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I own a Yamaha YS125 which is fuel-injected, and a lot of times I am flogging the bike on full throttle, which increases fuel consumption.

I was thinking of restricting the air flow intake of the engine, which I believe would yield:

  • Slower top speed and acceleration
  • Reduced fuel consumption on full throttle
  • Would not need remapping because fuel injection system will compensate automatically

Are my assumptions correct? Am I missing anything out?

  • How old is it / what year is the engine? I'd be worried about leaning out the mixture and causing problems later. Perhaps you should learn to stop putting the boot into it and ride more sedately? Certainly a cheaper fix. – Criggie Nov 21 '18 at 8:39
  • How do you plan to restrict intake air flow? If you're just restricting the throttle plate from opening all the way that's going to be very different from enforcing a lower Vmax through software. You should clarify this as the answers might change as a result. – Zaid Nov 21 '18 at 10:53
  • The power to improve the fuel consumption figures is in your hands, literally. Accelerate gently and avoid using full throttle. If you restrict the air intake you'll simply choke the engine which will either cause it to lose power or go into an overly rich running condition. Neither of which are good. – Steve Matthews Nov 21 '18 at 11:07
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Let's go through your assumptions about the effects of restricting air flow (assuming a mechanical restriction is implemented):

  • Slower top speed and acceleration

    Definitely. Restricting the air flow will mean there is less oxygen available to the cylinders for any given RPM.

  • Reduced fuel consumption on full throttle

    Yes, but only because you're not going to be able to reach the higher top speed that you would have previously.

    The fuel consumed while cruising at 50 km/h is less than the fuel needed to cruise at 70 km/h.

    Also, a slower-accelerating bike takes longer to reach any given speed, so it burns fuel for longer; total fuel consumption involves both fuel burn rate and the time spent burning it.

    For more information, see brake-specific fuel consumption (BSFC).

  • Would not need remapping because fuel injection system will compensate automatically

    True. The engine hardware is the same. Adding restriction is effectively limiting the usable region of the BSFC curve.

  • "So cruising at 50 km/h with a restricted intake will invariably consume more fuel than cruising at 50 km/h without a restricted intake." To cruise at 50kmh for a specific vehicle will need a specific amount of fuel/air. Can you explain how your statement works? – HandyHowie Nov 21 '18 at 12:13
  • The throttle butterfly valve is just restricting air. If you just add another restriction,it is just the same as not allowing the butterfly valve to not open fully. Or am I missing something? – HandyHowie Nov 21 '18 at 12:15
  • "If the EFI fuel management is already well tuned to begin with, I would expect to see a small loss in fuel economy after restricting air intake." - The fuel mapping will be read based on air flow detected either by the MAF or MAP sensors. Surely it wouldn't get confused by restricting the air flow. It would be like having a dirty air filter. You would just loose max power capability. – HandyHowie Nov 21 '18 at 12:18
  • The graph is showing that at the constant revs needed to go at 50kmh, you will need a specific air flow to get the torque required. This air flow will give a specific air pressure in the inlet manifold, and hence a specific efficiency. Cruising at 50kmh on the same incline cannot produce different intake pressures. The graph is showing how if you change the load on the engine, the pressure will change due to the change in air intake and hence a change in efficiency. – HandyHowie Nov 21 '18 at 13:23
  • Just realised the pressure on the graph is the pressure in the cylinder, however the argument is still the same that the pressure increased due to load. You can't have different pressures with the same load. – HandyHowie Nov 21 '18 at 13:35
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Short answer: No, you're just going to make it run badly and/or quit on you.

Longer answer: Engine RPM is regulated by controlling the amount of air and fuel that goes into the engine cylinders. Airflow is regulated by a butterfly valve, when you twist the throttle or push down on the accelerator pedal you are opening that valve either directly through a mechanical linkage or the engine computer does it for you. Opening the throttle lets more air through - more air and fuel, more bangs, more RPM. Rolling off the throttle closes the valve and you get less air, less RPM.

So airflow is already being regulated, if you restrict that airflow yourself then you'll starve your engine of air at higher revs, making the air/fuel mix too rich which will ruin performance, maybe cause it to quit altogether and lower your mileage.

If you want to reduce your fuel consumption then the best thing you can do is be less aggressive on the throttle - stop caning it and you'll use less go-juice. Also, make sure the bike is well maintained and tuned well.

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    Surely if it is an injection engine then the ECU will inject the correct amount of fuel based on the reading from the air flow meter or map sensor, so it shouldn't run rich. – HandyHowie Nov 21 '18 at 9:29
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    It depends on the ECU programming and how many sensors it has @HandyHowie. It may just limit fuel and therefore RPM, or it may fall back to a mapping if it is really out of expected tolerance. – GdD Nov 21 '18 at 10:06
  • So you are saying that when you close the throttle valve, unknown situations may occur? The ECU may start running rich? Since closing the throttle simply restricts air flow. – HandyHowie Nov 21 '18 at 13:26
  • I'm saying if you block the intake and then throttle up you may have problems because the ECU is expecting, but not getting a specific airflow rate. – GdD Nov 21 '18 at 14:11
  • Not following you fully. Why would the ECU be expecting anything? It only knows what its sensors tell it and if you restrict the air flow, the sensors will sense it and the ECU will adjust the fuel accordingly to keep the mixture correct. – HandyHowie Nov 21 '18 at 15:15
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If you reduce the incoming air, you will reduce the power output and increase the very fuel consumption you are trying to improve, ie to use more fuel instead of less for the same journey.

Make sure the bike is serviced correctly, clean filters, spark plugs adjusted corr4ectly etc.

You may find that your fuel consumption is because the route to work is arduous and not a route where you can cruise on light throttle.

  • Can you explain "increase the very fuel consumption you are trying to improve"? I don't see how reducing the power output will increase fuel consumption. – HandyHowie Nov 21 '18 at 9:30
  • @HandyHowie as the OP states that they are using full throttle already, reducing the power will not help.... – Solar Mike Nov 21 '18 at 11:22

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