blow off valve

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maddogrc

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hey all,
when ever i shift gears at 3000rpm the car makes a pssht sound, like from a ricer blow off valve. i know this happens because when i shift at 3000rpm, the waste gate also opens.

thing is, i never knew diesels had a blow off valve like on petrols? i thought they had like a slow release pressure valve or something. does anyone know where in the engine bay this blow off valve is? i've had a look in the manual and nothing is in mine, plus i have no idea how a diesel one would look like.

cheers,
george
 
The turbos just make that noise as the turbines unload, there is no blow off valve like a petrol as there is no need, a petrol has a blow off valve because of the butterfly in the throttle body that closes when you lift off the accelerator, which gives the boost pressure no where to go.

On a diesel when you lift off the accelerator the boost pressure just drops because there is little to no fuel being injected which causes a the drop in exhaust pressure, on the inlet side the boost is just depleted into the cylinders. Also more than likely the waste gate will already be open when you lift off to change gear, your engine might see max boost at 2500 rpm for example, its at this point that the waste gate is open keeping the boost pressure at a pre-set maximum pressure, and it will stay open until the exhaust pressure drops again regardless of rpm.
 

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