Dawes and needle valve clarification please.

[Chaz]

I have spoken to a few trusted mates some of whom are mechanics including the diesel specialist i use (who said they used to fit Dawes and needle valve but not any more) and some are just car nuts, they all say the same thing, and that is that they cannot work out how the Dawes valve could stop over boosting, they all say over boost is caused almost exclusively by the waste gate failing or as was explained to me today by the bloke i ordered my boost and EGT gauges from that u can think of overboost as a shop with a front and a rear door, say you can only have 6 people in at once (say 6psi) with standard boost control, as the 7th person (or 7psi) enters the shop, at this point the door at back (ie wastegate opens) and the first person (or psi) exits the rear and only 6 remain inside. However there can be a delay from when the signal to open the rear door (wastegate) and when this actually happens (ie open wastegate) and this is where the problem begins as 7,8,9 and so on can be forcing there way through the front door as the rear door is trying to open, they cant get out before more come in and presto over boost.
Every single person i have asked has said no you need to install an electronic boost controller as they actually take proper control of the waste gate by monitoring and controlling boost and take into account the pressure building via exhaust? What is the go?
Also i showed the people i asked the install diagram from this site and they all scratched their head in confusion as to how the Dawes valve really does much at all with regard to over boost? I am now confused as every one here says there great but i cant find any body else who will fit one or even knows what their purpose is, i am seriously thinking of going for the electronic boost control over the Dawes valve.

Ben81C,

It sounds like none of your mechanic mates or diesel specialist are familiar with the VNT controlled turbochargers used in almost all late model diesel cars. Variable nozzle turbochargers don’t use or have a wastegate and unlike a conventional wastegated turbocharger, they’re controlled by vacuum rather than pressure. Controlling boost with a wastegate is easy and in that case I would agree that an electronic controller is the better option, but in our case, and the ZD30 Patrols VNT unit, this doesn’t apply.

The main reason we fit a Dawes TDi controller is to enable bypassing the ECU control solenoid after blocking the EGR. The Dawes valve does control boost well and many people use them with good results. It’s just something fairly new that not too many diesel mechanics may be familiar with unless they frequent the Patrol or VW forums. Initially the TDi valves were produced for TDi VW Golf and Jeda cars in the USA, but they are also ideal for the ZD30 Patrol with its VNT turbocharger.