I'm new to all this turbo malarky. Can someone explain what blow off valves and turbo timers do, or point me so a website which explains such things.
Close...... The pressure on the intake side can stall the turbo (force it to stop), or at least create large 'shock loads' on the turbo shaft by this forced deceleration. The Wastegate moves purely to control the speed of the turbo, so what could happen is the wastegate being closed (directing exhaust gases over the turbine to spin the turbo) for a split second after you lift off... So the combined effect of the exhaust gases trying to spin the turbine and the pressure on the manifold side = lotsa stress on the shaft. The wastegate will open, however, to allow the turbo to decelerate so you are 99% correct, it just may not open that quickly.Without a dump valve when you lift off the throttle the pressure on the intake side builds up too much and stalls the turbo. At the same time cos the boost pressure has got so high the wastegate opens and directs the exhaust past the turbine so it really does stop.
Boost pressure will still get as high as without a dump valve (unless its set to release at lower pressures on an adjustable one), but it will release when necessary to stop the backpressure on the compressor. Turbo will spool up faster with a dump valve correctly fitted because there is not backpressure on the compressor when you change gear, so even though the wastegate will open as you change gear(releasing the gases spinning the turbine), the turbo will continue to spin thru its own momentum, so that when you put the foot down again it has less accelerating to do!With a dump valve the extra pressure is released on the intake side, so that turbo doesn't stall. In addition the boost pressure doesn't get so high, so the wastegate remains at least partly closed so the exhaust gasses keep the turbo spinning.
Not that I know of, but that certainly doesn't mean such a thing doesn't exist. However, it would only do the same job as a dump valve anyway.When you don't have a dump valve is there any other form of pressure release on the intake side ?
Thats it spot on mate.... The throttle butterfly (basically a round metal disc in the inlet plenum which is tilted on a shaft by the throttle cable) stops the air getting in, thats why the backpressure occurs on the compressor. All that air is trapped between the now closed throttle butterfly and the compressor.don't you then get massive overboost when you lift off the throttle, potentially causing detonation and damaging the engine - or does that fact that the thtottle butterfly is nearly shut stop too much air getting in ?
Thanks. I've got a reasonable understanding of engines already, but fuel injection and turbo's are pretty new to me.Man, you are learning seriously fast!