More efficient to cruise at a higher RPM?

  1. Zeffy94

    Zeffy94 Senior Member

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    I used to cruise at 2300ish RPM when commuting, but the engine always felt so meh when I applied throttle (not WOT) without downshifting. Lately I started cruising at around 3300 RPM instead, and the engine feels much more responsive and much more eager. Interestingly enough, I had thought my fuel economy would drop slightly from cruising at a higher RPM but it really hasn't. It's also way more fun shifting at 4.5k or higher :headbang:

    (Disclosure: Despite owning it for almost 2 years I still don't drive it that hard... that's slowly changing as I realize that it makes the car far more enjoyable)

    The only negative is my cabin is louder with a slight drone (HKS exhaust, not OEM), but it's nothing that would drive me insane.

    Any other's thoughts on this? I thought I remember something about smaller displacement engines actually being more efficient at higher RPMs.
     
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  2. Andre80

    Andre80 Senior Member

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    I am frustrated because I have so many things to say but my English is bad.


    yes it is, the engine consumes less torque, but you have more wear, more heat, more noise.

    and here you could start talking that if you understand this the clutch will last you as long as the car
     
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  3. CivilciviC

    CivilciviC Senior Member

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    I have zero empirical data to say one way or another, but, I would imagine if you're cruising at 3300rpm, you'd be using more fuel.

    At that point you're well into the boost zone and plus I always thought higher rpm means more fuel and more air required than say at 2300 rpm.

    Ive gotten used to piloting Honda V6s, so cruising around 2500rpm has become pretty normal for me. I don't like how buzzy the k20c engine gets when you're around 3500rpm. It's one thing if I was at the track or even purposely spiritedly driving, but for my commute to work, I wouldn't like the additional buzz.

    I believe our cars are in the boost window from like 2400rpm and up. Maybe you were previously cruising just outside it? For my normal driving, I cruise around 2500rpm and it seems the car is more than eager to fly when I mash the throttle.
     
  4. ez12a

    ez12a Senior Member

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    i'd say the most efficient is the least throttle to maintain a given speed.
     
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  5. JoYu

    JoYu Senior Member

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    Yeah, if you really want to know switch your gauge to the one that shows throttle and brake pressure and you can find out. Maybe with cruise control on and on flat ground.
     
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    OP
    Zeffy94

    Zeffy94 Senior Member

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    I find that easier at higher RPMs. That's what I mean by cruising though, if it wasn't clear.
     
  7. fiend busa

    fiend busa Senior Member

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    Cruise control doesnt show throttle percent on the gauge cluster.
     
  8. JoYu

    JoYu Senior Member

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    Change the gauges to the one that shows throttle percentage. IDK if the type R has it, but my si did, I will check next drive.

    Separately use cruise control to take your foot on the accelerator pedal out of the picture.
     
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  9. NapalmEnema

    NapalmEnema Senior Member

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    I've found with smaller turbo motors, a bit higher RPM ~3000-3500 is more efficient, and gives you better MPG's. The highest I've had in my Type R was on the drive home from buying it with a lot of highway in that RPM range. 28mpg average
     
  10. TheCanadian

    TheCanadian Senior Member

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    #10 TheCanadian, Feb 6, 2020
    Last edited: Feb 6, 2020
    When you are driving at steady state (cruising at the same speed), all of the energy your car is making goes into overcoming aerodynamic drag by the wind, and mechanical losses in the engine and drivetrain*. The combustion process itself is very complicated and also affects the efficiency at different RPM's - but my information here [ heavily simplified] will focus on wear and tear/mechanical drag.

    Driving at the same speed at 2300 rpm and 3300 rpm = same aerodynamic drag.

    Let's say the piston moving up the cylinder for one revolution produce 1 unit of resistance. For each RPM of the engine, you will get 8 units of resistance (2 strokes per cylinder per revolution if I recall correctly).
    2300 rpm = 18,400 "units"
    3300 rpm = 26,400 "units"
    26,400 > 18,400 = more friction + more wear

    But that's not it! The units most likely aren't same force because if the higher piston speed at 3300 rpm versus 2300 rpm. Engines of course are not perfectly balanced - and the forces on all sides of the cylinder will increase with a higher speed.

    So, the net energy emitted by the engine must equal all the drag forces. So that is,

    Net engine output = total mech engine output - aerodynamic drag - mechanical losses.

    As you have more mechanical losses in the engine at a higher rpm, you will technically need to burn more fuel to get same net engine output. That being said, its possible the combustion might occur better at a higher rpm and result in better fuel economy - but you will still have 43.4% more wear on the engine for the specific example. Again, the wear is at fractional, tiny, tiny levels.

    That being said, I would let the car to operating temperature and do as many redline pulls as you can [legally]. I believe in Honda's ability to make engines - the R is for fun. For cruising I would just use the highest gear possible.

    *
    I have no idea of the R's transmission design for each gear and did not mention it . But generally, higher rpm = more drag = more loss. Vehicles are more efficient on highway because you don't accelerate/decelerate often.
     
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  11. TheCanadian

    TheCanadian Senior Member

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    Not quite.

    The throttling of your engine is controlled by the throttle plate. The overall power requirement at steady state will not change regardless of your gear. The ECU will open the throttle at different % to vary amounts of air mass flow into the engine depending on power requirements .The air mass is balanced with the fuel injection to get combustion. When "punch" the gas, the throttle opens and lets in more air, which burns more fuel, which increases the rpm.

    To go the same speed in different gears the amount of air the engine consumes should be relatively similar. You can't consume more air and burn more fuel and stay at the same speed - you would have more mechanical power than you need for that speed- and you'd start accelerating.

    Of course, it's not exactly the same but relative.
     
  12. tinyman392

    tinyman392 Senior Member

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    Net engine output = total mech engine output - aerodynamic drag - mechanical losses

    To maintain speed at a constant speed would mean that you want the same net engine output and the aerodynamic drag is constant.

    I'd argue that a higher mechanical loss (friction within the motor) would require a higher mechanical engine output for the same net engine output to be achieved. The higher mechanical engine output would be what's using the gas if I'm not mistaken so I can see it being marginally higher at higher RPM. But it is a motor and the friction created and heat generated might have a "sweet spot" somewhere in the middle, I don't know. But with this sort of outlook, I'd probably argue that for something to be more fuel efficient you'd also need to do less damage to the motor (since it's most likely the friction that damages the motor).

    What I will argue is that throttle control can be very jumpy when in higher RPMs vs lower, so keeping that perfect net engine output could be difficult to do.
     
  13. TheCanadian

    TheCanadian Senior Member

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    I meant for both my replies to go hand in hand. With a higher rpm you will need to burn more fuel to overcome frictional - but the power made is the same (final pushing force after losses)
     
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  14. BABY NSX

    BABY NSX Senior Member

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    #14 BABY NSX, Feb 7, 2020
    Last edited: Feb 7, 2020
    Hi guys, here’s a video from Engineering Explained that might be useful. I just looked at it briefly. About midway in the video where you see 4 different rev ranges all going at the same speed if i remember correctly it looks like a slightly higher RPM is more efficient as some of you have mentioned.

     
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  15. willskiGT

    willskiGT Senior Member

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    Lower RPM = lower fuel consumption. Why do you think manufacturers are coming out with 8, 9, 10 speed gearboxes that shift up at like 2,000 RPM or less?

    Fuel economy and emissions are lower at lower RPMs.
     
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