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700/900 Series General Forum for the Volvo 740, 760, 780, 940, 960 & S/V90 cars |
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Boost drop as revs increase to red lineViews : 678 Replies : 16Users Viewing This Thread : |
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Feb 5th, 2024, 12:19 | #1 |
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Boost drop as revs increase to red line
After fitting a B200ft (8 PSI) actuator to my B230FK with an MBC.
I bought an NXS MBC, and installing it all was easy. I now boost to 12PSI happily. But as its an auto and as it holds gears with the throttle pressed. Once I go over 4K rpm the boost starts to drop off, to about 9-10 PSI. Any ideas how to keep 12 PSI till redline? |
Feb 5th, 2024, 20:55 | #2 |
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Personally, I am not into performance modding so you'll probably get better replies from others who are.
Assuming you have done a proper stage 0 service, the limiting factor is likely to be fuel or air. Your B230FK probably has an over-boost function built into its ECU. If it detects too much air for a given engine RPM then it will cut the fuel. This is more like a sudden loss of power rather than gradually hitting a limit. If the loss of boost is more gradual then something is probably restricting air flow. What sort of condition is the turbo inlet hose in? If it's too soft it might be collapsing under certain conditions and restricting air flow. I think it's also possible to hit a condition where you're passing so much air over the AMM that it cannot draw enough power. This is unlikely at relatively low levels of boost, but you could try carefully cleaning the AMM if you haven't already to restore its performance. Good luck! |
Feb 5th, 2024, 20:58 | #3 |
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Putting some washers behind where the actuator mounts is the only thing you can do. It's either a tired actuator or the wastegate hole is worn a bit, probably a bit of both.
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Feb 5th, 2024, 23:12 | #4 | |
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Feb 6th, 2024, 00:49 | #5 | |
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I thought people pre tensioned the actuator to avoid this? |
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Feb 6th, 2024, 05:52 | #6 | |
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Feb 6th, 2024, 14:54 | #7 | |
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I’ve searched but not sure where to put the washers. I feel people misname actuators and waste gates. Some pictures would be great Cheers |
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Feb 7th, 2024, 17:16 | #8 | |
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I still get a 15PSI spike then 12psi until about 4k rpm then it drops slowly back to about 8PSI. If it was manual I'd change gear sooner, but its auto and wants to hold the revs indefinitely |
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Feb 8th, 2024, 20:25 | #9 | |
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Feb 9th, 2024, 14:12 | #10 | |
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My car is the same, even with a stronger BSR actuator and MBC. It spikes to about 14 just like yours but drops to 12/13 to redline. If I adjust the actuator rod for more preload or try and add more boost via the MBC, it just results in a bigger spike. The turbo is capable of 15 psi (right on the surge line at 6000 rpm), and other people have run that. SalvadorP suffered the most extreme case of this with the worn wastegate, his car couldn't produce any more than about 1 psi!
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