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So, an increase in air flow with air box modifications will deliver a decrease in MPG? Can someone "dumb down" this answer for me? I can't wrap my feeble mind around this concept. I'm sure it's fairly straight forward, but I don't get it. Surely there's a part of the process I've overlooked.
 

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So, an increase in air flow with air box modifications will deliver a decrease in MPG? Can someone "dumb down" this answer for me? I can't wrap my feeble mind around this concept. I'm sure it's fairly straight forward, but I don't get it. Surely there's a part of the process I've overlooked.
This is just spit balling without thinking up the math and may be very incorrect, more air = more rich, more rich means sucking up more gas or burning more gas with the hungry for more.

I believe the bike is already suffocating trying to breathe, not enough air coming in on top of how lean it runs, of course over lean can cause bad MPG cause it has to struggle more to drive, but I imagine they got the formula just right to be hyper efficient versus over the edge of inefficient.

Just my thoughts with out calculating A/F, mass, resistance, consumption, etc.

Like a vacuum cleaner with a sock stuck in it.........although that might not be accurate since I imagine the sock stuck in it causes the motor to pull more amps, trying to pull the same volume of air.................I don't know, ignore me and this part, I think I killed whatever kind of rational thinking I just had.....lol :grin:
 

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In my simple mind its like this: It takes air and fuel to make a good bang and push those pistons down, give it more air and you need more fuel to get that air/fuel ratio right, thus more air = more fuel = lower mpg. Also it is my understanding all bikes including the N400 come very lean from the factory to meet air pollution requirements, if you get your ECU flashed it will run richer (and better) and will lower mpg as well. Of course all of this is dependent on how much you want to restrict your right hand from twisting the throttle, I have minimal restrictions in this area ;-)
 

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Okay- let's put it down from a professional's point of view. Adding more airflow through the airbox will be detected by the oxygen sensor, which, in turn, will cause the ECU to see a lean condition, which will (to a point) cause it to keep the injectors open longer for a given injection event/load, which, yes, causes a decrease in fuel economy, as more fuel is being burned.
The second part of this is that these bikes, as has been mentioned, run quite lean (less fuel than is optimal for a given load range) from the factory, especially under 5-6K RPMs. This is painfully common in modern cars and bikes, as manufacturers do this to meet strict emissions regulations. Unfortunately, the bike can only compensate for so much added airflow (injecting more fuel), as modifications can push the bike outside of its ability to learn (bike ECUs and fuel systems are much more rudimentary than on modern cars and trucks- only one oxygen sensor, no knock sensor, etc.). This is where you can run into problems- running too lean will raise combustion and exhaust temperatures, which will lead to early component failure- catalytic converter, oxygen sensor, and, in extreme cases, internal engine failures.
This is why a fuel programmer is strongly recommended on these bikes, even in factory trim. As you add a slip-on, high-flow air filter, velocity stacks, and header, you are pushing the fuel trim further and further into the lean range. A reflashed ECU or add-on programmer will have adjusted fuel trim tables (and ignition tables, but that's less critical to fuel trims, and this discussion,obviously) that will compensate for these modifications. A good tuner can adjust fuel and ignition tables so that the fuel economy lost due to more fuel being injected is offset (to varying extents) by improved combustion and power via ignition tuning, which will often give the magic combination of improved power and torque AND improved fuel economy, up to a point. It is typical to see SOME loss in fuel efficiency with a tune on a modern vehicle, but the rider often compounds this loss because he/she is twisting the throttle more to enjoy the added power, so it's not so much a net "loss." Also, there is a reliability gain from reduced temperatures from getting rid of the lean condition, which is a further benefit.
Overall, the TL;DR of all of this is get a fuel tuner/programmer. Best mod you can do for your bike besides getting to be a better rider. Cheers,
Mark W.
 
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