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Old 01-17-2006, 10:10 PM   #1
Azn-Jap STI
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Default how much boost Q!

Hey guys, im having some boost problems right now and i found out the reason why. the stock hoses from the turbo to the waste gate was changed. my cousin from WRC just came over and he gave me a small "pea" (a hose restrictor) off an evo9 to put in the hose and it pushed my car to 25psi's... not good. i took it out and he told me to expand the hole of the "pea" to 1mm but the psi was still too high. on first gear, i can get up to 10psi but on second and third gear, it goes all the way up to 20psi's. is that normal?? or should i just go back to subaru and get the stock hoses?

NOTE: when i got the car, my boost read at 7psi. with the extra mods, i got it up to 12, but then the engine cuts off at 12psi because there's nothing stopping the air from opening the waste gate.

I NEED HELP PLEASE!
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Old 01-17-2006, 10:17 PM   #2
tora
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It might help if you said what your mods are, what turbo you are running, engine management etc.
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Old 01-17-2006, 10:44 PM   #3
Azn-Jap STI
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UP, DP, intake, no EM... well ya thats it i guess. well for the engine i guess.
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Old 01-19-2006, 12:02 AM   #4
patr
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it works like this

there is a nipple from the pressure side of the turbo (compressor outlet) which has boost on it. It is t-d to the wastegate actuator and the boost solenoid. Before that tee, there is a brass restrictor.

Those of you who have had your cars tuned have seen my "box of pills".

Now, you ONLY need to change the pill if you significantly change the flow characteristics of the exhaust. Because for a given wastegate duty cycle, you can build more boost with a freer exhaust.

If you change ANY of these hoses, in diameter, or the i.d. of the tee, or the restrictor, or the UP/DP/TB, without a pill, or some sort of tune, you have boost control issues. normally this manifests itself in the form of boost oscillation. Some people dont even know it is happening. On the extreme side, you have boost osillation so bad you will get boost cut.

If the pill is too small, the solenoid opens up and tries to build boost. Then it realizes it overshoots and closes. Then it realizes it overshoots and opens. this is why you get oscillation. you can tune around this, but best setup for the cars is around 70% wastegate duty cycle (you can run other target DC but there are a slew of heuristics to tune including atmospheric compensation, temp compensation, instantaneous boost error, and continuous boost error). In any case

if you run the hose right off the compressor the the wastegate, you get spring pressure.

Unless you know what you are doing, do this and get it setup by someone. Bad oscillation will be very, very bad for your car and even can cause an accident if you try to pass someone and it goes the wrong way. People who have tb and up and no em get this, as do some folks with a popular certain brand of engine management which shall remain nameless.

If you can change the ECU, you can tune it to your hoses/pill.

If you have stock hoses, put those on and put the oem pill back in. If you have any sort of pill in there, if it is wrong you will have lazy boost (too big) or boost oscillation (too small). 1.03mm is stock. But it all means nothing if anything is changed, because it is a system. In other words, you need to get it setup to your tb/up etc. this is why putting on a tb with no EM is a bad idea. I'm not saying this to toot my own horn, this is to save you from disaster. but really, your overboost will get caught by your fuel cut.

your overboost danger is even exacerbated by the fact that you have an intake, which leads to erroneous MAF readings, which, in the case of an overboost situation, can easily hurt your motor.

stock, depending on temperature and altitude, is around 14-15 psi. If you get 12, I would say the system is working. Run it in fourth up a hill and see where it goes. if it goes high, if you hold on to it, it will probably come back down, or else hit fuel cut. Regardless, it needs to get setup. going all stock will not help with those mods, but it may make it 'liveable' - most folks will not experience the osicllations unless they really get into it. GO SPRING PRESSURE UNTIL YOU CAN TUNE IT.

-Pat

Last edited by patr; 01-19-2006 at 12:11 AM.
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Old 01-19-2006, 12:16 AM   #5
buster
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Quote:
Originally Posted by Azn-Jap STI
small "pea" (a hose restrictor) off an evo9
NOTE: when i got the car, my boost read at 7psi. with the extra mods, i got it up to 12, but then the engine cuts off at 12psi because there's nothing stopping the air from opening the waste gate.

I NEED HELP PLEASE!
Using a part off the Evo for your suby should have been the first hint this wasn't right.

You bought the car used? From where, just curious?

Go to the dealer, or bribe Pat for a pill and hose if you don't have the stock hose.

Pat, I didn't get that last part about the spring pressure?
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Old 01-19-2006, 12:30 PM   #6
patr
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if you do not run a line to the solenoid, and just run a line from the compressor outlet to the wastegate actuator, then all of the compressor side air goes to the actuator, meaning there is no bleed off (which is accomplished by the wg solenoid), meaning you run wastegate spring pressure

this is totally safe and a totally good backup plan if you have a problem, just low boost (whatever the spring is set to)

now you can understand... the solenoid bleeds air from the wg actuator, letting it build boost. If you change the size of the lines, the tee, the secondary fitting on the solenoid, the pill, etc. etc. your boost behaviour changes. You can map the solenoid to match any of those changes, or you can leave it stock and mess with all of these to find something that works (but 95% of the times, what I have seen is what someone thinks works, works in only a small set of circumstances).
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Old 01-19-2006, 01:20 PM   #7
Kevin
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Don't screw around, got to Pat for a tune or get a Cobb Accessport.
IMO you should have the engine management and TB exhaust installed at the same time.
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