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beepbeep beepbeep is offline
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Join Date: Jun 2001
Location: Sweden
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Originally Posted by fredmeister View Post
What causes the boost creep?
Please explain the actual physics behind it, because it does not sound like a symptom of which wastegate you own but rather the exhaust header design.
There are many names for different things that can happend.

Wastegate creep: when wastegate spring is too soft and is cracked open by exhaust backpressure before wanted boost is obtained. Usually leads to more time before full bost is online. Cured by fitting harder spring or using EBC with pressure delivered on both sides of membrane.

Boost creep: condition when boost "creeps" upwards from desired value. Often provoked by too small wastegate and big turbo. Example: engine that spins at 3000 RPM, boosting 1 bar will need certain amount of gases evacuated trough wastegate to keep that boost steady. Wastegate will have sufficient diameter to obtain flow and everything is OK. When revs rise, amount of exhaust gases needed to flow trough wastegate also rises, increasing the backpressure trough wastegate. Instead of being dumped aboard, gases are forced to pass trough turbocharger instead and boost starts "creeping" upwards at higher revs. Cured by fitting bigger wategate.

And then there is typical syndrom of "my 0.7 bar wastegate now delivers 0.9 bar of boost after I fitted free flowing exhaust/bigger turbo. The reason for this is abvious: if you decrease exhaust backpressure for turbo/exhaust combo, gases will more likely flow trough turbo than trough wastegate. Result is same as fitting harder spring: higher boost troughout the range, but without "creep". Cured by fitting softer spring.
Thank you for your time,
Old 09-19-2007, 01:39 PM
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