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Why Does My Boost Spike Then Drop Off.

Duncan

By Duncan,
in Engines & Forced Induction

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Duncan Grand Master

Duncan

Posted
Posted

Recently got the car tuned with more than stock boost for the first time, and I noticed it spikes and then drops off badly at higher boost.

12psi boost graph

12psi.jpg

18spsi boost graph

18psi.jpg

boost controller is profec B, turbos are r32 n1, wastegates have standard 7psi springs. boost controller taps into the FPR pressure source and the solenoid is using the boost line at the rear of the plenum routed to the wategates via a T

Its not a huge problem really, I need boost to be super stable at 12psi so it doesn't open the pop off valve but at 18 (for non cams events) I don't really care so much about the 18psi tune. But if there is any easy fix it would be good to know. It does strike me that the pressure signal from FPR to head unit on the steering wheel is pretty long.

Swiper the Fox Grand Master

Swiper the Fox

Posted
Posted (edited)
Recently got the car tuned with more than stock boost for the first time, and I noticed it spikes and then drops off badly at higher boost.

12psi boost graph

12psi.jpg

18spsi boost graph

18psi.jpg

boost controller is profec B, turbos are r32 n1, wastegates have standard 7psi springs. boost controller taps into the FPR pressure source and the solenoid is using the boost line at the rear of the plenum routed to the wategates via a T

Its not a huge problem really, I need boost to be super stable at 12psi so it doesn't open the pop off valve but at 18 (for non cams events) I don't really care so much about the 18psi tune. But if there is any easy fix it would be good to know. It does strike me that the pressure signal from FPR to head unit on the steering wheel is pretty long.

can you increase the spring pressure or change the actuators Dunc's? the turbine pressure is forcing the wastegate flap open as the spring holding it closed cannot stop the extra pressure from opening it.

Edited by DiRTgarage
Swiper the Fox Grand Master

Swiper the Fox

Posted
Posted (edited)
how many of you guys have your wastegates plumbed directly into the exhaust housings of the turbo?

they are internally gated low-mounts.

Dunc's HKS make two types although im sure they wont let you run them, one has a yellow dot on them if you seperate the actuator from the bracket they are 1.2bar spring the other are 1bar and have a green dot.

See if GCG can rebuild them for you with a bigger spring.

Edited by DiRTgarage
SLAPS Rising Star

SLAPS

Posted
Posted (edited)

Internal gates that spike then tail off increasing preload will only exsasabate the problem.

Best off with new set of actuators that are stiffer and setup them up with 2-3mm max preload.

Duncan,

Tested a regular bleeder to see how it reacts?

Turned off the ebc to see if the spike is still there 'slightly'.

It might just be the ebc is on its way out or the the <b>start boost</b> is set too high.

and last of all... have a little fiddle with cam timing.

I would assume the tuner knows how to setup the ebc correctly. Incorrectly set start boost and gain can cause this exact problem.

Edited by TheRogue
Duncan Grand Master

Duncan

Posted
  • Author
Posted
can you increase the spring pressure or change the actuators Dunc's? the turbine pressure is forcing the wastegate flap open as the spring holding it closed cannot stop the extra pressure from opening it.

yeah I was afraid that was the real issue....was hoping there was something I can do with the boost controller. basically the boost controller is not bleeding off enough boost, right?

I can't legally change or modify the actuators, and even if I did I need it to hold a good 12psi for 80% of its use. Its only the other 20% of the time I can run more boost so its not worth compromising the setup for.

sirracer my pressure source is at the rear of the plenum (standard spot) not turbo compressor cover. Not sure how turbo cover source would work with twins anyway, but in any case I can't modify the intake in any way.

Duncan Grand Master

Duncan

Posted
  • Author
Posted

sorry missed the other responses.

therogue, interesting points. the cam timing has been extensively "fiddled" that is where we have made big gains on this tune - taking the time to get the right results (ie early boost not top end). as usual it looks like there are compromises which might be affecting boost.

as for the boost settings, I did them so it is likely the "tuner" doesn't know what they are doing lol. I might set the start boost lower for high boost and see if it helps - I love the kick it gives as it comes on though :sick: Am I correct that higher gain but lower start boost might help?

overall it doesn't really matter too much, but if I can straighten it out without compromising other things I would like to.

Swiper the Fox Grand Master

Swiper the Fox

Posted
Posted (edited)
sorry missed the other responses.

therogue, interesting points. the cam timing has been extensively "fiddled" that is where we have made big gains on this tune - taking the time to get the right results (ie early boost not top end). as usual it looks like there are compromises which might be affecting boost.

as for the boost settings, I did them so it is likely the "tuner" doesn't know what they are doing lol. I might set the start boost lower for high boost and see if it helps - I love the kick it gives as it comes on though :sick: Am I correct that higher gain but lower start boost might help?

overall it doesn't really matter too much, but if I can straighten it out without compromising other things I would like to.

it still holds almost 18psi (17.5) of boost on the 18psi setting...try knocking a little gain out of it to get rid of the spike first.

remember...spiking less gain---not reaching target boost needs more gain

Edited by DiRTgarage

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