyclistonline.com/all-about-tuning-your-bikes-efi-adding-power-commander-or-reflashing-ecu-motorcyclist-mc-garage-tech

  • I'd be surprised if bikes run lean from the factory. On the automobile side, they run rich from the factory and can usually benefit from aftermarket tuning in both power and economy. Secondly, even if they do run lean from the factory, introducing more air should not change the air fuel mixture. If the bike is fuel injected, the ECU will accommodate the extra air by adding more fuel. If the bike is carb'd, the carb naturally provides more fuel as air is increased. Just adding an air filter will not cause the issues you are specifying. If you have references proving me wrong, please show us! Feb 9, 2017 at 18:54
  • It's also possible a number of motorcycle EFI systems are open loop. That seems to be the case with my bike.
    – raydowe
    Feb 10, 2017 at 10:44

Motorcycles require either a rejet (carb) or remap (fi) when changes are made to the intake or exhaust.

A carburetors fuel output is not linearly proportional to the air flow it receives. While it is true that the carb will provide more fuel as air is increased the fuel will not be added correctly since the carb is a complicated device with many non-proportional overlapping circuits and can only be tuned to perform correctly over a relatively narrow range of airflows.

Similarly the ecu of a fuel injected motorcycle typically works from a fixed map at high loads. If this table (air mass / rpm) has not been extended beyond the normal operating range (i.e. for larger air masses ) properly, then the incorrect fuel will be supplied. This is likely to produce a dangerous lean condition in motorcycles which typically do not have knock sensors or run fuel correction algorithms at high load.

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