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1994-2002 [Subscribe to Daily Digest] |
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Boost pressure is the pressure that is developed in the intake manifold at wide open throttle (WOT).
The wastegate allows some of the exhaust gases to bypass the exhaust turbine of the turbo charger. The more gases that spill via this alternate path, the less power is developed by the turbine which spins the turbo compressor. So when you go WOT, the wastegate is initially closed. As the boost pressure rises, pressure is applied to the wastegate actuator to start opening the wastegate and this limits the boost pressure to a certain value. The pressure applied to the actuator can be varied by a boost controller to lessen the pressure that the actuator sees, which will create an increase in boost pressure, as the actuator sees less of this negative feedback signal. If the contoller does not lessen the pressure at all, this will be the lowest boost that can be set and this is the base boost. If the controller reduces the pressure seen by the actuator, the pressure csn be increased above this base amount.
The pressure is adjusted by bleeding part of the flow off and using restriction orifices. The Saabs and many other makes use a solenoid controlled variable bleeder valve boost controller. Called the BPC, boot pressure controller. It in turn is controlled by the ECU. The ECU limits boost in the first two gears to make driving a bore. Actually it prevents wheel spin and protects the gear box from stress and warranty claims.
Many are fitting modified ECU's which have higher boost settings. These are expensive. An inexpensive manual boost controller (MBC) provides great results too. But you have to control wheel spin with your right foot. This is really a drag. It is like owning a real powerfull vehicle. :)
When your foot is off the throttle there is simply not enough exhaust gas flow to develop the power required to develop any boost pressure. When on boost, when you close the throttle, manifold vacuum is applied to the bypass valve which causes it to open which 'dumps' the boost pressure back into the intake. This prevents the high pressure high velocity flow from waterhammering (bouncing) off of the closed throttle and reversing though the turbo compressor stage. That can cause shock waves which have the potential to destroy the compressor wheel. There are a few variations of this device. Some 'dump' valves discharge the air directory to atmoshphere which makes a loud noise and some take great pleasure in their ability to startle people and frighten animals.
The simple bleeder valves, including the factory units, have a problem of being too linear. They start applying pressure to the wastegate before the pressure gets near the pressure objective. So before boost pressure is achieved, some of the exhaust energy is leaking through the partially open wastegate. This reduces the ability of the turbo to spin up, or spool up. So the inherent boost lag of the turbo it self is greatly exagerated. By using an advanced manual boost controller (MBC), the wastegate does not see any pressure until the pressure gets near the boost pressure. This is a non linear response. The turbo then will develop pressure much faster. The MBC's are relatively simple devices. The non linear characteristic is created with a spring and a ball bearing, or other spring loaded valving, that prevents flow until the pressure can overcome the spring force.
Other concepts involve heat and heat transfer. The temperature of the charge air affects the air density. Hot air at boost pressure creates less power as it is less dense. The turbo compressor creates heat by compressing the air and from its inherent inefficiencies. Passing the air through the intercooler gets rid of some of the heat. Pressure losses in the air flow make the turbo work harder to create the boost pressure, which increases heat. So there is a focus on reducing pressure losses. Hence the K&N filters, removal of disharge silencer, throttle body transistion casting rework, Sports exhausts etc. Heat can get into the charge air from the engine bay, so some insulate the ducts and pipes.
So those are some of the starter concepts. There are some good books but they are highly technical.
Eventually you will become familiar with this stuff and will be comfortable with modifying this stuff if you want to.
One thing that you will not get to experience is the social havoc that arose when some nut presented the MBC option to this BB 1.5 years ago. The Abbott ECU crowd was quite hostile, or threatened... But no one cares anymore. The ECU mods develop more power and make 1st gear more controllable. So there is a choice. I suggest an MBC an an inexpensive intro for someone who is unsure about spending $550 to $800+ on a modded ECU. It is only a $35 stepping stone, or a permanent addition if you want.
posted by 209.172.2...
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