This article covers most boost related issues including a short introduction on how your boost systems work. This information is based on the Classic Impreza's but will cover the newer cars too, to a certain extent,
Safe boost levels
When modding your car without mapping (full de-cat and high flow induction etc) you increase the efficiency of your turbo which could result in engine damage due to lean running at high rpm / max boost. To prevent damage always try and keep your boost level as close to standard as possible until your car is mapped for the increase in boost pressure.
For a list standard boost pressures see our Boost Pressures FAQ
Is my boost control system connected correctly?
2 Port Solenoid System
Starting at the turbo there should be a short pipe from a tapping in the turbo outlet duct to a plastic ‘t’ piece, this short piece of pipe should have a brass/alloy restrictor pill within it, this restrictor pill is vital to the correct operation of the 2 port solenoid system. From the ‘t’ piece you should then have another short pipe to the ‘Wastegate Actuator’, the other long pipe from the centre of the ‘t’ piece should go to the wing and connect to the side port on the 2 port actuator, it is important that it is connected to the side port as if the solenoid fails it fails safe and prevents any bleed as the pressure from the turbo will hold the solenoid closed preventing any bleed and as a result the wastegate will open at its spring rate pressure normally 0.5 BAR. The final pipe in the system should connect the bottom port of the 2 port solenoid to the intake pipe post Mass Air Flow Sensor.
3 Port Solenoid System
From the turbo outlet there should be a long pipe that goes straight to the top port of the solenoid, then another pipe connects the side port of the solenoid to the wastegate actuator, the final pipe connects the bottom port of the solenoid to the intake/resinator box, this section of pipe should have a brass/alloy restrictor pill within it.
How the systems works ‘Basic’
2 port system ‘Bleed System’
If the turbo outlet was connected by pipe straight to the wastegate actuator the wastegate actuator would always open at its spring rate (normally 0.5 BAR) to achive higher than wastegate ‘spring rate’ pressure you need to bleed air away from the actuator to prevent it from opening the wastegate at its spring rate pressure, to do this on the 2 port system a ‘t’ piece is inserted in the middle of the pipe that connects the turbo outlet straight to the wastegate actuator then a pipe is run from this ‘t’ piece to the solenoid, what the solenoid does is open and close a port to allow a controlled bleed (governed by the ECU-solenoid duty cycle) out of the pipes to keep the air pressure within the pipes below the 0.5 BAR that opens the wastegate, without the restrictor pill in the first pipe between the turbo and the ‘t’ piece the air passing from the turbo into the pipes cannot be bled off quickly enough to prevent the wastegate actuator opening the wastegate the result of this would be poor boost responce, so what the restrictor does in effect is cause a pressure drop and slows everything down to allow the solenoid to deal with a much lower/slower volume of air, the smaller you make therestrictor hole the greater the pressure drop and the easier it is for the solenoid to keep the pressure in the system below 0.5 BAR (spring rate pressure)which will result in higher boost output, a larger restrictor hole the smaller the pressure drop will be and the harder it is for the solenoid to keep the pressure in the pipes below (spring rate pressure) which will result in lower boost output. (Please do not adjust the restrictor hole size unless you know what you are doing)
3 Port System ‘Direct Interrupt Bleed’
Unlike the simple bleed system that 2 port solenoid uses, the 3 port uses a direct interrupt bleed setup, what this means is that the feed from the turbo outlet duct has to pass through the solenoid to get to the wastegate actuator, this is a much more stable way of controlling boost than the 2 port system, how it works is simmilar to the 2 port as in the idea is to prevent the wastegate actuator being moved until your desired boost level is achieved, to do this the solenoid again bleeds air away from the system using a controlled bleed ( governed by the ECU-solenoid duty cycle), the restrictor in the bleed/return pipe determines the rate of bleed, a large restrictor hole has a high bleed rate making it easier for the solenoid to keep the system pressure below the actuator spring rate pressure which will result in a higher boost output, a small restrictor has a low bleed rate making it harder for the solenoid to keep the system pressure below the actuator spring rate pressure which will result in a lower boost output. (Please do not adjust the restrictor hole size unless you know what you are doing)
What Is Boost Creep?
Boost creep is a situation where your wastegate port is not large enough to allow the exhaust gas to bypass the turbo, what happens is the exhaust gas will choke the wastegate port preventing further gas flow through the port then the exhaust gas has to take the path of least resistance which is through the turbine of the turbo, this will spool the turbo ‘uncontrolled’ beyond your normal controlled max boost level, the turbo will be spooling past wastegate spring rate pressure even though the wastegate is fully open hence it is uncontrolled, the best way to check for boost creep is to connect the turbo outlet port directly to the wastegate actuator port and go for a drive, in 4th gear you should normally get a stable boost level of about 0.5 BAR, if you have boost creep the boost will hit 0.5 BAR and will continue to rise with rpm until you either back off or hit overboost fuel cut. Boost creep should only be present on a turbo that has very little restriction ie a full decat and high flow induction, I have found that the fast spooling IHI VF35 is very prone to boost creep, the cure is to remove the turbo and enlarge the wastegate port, then to fit a stronger actuator 0.75 BAR the reason for this is because you have made the wastegate port larger the effective size of the wastegate plate acting against the exhaust gas flow is larger which allows the exhaust gas excert more force on the wastegate plate which in effect weakens the effectiveness of the actuator, ie before the increase in size of your wastegate port the actuator would open at 0.5 BAR, after the increase the actuator would open earlier at 0.3-0.4 BAR, after these changes are made to the turbo either a boost controller or a remap (to adjust solenoid duty cycle) should be sought to control the boost to a safe level.
What Is Wastegate Creep?
Wastegate creep is a very common fault on the 2 port solenoid system, on both modded and standard cars, the most common cause is a lazy or dirty 2 port solenoid, what this does is reduce the bleed rate through the solenoid which allows the pressure in the system to build up enough for wastegate actuator to creep open, this reduces boost then the wastegate will close again then the boost will increase again and the wastegate will creep open again and so on, in this situation a simple clean of the solenoid would normally cure the fault, the symptoms of this fault are lumpy, flat and hesitant boost , on a standard car wastegate creep will feel like the the car is pulling then backing off repeatedly like a surging effect whilst you are accelerating. This issue can also be caused by a poorly setup boost controller or a weak actuator spring (I have yet to come across a weak spring but it can happen).
What's the Common Causes For Overboost
Decat + High flow induction – Cure ( Reduction of the solenoid duty cycle or alteration of restrictor size will help return boost output to its normal level)
Split, poor fitting, or disconnected pipes – (Cure replace or refit pipes, the pipes that will cause this issue are between the wastegate actuator, solenoid (including upto the restrictor on the return pipe of the 3 port solenoid) and the turbo.)
Manual Boost Controller – Electronic Boost Controller set too high – Cure (Dont be so greedy and back the boost duty/adjuster off to a safe level smile.gif )
Restrictor Pill not fitted / size incorrect – Cure ( Ensure restrictor pill is fitted (3 port) if so on a 3 port reduce the restrictor size and on the 2 port increase the restrictor size to reduce the boost to a safe level)
Clogged 3-port solenoid It is possible that the flow of air through the 3-port solenoid could be restricted between the turbo outlet port and the wastegate actuator port if the solenoid is very dirty (usually oil vapor from the intake system), this allows the wastegate to remain clamped shut longer than it should be causing a potential overboost situation. Cure (Clean with carb or clutch/brake cleaner)
Loss of solenoid funcation Although this is not Overboost it shows itself with very simmilar symptoms, its an interesting senario I had brought to my attention today ( Thanks Mark wink.gif ), is it is possible for the solenoid to fail or even stick shut whilst under boost, this would result in a rapid reduction of boost pressure to wastegate pressure approx 0.5 BAR , so if you were running at full boost 1.0 BAR for example and the solenoid was to fail shut it would feel just like Overboost as the wastegate rapidly opens due to the solenoid blocking off the spill from the wastegate. Cure (Either clean the solenoid with carb or clutch+brake cleaner or replace the solenoid)
What's the Common Causes For Underboost?
Solenoid lazy or not working – Cure ( Clean (use carb cleaner or simmilar) or replace the solenoid)
Restrictor Pill not fitted / size incorrect – (Cure ensure the restrictor pill is fitted (2 port) if so on 2 port reduce the restrictor size and on the 3 port increase the port size to increase boost to reach normal safe max boost level.
Weak or broken actuator spring – Cure ( Replace the actuator ) this can be tested for by connecting the turbo outlet straight to the actuator and seeing what max boost level you get this should be about 0.5 BAR
Leaking exhaust manifold and/or up pipe The production of boost via the turbo is very dependant on exhaust gas flow over the turbine so if there are any leaks in the exhaust pre turbo your spool and peak boost will be affected, obviously the larger the exhaust leak the worse the effect on the turbo output will be.
How to test your actuator spring rating.
You will need a boost gauge for this unless you have an electronic boost controller (it should have an on screen boost display), all you need to do on the standard boost control system is remove the boost control pipes from the turbo outlet nipple and the nipple on the actuator and fit a short piece of pipe direct from the turbo outlet nipple to the actuator, this allows an uninterrupted flow of air to the actuator and will limit the boost roughly to whatever your wastegate spring rating is, normally 0.5 BAR on standard setups.
On an electronic boost controller all you need to do is follow the instructions on how to switch your boost control off and this will have the same effect.
Boost Controller Info.....Links
Apexi Avcr info + pdf.....links
Blitz Boost controller Info....
Turbosmart Boost Controller Info + pdf....
Information source: [Big 'D'] SIDC thread Having Boost Issues?, This info may help :)