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Discussion Starter · #1 ·
i've noticed somethign for some time, but never bothered to look into it.

if i'm on the gas, and then shift, for a *split* second...right after I let off the gas and start to shift, I feel a momentary spike in power. as if the car built up more boost for just a second.

i'm assuming that as I let off the gas, the intake must shut, and all the boost that is built up must momentarily get "bottled up" in the turbo, and cause the boost to spike for a split second - just until the bypass valve can open and bleed off the pressure....

i assume this is normal and it's wont' damage antyhing, but i figured i would ask you all if you notice the same thing.
 

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Re: Burst of power after shift (23109VC)

I have noticed a similar phenomenon in my car as well. I almost have to anticipate it if I am bring the shifts to redline shince the "momentum" seems to bring the revs even higher than wanted.
 

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Re: Burst of power after shift (23109VC)

I think what you're describing is a boost spike or flare.

MTE specifically made modifications to their ECU code for cars with DPs to prevent this from happening with theirs. Scott could probably comment in greater detail though.
 

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Re: Burst of power after shift (StaggRLee)

You think it's bad in an R, try it full throttle in an ECU upgraded 4.4 motronic (T5 etc.
It's almost violent
 

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Re: Burst of power after shift (23109VC)

Quote, originally posted by 23109VC »
just until the bypass valve can open and bleed off the pressure....

The bypass valve doesn't bleed pressure, it just sends it round and round. This is intentional, and better than blowing off (the old method). Yes, there can be a spike after closing the throttle. Remember too, the turbo has a lot of angular momentum and when the CBV is open, it isn't doing much work so it's spooled and ready.

Tom.
 

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Re: Burst of power after shift (JRL)

Quote, originally posted by JRL »
You think it's bad in an R, try it full throttle in an ECU upgraded 4.4 motronic (T5 etc.
It's almost violent

Put some WD40 on the throttle linkage, the the nylon spherical connections.

Pretty sure they are sticky. That or a sticky throttle cable.
 

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Re: Burst of power after shift (tmtalpey)

Quote, originally posted by tmtalpey »


The bypass valve doesn't bleed pressure, it just sends it round and round. This is intentional, and better than blowing off (the old method). Yes, there can be a spike after closing the throttle. Remember too, the turbo has a lot of angular momentum and when the CBV is open, it isn't doing much work so it's spooled and ready.

Tom.

You are explaining well but I don't think this is happening in this situation. When shifting fast at certain RPM a sort of after throttle can be noticed during shifting, almost like blipping the throttle pedal. I have noticed this on many occasions mainly when going full throttle uphill and shifting.
Feels like a combinations of slow CBV and throttle plate, not enough vacuum to open the CBV.
 

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Re: Burst of power after shift (Johann)

Okay, I'd buy it being a sticky (or slow) valve too. Put it this way, if it's a little, I say it's just because. If it's a lot, it might be worth checking the vacuum lines especially where they connect to the valve (below the airbox).

Tom.
 
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