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Old 01-28-2004, 01:22 PM   #1
Undepelo
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Question Ok what's with the rpm spike when foot's off the gas?

Ok I remember posting this question a long time ago and never have gotten a straight answer, just theories. I also have been looking in the archiuves and find nothing. Now does anyone know what the hell are the rpm spikes that occure as you remove your foot from the gas to up shift? It's kind of anoying. I've tried to modify my timing as to when I step off the gas and disengage the clutch but it's not practicle.
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Old 01-28-2004, 01:33 PM   #2
dmross
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Default on a wrx?

It's funny, I drive an RS but whenever I drive my brothers WRX, I get an RPM spike when shifting. In my case, I am pretty sure it's just my own timing. I am probably compensating for a slightly different clutch engagement point. Does it do this consistently or only sometimes? Did you drive a different car for a long time prior to the WRX?
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Old 01-28-2004, 02:09 PM   #3
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It is most likely due to pedal timing. The engine will rev like crazy if you depress the gas pedal with no load on it.

To avoid this, try completely removing your right foot from the gas pedal before you put your foot on the clutch. The engine will gradually slow initially as the throttle closes, and you may then see a very slight blip in RPM as the clutch plate engages and unloads the engine, then the RPMs will drop very quickly as the unloaded engine spins down.
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Old 01-28-2004, 02:19 PM   #4
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My theory on this is that it's turbo surge due to a delay in the bypass valve opening. I had it on my '02 WRX and still have it on the STi. I've driven nothing but sticks for the past 16 years and have never had this problem on any of my non-turbo vehicles.
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Old 01-28-2004, 02:22 PM   #5
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Same in my Baja Turbo 5MT as well. I re-learned to pause before I disengage the clutch. Got used to it now...
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Old 01-28-2004, 02:28 PM   #6
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I would chalk it up to timing, my gf does the same thing and blames it on the clutch.
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Old 01-28-2004, 02:47 PM   #7
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it's not just clutch/opperator error..
some times i can rev, feel a little boost, quick give it a little quick dap on the pedal... the rpms will hang a bit longer than if there was no boost build up...
at first i thought my FLOOR mats had scooched down... but no.

Last edited by DISCOPOPE; 01-29-2004 at 02:13 PM.
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Old 01-28-2004, 03:27 PM   #8
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I thought it spiked to maintain boost during an upshift.

Justin.
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Old 01-28-2004, 03:35 PM   #9
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I thought this happened on all MT cars... You let go of the gas, declutch, and the tach will rise a little bit and fall -- and you engage the gear when it hits the right RPM as its falling...

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Old 01-28-2004, 03:58 PM   #10
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It's the turbo pressure combined with no-load when you press the clutch.
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Old 01-28-2004, 07:42 PM   #11
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Try this just for kicks. When you go to shift, completely let off the gas until you feel the car slowing, then engage the clutch. Does it still do it?

Never driven a turbo nor know much about them, so no help here on whether or not the turbo is a factor. I'm still banking on operator timing.
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Old 01-28-2004, 10:49 PM   #12
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I've driven dozens of MT N/A cars and never had it happen.

I agree with the excess turbo pressure partially heading upstream into the intake when you shift. Not all the pressure is released cleanly through the BOV...

The net effect is very unnatural upshifts - in order to drive smoothly, you really have to slow down between shifts.

Does anyone know if it still happens with aftermarket turbos on the WRX?
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Old 01-28-2004, 10:58 PM   #13
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I have the same issue with my 04 Forester XT M5, and driving the same exact way my daugthers 02 WRX M5 does not have the problem.. I do think it is both car and driving style issue... It sure is a PIA

My 1993 Eagle Talon AWD turbo M5 does not have the problem, this is a subie unique issue...
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Old 01-29-2004, 03:25 AM   #14
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This was answered a while ago. The consenus was that in order for subaru to get all it's ulev ratings (subaru was recently given a ulev award by an environmental group), subaru programmed the cars to burn off the small amount of fuel still in the engine instead of having it dumped directly to the exhaust.
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Old 01-29-2004, 05:09 AM   #15
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I think it's more with the car, too. When I shift, I'm totally off the gas then in on the clutch, and it still revs a little (more so during hard accelleration, or I miss time the clutch).
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Old 01-29-2004, 07:44 AM   #16
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I get the "surge" as well, usually when I shift under the 4500 RPM through all gears.. Really negligible amount, maybe 250 RPM in surge. I'd agree it's timing, because as a test I fully let off the throttle pedal and then pushed the clutch in - it doesn't happen! So there...
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Old 01-29-2004, 08:17 AM   #17
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I would also say emissions/ECU related. Driving other turbo cars (older, or custom) this does not occur, so any pressurization of the turbo, etc...makes no sense.

If the throttle plate is close, there is NO WHERE for the the air to go, except out a bypass valve

If the idle cotrol circuit was to open for a split second, squirt a smidge of fuel...

- b
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Old 01-29-2004, 10:50 AM   #18
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Quote:
Originally posted by Impreza01
This was answered a while ago. The consenus was that in order for subaru to get all it's ulev ratings (subaru was recently given a ulev award by an environmental group), subaru programmed the cars to burn off the small amount of fuel still in the engine instead of having it dumped directly to the exhaust.
is that corrected with an ecu flash?
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Old 01-29-2004, 11:06 AM   #19
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I think it has something to do with the heavy a$$ flywheel. I recently had my clutch and flywheel replaced under warranty for the shudder problem. My clutch is very smooth now but I noticed my rpm's "rev up" a little bit when I shift. And I'm not keeping my foot on the gas, this happens with my foot completely off the gas pedal when depressing the clutch. It is kind of annoying. My theory is the flywheel, but I could be wrong.
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Old 01-29-2004, 01:40 PM   #20
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It's simple physics... momentum.

Conservation of momentum:
mass1 * velocity1 = mass2 * velocity2

When you depress the clutch you disconnect some of that mass from the system, therefore the velocity of the system instantaneously increases.
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Old 01-29-2004, 01:47 PM   #21
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i think i heard somewhere on the board that it was a feature built in to the ecu. so thats what i always assumed.
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Old 01-29-2004, 02:06 PM   #22
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Quote:
Originally posted by Impreza01
This was answered a while ago. The consenus was that in order for subaru to get all it's ulev ratings (subaru was recently given a ulev award by an environmental group), subaru programmed the cars to burn off the small amount of fuel still in the engine instead of having it dumped directly to the exhaust.
This is the theory that makes the most sense to me. VW/Audi 1.8 turbos exhibit the same characteristic, as does my buddies S4 (it's not the new one with a V8, it's the V6 bi-turbo).
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Old 01-29-2004, 02:33 PM   #23
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Quote:
Originally posted by rexxer
It's simple physics... momentum.

Conservation of momentum:
mass1 * velocity1 = mass2 * velocity2

When you depress the clutch you disconnect some of that mass from the system, therefore the velocity of the system instantaneously increases.
If you disconnect some of the mass then the momentum should DECREASE. Your theory is not valid in this case. Also u cannot create more momentum without some outside force acting upon the mass.
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Old 01-29-2004, 03:39 PM   #24
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Quote:
Originally posted by rexxer
It's simple physics... momentum.

Conservation of momentum:
mass1 * velocity1 = mass2 * velocity2

When you depress the clutch you disconnect some of that mass from the system, therefore the velocity of the system instantaneously increases.



That logic is flawed. The mass you remove is still spinning. It's momentum is not magically transferred to the flywheel upon clutch engagement.

The rpms rise because power is being generated in the engine (fuel igniting) without any load.

The power either comes from the ECU dumping fuel for a split second until the intake pressure is relieved (maybe to prevent lean combustion that would increase the EGT's and output of nitrous oxides) or a slow reaction between the release of the gas pedal and the closure of the TB.
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Old 01-29-2004, 04:18 PM   #25
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i thought it was my bad driving for the longest time. thanks guys.
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