There are a couple of ways to control boost pressure in a turbo system. The most common way to prevent overboost in a throttle open condition on older turbo systems is a wastegate. A wastegate is a valve in the exhaust stream before the exhaust gasses reach the turbo. This valve opens and diverts exhaust away from the turbo, slowing the turbo vanes. Typical ways to actuate the wastegate is to bleed a little turbo pressure to a spring loaded diaphragm in the wastegate. As turbo pressure builds, the bleed air pushes on the diaphragm and moves the wastegate valve. This reduces the exhaust gas pressure on the turbo vanes, slowing the turbo and limiting the boost. Wastegates can also be electrically actuated. Another option is a dump valve. A dump valve is a spring loaded diaphragm on the output side of the turbo. The spring preloads the daiphragm to a specific pressure over atmospheric. If the boost pressure rises above the preload, the valve opens and vents boost to the atmosphere. They are non-adjustable and are usually used in conjuction with wastegates. The wastegate will typically open before the dump valve, because it is more driveable to slowly limit boost than to just vent the turbo output to the engine compartment. So why have a dump valve? As Martin notes below, most turbo systems blow through the throttle plates. When the throttle plates are closed quickly, such as when you shift or when you brake, there is a reflected wave of air that is sent back through the intake system to the turbo impellor. This wave of air can be dense enough and fast enough to actually damage the impellor vanes. This extra pressure will also slow the turbo impellor down, adding to turbo lag. Since you are no longer accelerating, it is reasonable to vent the excess pressure suddenly. This protects the vanes and allows the turbo to keep spinning. A blow-off valve is similar to a dump-valve in that it vents turbo boost out of the intake. The difference is that a blow-off valve is usually adjustable so you can vary the maximum boost, and it is more progressive in it's action. It will also use other control mechanisms (vacuum, boost pressure, ECU signal, etc) to determine when to actuate. -- Mike To address comments privately to the moderating team, please address: moderators@xxxxxxxxxxx For more info on the list, tech articles, cars for sale see www.dmcnews.com To search the archives or view files, log in at http://groups.yahoo.com/group/dmcnews Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/dmcnews/ <*> To unsubscribe from this group, send an email to: dmcnews-unsubscribe@xxxxxxxxxxxxxxx <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/