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Boost cut?

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Old Apr 21st, 2019, 00:25   #1
SamP94
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Default Boost cut?

My 98 940 2.3 lpt is hitting a boost cut at 0.6 bar(8/9 psi). I have read about people taking the little 13c to 1bar without any issues. Anything specific to check that could be causing this? Or is there any way to bypass it? Or is this normal? Tia
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Old Apr 21st, 2019, 10:44   #2
Laird Scooby
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The clue is in the "LPT" bit - Low Pressure Turbo.

Most only run to about 4-6psi from what i understand and the HPT runs to about 8-9psi.

The other question is, do you really mean boost cut? That is controlled by the ECU and (depending exactly how it's set up) cuts the fuel and/or spark when boost exceeds a certain level.

It sounds as if you already have an HPT wastegate in there, allowing it to boost to 8-9psi, i might be wrong on this as i'm not that well up on the turbos - i subscribe to the "there ain't no replacement for displacement" theory!
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Old Apr 21st, 2019, 11:21   #3
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Are you using a manual boost controller to set the boost?
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Old Apr 21st, 2019, 12:11   #4
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Sorry should have mentioned running a manual controller. Definitely some sort of boost/fuel cut just like hitting a wall for a brief moment
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Old Apr 21st, 2019, 14:59   #5
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Quote:
Originally Posted by SamP94 View Post
Sorry should have mentioned running a manual controller. Definitely some sort of boost/fuel cut just like hitting a wall for a brief moment
You're right in your inital post about running the 13c to 1 bar easily, I did so in my LPT without issue. Can you boost past 8/9PSI in any gear? Like drive at 40 in 3rd and do a slow, smooth acceleration to slowly raise the boost?
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Old Apr 21st, 2019, 16:34   #6
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You're right in your inital post about running the 13c to 1 bar easily, I did so in my LPT without issue. Can you boost past 8/9PSI in any gear? Like drive at 40 in 3rd and do a slow, smooth acceleration to slowly raise the boost?
Unfortunately not, however its driven it will do it. Only way around is to turn the boost back down
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Old Apr 21st, 2019, 18:44   #7
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Manual control is prone to produce a peak of boost while raising up the boost.
Long and small diameter hoses will help to produce this mail function.
With a electronic one or the HPT waste gate you won't have this problem.

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Old Apr 21st, 2019, 20:22   #8
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You should definitely not be hitting boost cut at 10psi or even 14 psi on a 13c turbo as it doesn't flow that much air.
The amm reaches it's voltage limit as it needs more power to heat the wire ia a large mass of air is moving across it.
If it feels like you head is about to hit the windscreen then that is boost cut. Juddery movement is probably the mbc working .
Have you got a proper boost gauage fitted?
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Old Apr 21st, 2019, 20:40   #9
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Quote:
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You should definitely not be hitting boost cut at 10psi or even 14 psi on a 13c turbo as it doesn't flow that much air.
The amm reaches it's voltage limit as it needs more power to heat the wire ia a large mass of air is moving across it.
If it feels like you head is about to hit the windscreen then that is boost cut. Juddery movement is probably the mbc working .
Have you got a proper boost gauage fitted?
Boost gauge fitted, chinabay thing but works, its 100% a boost cut it only does it at 0.6 bar for a split second. Any other time it runs and drives spot on
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Old Apr 21st, 2019, 20:58   #10
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Quote:
Originally Posted by mocambique-amazone View Post
Manual control is prone to produce a peak of boost while raising up the boost.
Long and small diameter hoses will help to produce this mail function.
With a electronic one or the HPT waste gate you won't have this problem.

Happy Easter, Kay
Hose diameter won't make a difference.
Nor will using a different actuator, boost cut is in the fuel/ECU chips, changing mechanicals won't make a blind bit of difference.
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