Why does the WRX tend to rev an extra 500 rpm during shifts?
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This is a discussion on Why does the WRX tend to rev an extra 500 rpm during shifts? within the Transmission & AWD forums, part of the Tech & Modifying & General Repairs category; Alright, I have been driving stick now for about a half year and I feel that I'm relatively good at ...

  1. #1
    Registered User Shibby's Avatar
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    Why does the WRX tend to rev an extra 500 rpm during shifts?

    Alright, I have been driving stick now for about a half year and I feel that I'm relatively good at it. My shifts at full throttle and when I'm barely accelerating are just perfect. However, when I drive at about half throttle, I encounted this "extra 500 rpm" in between my shifts.

    Heres what happens:
    1. driving in 2nd (i seem to only encounter this problem on the 2-3, 3-4 shift)
    2. clutch in, gas off
    3. rpms jump about 500 revs
    4. this little increase messes up the whole shift.

    My attempts to stop this:
    1. I've started to let off the gas about .5 - 1 sec earler before I clutch in. This seems to take the momentum off the flywheel (maybe?) and I don't get a little rpm jump. As soon as I declutch, my rpms fall straight down

    2. If I stay in gear a little longer (say maybe to 5-6K), this "extra 500 rpm rev" doesnt happen

    Basically, have you guys encountered this problem? Why is this so? I think what bothers me most about is the unsuredness about how my shifts are going to go when I'm driving at half thottle. It just doesn't seem very "solid" or "predictable" i guess. I really can't describe it. Help guys
    --Jason-- 2004 MT WRB WRX
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  3. #2
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    Well I don't think there is any "problem" at all.. The "problem" is just how you shift.
    As you said "clutch in, gas off" has to make the RPM jump alittle. When you are at fullboost you may not notise it bacause you may shift faster or something..

    If you really have to get rid of this "problem" I guess you just have to revese the way you shift.. like gas off, clutch in, or do like me, find something in the middle. gas off and clutch in just a split secon later.

    On a few shifts I myself experience this rpm jump, but mostly I don't however this is no real problem, If you are worried just adjust the way you shift..
    Silver -02 Impreza WRX

  4. #3
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    I notice it too. It seems to be when you shift quick after hitting boost. Let off the gas a bit sooner before depressing the clutch and it will not happen.

  5. #4
    TheWRXstore's Avatar
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    If you have a Recirc bov, the air being pumped back in on a shift will bring your RPMs up a little bit. It takes a little time to get used to, but it's not that bad
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  6. #5
    Registered User Retro's Avatar
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    I have gotten this effect even if its gass off, then clutch. There's something fishy going on, like the recirc. valve, or something causing it to do this.

  7. #6
    Registered User ShotgunTC88's Avatar
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    Flywheel inertia?



    Shotgun

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    Registered User NJwrbT-WRX's Avatar
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    I have it, and I have the upgraded (HEAVY AS HELL) flywheel, so flywheel inertia makes sense.

    -joe
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    Moderator GV27's Avatar
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    Nope - definitely NOT flywheel inertia. Inertia makes the revs drop slower, but in this case the flywheel (and the rest of the motor) are accelerating. Inertia keeps things going at the same speed rather than accelerating things (in this case anyway - in the case of a spinning figure skater inertia does accelerat her as she changes the position of the inertia relative to her CG - but that's an entirely different topic!).

    It's a combo timing (foot timing, not spark timing) and boost thang.

    C
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  10. #9
    Registered User Retro's Avatar
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    Thank you for the lesson on inertia, I have been annoyed nearly to death whenever someone says it's the flywheel. But there is some fueling/boost oddities going on in the car. I have had the car act up regularly before and for 15 minutes I did nothing but practice letting off the gas first, yet it would bump up a couple of rpms. It could be lag on the throttle control.
    clutch, shift, gas, clutch, shift, gas, shift, clutch, gas, clutch gas, shift... oops!

  11. #10
    Registered User ShotgunTC88's Avatar
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    GV,

    I guess what I was thinking was that if one pushed in the clutch while at 5000,etc RPM the engine would no longer be under load from the drivetrain and speed up since it would in effect be free revving IF the drivers timing in footwork were off.

    Shotgun

  12. #11
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    My memory is a little vague on this but I seem to remember reading that this is programmed intentionally to reduce emissions.

    Something about not letting unburned gases out the pipe that would result from the sudden lifting of the accel.

    Ever notice that when racers (Real ones, not like us) back off htere's frequently some fire coming out of the pipes. It's to eliminate that.
    I made it idiot-proof and they went and hired a better idiot.

  13. #12
    Moderator GV27's Avatar
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    Bingo! You beat me to it. I was thinking about it last night and came to the same conclusion. Those who want to know FOR SURE might want to check this with the regulars in "Engine Performance" as they know a heck of a lot more about it than I do, but here goes:

    All modern engines continue to spark after you let off the throttle in an attempt to burn off as much hydrocarbons as possible for emmisions reasons as OldGuy stated. Now in a N/A motor this doesn't produce much energy at all and you don't notice it. In a Turbo things are a bit different.

    The BOV is on the intake side of the throttle plate, so it bleeds off pressure in the intake. However, this leaves a considerable amount of pressurised air on the manifold side of the throttle plate. Just like the pressure helps create a bigger explosion on throttle, so it also creates a bigger explosion during this hydrocarbon burn-off phase. This doesn't produce all that much energy, but enough to accelerate a free-running motor slightly.

    I'm not sure if the recirc BOV has anything to do with it or not - again it's on the intake side. But the throttle plate never closes completely - the idle screw sticks it open slighltly - so some of that air could make it's way back to the manifold. I guess it would help keep pressure in the manifold rather than backing out around the cracked-open throttle plate.

    GV,

    I guess what I was thinking was that if one pushed in the clutch while at 5000,etc RPM the engine would no longer be under load from the drivetrain and speed up since it would in effect be free revving IF the drivers timing in footwork were off.

    Shotgun
    I'm sure that's part of it to - if your timing is off a bit it's going to exagerate this as the throttle plate will still be open a bit and the BOV wouldn't have gone off so the turbo would still be feeding pressure to the motor. You'll always have the throttle open slightly when you push in the clutch if you're getting a smooth shift - since neither clutch actuation or throttle closing is truly instantaneous, if you waited to actuate the clutch 'till the throttle was completely closed you'd get a big jerk.

    C
    Last edited by GV27; 11-23-2003 at 07:57 AM.
    "Inasmuch as ye have done it to one of the least of these my brethren, ye have done it to me." -Jesus

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  14. #13
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    This is definitely not just foot timing. I've got the same thing in my 04 and I'd consider myself pretty good at manual transmissions. I've owned probably 30 manual transmission cars and driven many more including various turbos and never felt another car that does what the WRX does. It definitely does feel like something the computer is doing on purpose so I buy the argument that it is for emissions. It is annoying though.

  15. #14
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    I had/have again same problem. I went to a 100% atmos. BOV and this RPM surge was nearly non existent.(99%). I had to switch back to stock Rec. BOV and it is back just the same. All that extra air and fuel being dumped back in has to be burned off. When I had the 100% atmos. BOV on initially, I found I was pushing in the clutch too late. Now, back to waiting around 2-3 seconds after I let off gas before I engage clutch. The combo of lighter flywheel and atmos. BOV should be interesting. I found it easier to shift w/100% atmos. BOV. Lighter flywheel should enhance this feeling. I hope.

  16. #15
    Moderator GV27's Avatar
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    Just a technical note: when the BOV recirculates it only dumps air back in, NOT fuel.

    C
    "Inasmuch as ye have done it to one of the least of these my brethren, ye have done it to me." -Jesus

    1990 Alfa Romeo Spider Veloce
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