Re: Dump-valve on a JTD
Ok, I know a bit about this having had turbo'd petrol and diesel engines.
First thing to get straight is the way the dump valve is activated. As has been mentioned on here, petrol engined cars have a butterfly valve in the inlet, somewhere between the air filter and the inlet manifold. On the VW 1.8T 20v engine this is placed right on the end of the inlet manifold casting after the intercooler and before the injectors. The butterfly valve opens and closes with throttle movement causing a near complete seal when throttle is at zero. There is a vacuum feed pipe taken from the inlet manifold after the butterfly valve that goes to the dump valve. When throttle is at zero, or is backed off sufficiently at higher revs, the vaccuum created in the inlet is used to activate the dump valve by means of a piston or flexible diaphragm. (For recurculating valves) The piston or diaphragm opens up a bypass circuit that links up the inlet system just before the butterfly valve, with the inlet system just before the turbo intake, essentially linking up the turbo's output with the input (all on the inlet side) so that the turbo doesn't stall. Vent to air system simply redirect the inlet just before the butterfly valve to air.
Ok, so with diesels, as has been mentioned, they don't have a butterfly valve on the inlet. Don't know why this is! But this means that there will be no vaccuum on the inlet in the event of zero throttle or backed off throttle as the turbo pressure will constantly be fed in. In this case, if you want to fit a dump valve it must be activated 'manually'. The ones I have seen (306 diesel) are activated by a microswitch on the throttle. When the throttle is closed the microswitch activates a solenoid which switches a feed to the dump valve from pressure to vacuum, opening the valve and Hisssss! So if you want to fit a dump valve to your JTD, this is the kind of solution you will be looking at. However, I can't see that it will make any difference. On petrols it stops the turbo stalling against the back pressure caused by the butterfly valve. On diesels you don't have this, so the pressure goes into the engine, out of the exhaust and continues to drive the turbo, which should reduce lag more than sending the pressure to air or back into the inlet.
Now, why diesels don't have a butterfly valve I don't know. I suspect it is because petrol turbo's spin faster in order to shift more air. Although diesel and petrol turbos create the same amount of pressure, the petrol engines rev faster and will therefore require a greater volume of air to acheive the same amount of pressure. I guess this may be something to do with the requirement for the throttle plate and dump valve.
erm, sorry to waffle on!