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Discussion Starter #1
Blow off valves (aka dump valves) have a very simple yet useful function....

When the turbo is 'working', ie you have the foot on the gas and its producing 'boost', what the turbo is doing is compressing air (to get more into the cylinders - more air in (with fuel) gives more power). This air is passed thru the intercooler to cool it (cos when you compress it it get warm, but the engine needs air thats as cold as possible to get power) then it goes thru the throttle body and into the engine. Now, if the turbo is compressing this air, then as you can imagine there's a lot of air under pressure whooshing towards the throttle butterfly (go faster device) which is open when you are pressing the go pedal. If you lift off the throttle to change gear or slow down, the throttle butterfly closes. Where does the air go???? There's a lot of it under pressure, remember. Well, normally it get trapped and produces backpressure (ie it pushes back against the turbo) which can damage the turbo over time. It forces the turbo to slow down/stop completely very suddenly. So, when you put the foot down again, it takes a wee while for the turbo to spin back up to operating speed.

A dump valve vents this excess pressure created when you lift off the throttle. It sounds wicked too, as well as keeping the turbo spinning and preolonging its life.

A turbo timer is another cool device. Your turbo spins incredibly fast (up to 80000rpm for some), and its bearing/shaft is usually lubricated and to an extent cooled by engine oil. If you are running hard for a while, things get hot in there. If you were to just stop the engine and leave it, then the hot oil sitting in the turbo shaft and bearings slowly bakes the turbo, which generally isn't good for it. A turbo timer will keep the engine running at idle after you take the key out and lock the car, for a preset amount of time. SOme can even work out themselves how long the car needs. This ticking over allows cooler oil to circulate around the turbo, as well as generally letting the car cool down inside the engine itself. Great bits of kit, again, prolonging the life of your turbo.

However, it is illegal in the UK to walk away from your car and leave it with the engine running. Even if it is locked.
They usually have a hadbrake sensor which, when the handbrake is let off, will kill the engine immediately (in case someone goes in thru the window and tries to steal your car), although it'd stop anyway when the timer ran out.
 
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