strange boost problem
 

i just finished installing a profec b spec-II today and was out adjusting and setting the boost levels. before i installed it i had a boost fluxuation on the primary turbo. it would go to 12 psi and then drop off 1 psi and back up creating a wawa sound all the way to the transition at 4500 and then stop. (if that description doesnt make sense let me know and i will re-write it.)

putting the boost controller on was an attempt to get rid of this. now i put it on and working and it fixed it, well, sorta. i have it set to 8.5 psi for right now in the testing stage. now what it does is is when i floor it it will go to 10 psi fluctuate down and up one psi (like above) once or twice, then drop down to 8.5 and hold solid all the way through the transition and to redline. why does it do this?

and the kicker is if i turn turn the boost controller off and just let it run off wastgate it doesnt do any of the above at all. just boost completelly normal, but only 7.1-7.3 psi (which sucks!!!)

mods are basic DP, MP, cat-back, intake, 99-spec turbos (if it matters), pfc and a good tune to 12 psi.

I think you are experiencing boost spike, which means over boost and ECU shuts things to be safe and builds back up again

the ecu has nothing to do with controlling boost.

try turning your boost down and your duty cycle up. fluctuating boost usually means your duty cycle is too low. not sure why it was doing that without the boost controller though (and fluctuating so much)... it sounds like an ECU problem.

ok, would someone that knows what they are talking about please post!

anybody?

with a reply like that no one should help you douche bag

You really should be more polite to people who are only trying to help you. That said, I think your problem has nothing to do with the ECU. I also have the profec B spec ii and my boost fluctuates similarly to the way you mentioned. The problem should be fixable with further tuning of the boost controller but I haven't had time to investigate too much yet. Since I don't yet fully understand how each of the setup parameters on the profec relate to the feedback loop, I can't tell you exactly what to adjust, but try playing with your start boost and other gain parameters. I suspect that lowering the start boost number will improve the situation, but at the expense of spoolup time.

I started writing a somewhat technical explanation of what is going on, in case you don't know, but it got wordy so I'll leave what little I wrote below for you. The boost controller is running a feedback loop which probably adjusts the boost control solenoid duty cycle with a function with components proportional to the error, the derivative of the error and the integral of the error (PID). The fluctuation is indicative of an underdamped condition where the derivative component is too small. The boost overshoots the setpoint (desired boost) and then quickly overcorrects (due to excessively large integral or proportional components) and undershoots it. This phenomenon is probably most pronounced as the turbo is spooling up at lower rpms, particularly with only the primary turbo online. I think I'd have to write a lot more to make this remotely understandable.

thats what i was able to correct by raising my duty cycle

thank you and what you posted does make sense, i guess i will have to play with the other settings a bit more and figure it out. my biggest fear is the fact that it starts out high and then comes back to the set pressure. the car is only tuned for 12 psi, so i dont want it to go to 15 then back down when i finally turn it up. but who knows, maybe when i turn it to 12 it will have an easier time managing it.

Yeah, I thought you might have been on to something rotarybred, but I don't think what you were doing was raising your duty cycle because that would simply raise the boost. Increasing the frequency of a solenoid pulse might solve this problem, maybe we just have a terminology issue. But I don't think that is adjustable with a profec b spec ii.

Thats one of the reasons I went with the Blitz SBC-id boost controller. Its one of the only controllers that work properly on a sequential car. You controll how fast the boost comes on, the boost spike. All you do is just keep playing with the controller until the boost settles out.

On a side note with the boost controller off you should only see 7psi of boost because thats the stock wastegate pressure. Its a nice thing to have especially when taking it in for service. That way the tech can't bag on your car because the controller is off and all he gets is 7psi of boost lol.

R.K.