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6 sp manual downshifting effect mpg?

Discussion in 'Technical Chat' started by MMazz365, Apr 8, 2011.

  1. Apr 8, 2011 at 10:18 AM
    #21
    skunk

    skunk what did I miss?

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    The ECU control with the manuals makes the EPA MPG rating worse on the 6 spd than the autos.

    I have noticed that when coasting in neutral the speed sensors must tell the RPMs to stay elevated (~1200) until you come to a stop and then it goes to warm idle (~800).

    Because of this I usually leave it in gear while braking to keep the RPM's down and then I put in Neutral as I come to a halt.

    I just wish an EE would come up with some sort of Flash fix to take the ECU emission control out of the equation. It takes the fun out of a manual when the RPM's don't drop when you let off the gas.:(
     
  2. Apr 8, 2011 at 10:22 AM
    #22
    buck

    buck TheEh-Team.ca

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    Your statements make no sense.

    Downshifting does slow the vehicle due to engine braking, requiring less brake application to get the same amount of deceleration.

    Downshifting with no accelerator input cuts injector pulses above certain RPM's. Downshifting does have potential to save gas. When your engine is at idle speed while coasting to a stop, the injectors are pulsing thus using gas to maintain RPM's. Downshifting does put additional stresses on the drivetrain and clutch plate, but that can be minimized with reasonable rev-matching.

    Your talk about heel-toe has nothing to do with this thread. Heel-toe for your application on a track where grip is at it's limits is completely different in purpose for street driving and downshifting. How often are we downshifting while at the limits of traction on the street? You match-rev on a track to disrupt as little as possible the contact patch friction limits.
     
  3. Apr 8, 2011 at 10:23 AM
    #23
    Hondarider08

    Hondarider08 "That's what she said..."

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    While I'll let the others argue about something completely off topic... I'll add my two cents into the actual question. :rolleyes:

    I have a 4.0 liter 6spd and I was always curious about the effects of downshifting on MPG's. I broke down about a month ago and bought an ultra-gauge. This gauge has an instantaneous MPG window and also has a open loop closed loop indicator. When driving normally down the road it shows an closed loop. As I go to slow down at a stop light, I put it into neutral and watch the mpg's as I'm coasting. They take a large jump up, but then slowly drop down as my speed drops as well. Then at the next stop light I try it with a downshift. My loop indicator then shows an open loop (injectors are cut off). My mpg's go up quickly, but they dont seem to drop down quite as quickly.

    So to sum it up, downshifting does seem to help with milage. I dont think it's hugely significant, but enough to notice. As long as your not revving the shit out of your motor or going from an extremely high speed to a slow speed by downshifting, then you should see an increase. As far as clutches go, like I said, as long as you're not abusing it and be gentle, you shouldn't have any problems. I downshift everywhere and my clutch still feels the same as the day I got the truck.

    Thats my two cents.
     
  4. Apr 8, 2011 at 10:27 AM
    #24
    Evil Monkey

    Evil Monkey There's an evil monkey in my truck

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    It does save gas according to my aeroforce gauge. The injectors don't shut off as some have claimed (Injector pulse width drops to about 2ms in a 20ms period meaning it's pumping gas about 10% of the duty cycle). It isn't much better than coasting though. So I doubt you'd save much over just coasting and braking.

    Engine braking does slow the vehicle down though. To test it, put the vehicle in neutral and coast. The put the vehicle in gear and let out the clutch. You'll observe the vehicle slowing much quicker.
     
  5. Apr 8, 2011 at 10:32 AM
    #25
    Evil Monkey

    Evil Monkey There's an evil monkey in my truck

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    It's not burning more fuel. In fact the injectors are at their lowest fuel rate when you're off the accelerator. The reason the rpms are higher is because the drive train is mechanically linked to the engine. The engine has to turn at the same rate which is why the rpms jump.
     
  6. Apr 8, 2011 at 11:35 AM
    #26
    skytower

    skytower Well-Known Member

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    Close, but not quite. With the throttle plate blocking the intake, you still save gas and use engine braking on gas engines. Diesel engines are another story, since there is no throttle plate. Diesel controls speed by fuel delivery. Jake/compression brakes contol valves to affect engine braking.
     
  7. Apr 8, 2011 at 11:36 AM
    #27
    macgyver

    macgyver Well-Known Member

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    They aren't two completely different matters. It's 100% the same concept. You are using the engine to control the speed of the vehicle against momentum and gravity. Decelleration is decelleration regardless of the grade.

    Your 10% comment is probably accurate. However it is possible in the Tacoma, I do it all the time. Sure it was an adustment compared to my previous vehicle but after a little getting used to it, I can heel-toe just fine in the truck. I do it to make downshifting a smoother, also similar to driving a racecar, it's makes downshifting for acceleration smoother.

    Slowing down is the same thing as controlling forward momentum. Otherwise you would continue to move forward at the same rate of speed or faster if you're on an incline (obviously gravity and wind resistance affect this)

    I have personally never spun out of control going down an icy hill, just using that as an example. Gearing down in low traction is not an epic fail, its actually better than using your brakes, there is no risk of locking up. Same concept that ABS accomplishes, which is modulating the brakes to even braking pressure to allow the wheels to continue to spin and to slow you down without locking up.

    Anyways no more arguing for me, just based on the comments from others, it appears that you're in the minority here.

    So according to people with a scanguage and other AFR monitoring devices, the consensus seems to be that downshifting does save gas over braking in nuetral.
     
  8. Apr 8, 2011 at 12:11 PM
    #28
    Evil Monkey

    Evil Monkey There's an evil monkey in my truck

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    It would be miniscule though. In gear, my gauge indicates 1.9-2.0 ms out of 20ms. In neutral, 2.0-2.3 ms out of 20ms. 300 microseconds difference. You'd probably never notice it at the pump. There are too many other variables that would affect it more.
     
  9. Apr 8, 2011 at 12:24 PM
    #29
    macgyver

    macgyver Well-Known Member

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    hmm..interesting. I wonder if the narrowband sensor vs. wideband sensor has an effect on this.

    When I had my wideband it would read "lean" (17:1+ AFR) if I decellerated in gear but if I were in nuetral it read 14.7ish. It wasn't a tacoma though but I doubt for it would be that big of a diff. Most cars idle at 14.7 (stoich)
     
  10. Apr 8, 2011 at 12:28 PM
    #30
    Tacomadude89

    Tacomadude89 Well-Known Member

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    That's what I was just going to say. The injectors shut off when you're 'coasting' while still in a gear.

    I usually don't down shift until I know the rpm's are going to be 2.5k or below though.
     
  11. Apr 8, 2011 at 2:02 PM
    #31
    thinkingman

    thinkingman Well-Known Member

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    Foglights are for fog, not oncoming traffic!
    Well, since you're a genius, I won't bother pointing out the ridiculous innaccuracies in your post.
    No, I can't ignor them.
    number one, you can't shift into second from 70mph.
    Number two, you don't even know what a Formula Ford is, let alone posess the intelligence to know whether it's a POS or not...I'm pretty sure you've never driven a non-syncro close ratio 4spd.
    Number 3, use some of that genius to show us your calculations for saving 'A TON' of gas.
    301.783 gals?
    329.326 gals?
    I'll leave that question to the genius...bet you can't or won't figure out the what I'm even referring to.
    4...I hypermile my 6sp...never rev over 2k except driving on the freeway where the pathetic 6th gear requires 2.4k rpm to maintain freeway speed.
    thanks for our insight and wisdom....look forward to your response.
    sorry for the hijack.
    some people can't figure out the difference between deceleration and maintaining control of forward motion using engine compression.
    BUT THEY'RE GENIUSES!
     
  12. Apr 8, 2011 at 2:09 PM
    #32
    thinkingman

    thinkingman Well-Known Member

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    Foglights are for fog, not oncoming traffic!
    I'll help you with the difference between deceleration and maintaining downhill speed.....deceleration means your velocity decreases, maintaining downhill speed means your velocity remains constant.
    There, see?
    In re using engine compression to ease down an icy hill, stop fooling yourself into thinking you have better control using engine compression, you can't modulate the inputs as precisely as you can pressure on the braking system.
    I learned this 30 years ago before ABS was prevalent.
     
  13. Apr 8, 2011 at 7:49 PM
    #33
    krap22

    krap22 Well-Known Member

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    I've been driving a manual for 16 years. I put nearly 300,000 miles on 2 vehicles. I replaced 1 clutch. My 1st Gen taco had 196,000 miles (i put on the last 116,000 miles) on it when i traded it in, and still had very strong original clutch in it. I downshifted all the time. It was just part of how i drove the vehicle. IF you know how to drive a manual, downshifting doesn't cause any extra wear on the transmission or clutch.

    Downshifting shouldn't waste anymore fuel. If it uses extra fuel, it shouldn't be enough to make you lose 2-3 mpg. The fuel injectors shouldn't be pumping out much fuel while you are out of the throttle.

    If your getting bad gas mileage, change the way you drive. Easy on the gas, shift early, don't drive over 60 mph. Start with that and see where it gets you.
     
  14. Apr 8, 2011 at 8:56 PM
    #34
    skytower

    skytower Well-Known Member

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    Until they come up with a commercially available full rotor contact pad, engine braking trumps brakes. Brakes will lock long before you lose traction with the road, it's just the nature of the beast.
    ABS corrects problems caused by the average driver. A good/expert driver can out-brake ABS every time.
     
  15. Apr 9, 2011 at 8:09 AM
    #35
    Evil Monkey

    Evil Monkey There's an evil monkey in my truck

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    It's the same principle with engine braking whether you're going downhill or running on a straightaway. The reason the speed stays constant going downhill is because there is extra force (gravity) being applied. Without the extra force, the vehicle would slow.

    A particular gear is only going to allow the truck to go so fast without extra fuel input. The gear that keeps the vehicle at say 20mph downhill is also going to slow a vehicle to 20mph on a straightaway and it will do it much faster than letting it coast. The point is that the engine is doing it.
     
  16. Apr 11, 2011 at 5:33 AM
    #36
    rwilso03

    rwilso03 Well-Known Member

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  17. Apr 11, 2011 at 6:36 AM
    #37
    skytower

    skytower Well-Known Member

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    True. The throttle plate will not completely stop the flow of air. The throttle plate stop screw is what sets idle on carbs. It still offers a great deal of restriction, thus engine braking. A carb will not give you the control of fuel injection, and will constantly be fueling the engine regardless of speed.
    If you put the trans in neutral on a carb engine, you save gas, but lose engine braking.

    I'm unsure of the point you are trying to make. We seem to be in agreement.
     
  18. Apr 11, 2011 at 10:55 AM
    #38
    jspadaro

    jspadaro Well-Known Member

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    I'm going to just avoid all of the crap you spewed afterwards, and rehash what everyone else has said because I can.

    Downshifting slows you. Your assertion is provably false. Go 40mph, then downshift to second. Then, go 40mph and go to neutral. You will slow faster in second, period. :rolleyes:

    Whether it saves fuel is dependent on the automobile design. As others have mentioned, if the designers allowed for fuel shutoff while coasting in gear as the wheels drive the engine to turn over (which a source on this forum, I can't be bothered to search where, indicated that they did), then it is possible that it will save some fuel over coasting in neutral.

    Make your Tacoma a Prius? No. Good idea to do? Up to you. But does any of that make it untrue? Absolutely not.

    Shut the fuck up.
     
  19. Apr 11, 2011 at 11:12 AM
    #39
    thinkingman

    thinkingman Well-Known Member

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    Foglights are for fog, not oncoming traffic!
    Thanks...I spewed coffee out of my nose.
    Are you all so obtuse as to think I don't know what my 6speed Taco does when I let off the gas?
    Apparently so.
    Thanks for the education.
    For those of you experts that think using the gearbox and clutch to slow your car is an effective use of the machinery, keep going. Your repair shop thanks you.
    In re Formula Ford=POS, you lack the intelligence to know what you don't know.
    I love you idiots.
     
  20. Apr 11, 2011 at 11:14 AM
    #40
    jspadaro

    jspadaro Well-Known Member

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    Effective? Arguable. Does it work? Yes Did you say it doesn't and then call all of us idiots for saying that it does, when it in fact does? Absolutely.
     
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