Dawes and Needle Valve?

[the evil twin]

Some word terminoligy differences here . Actuator adjustment shaft is my description of it , but the grub screw would be the limit adjust . This is how I have understood and answered it , so my blew if this is wrong .

I could be wrong too, Cobber, but thats my understanding of it all. AFAIK the boost pressure is controlled electro-pneumatically from the ECU sensing the pressure at the intercooler and porting the boost solenoid whne max boost is sensed. Trying to control spool the same way would be doable but more difficult.

I still haven't got my head around that "other" shaft on the Turbo and adjustment beside the VNT... the one with the flat plate either side of the threaded shaft but everything I have researched says "whatever you do don't fiddle with it"... I sure don't have the guts to do it... ROFLMAO

Bottom line is I guess... pre CRD do both (Dawes and Needle) post CRD try a tweak on the VNT if it doesn't work or if you want to go fully manual anyway then, again, do both.

P.S. ... I think its a bit like flying a chopper... anything you do affects everything else anyway

[BillsGU]

Have a look at the two boost curves that Chas has on yellowfoot.com. You can see the difference between the OEM curve and the Dawes curve. The Dawes curve is much smoother and more predictable. Once the Dawes valve is installed it sets the absolute maximum boost. It is then impossible to overshoot.

You won't regret doing the mod the Patrol just runs much better, and it is about the cheapest mod you can do as well. $45 for the valve - delivered.

[threedogs]

X 2 with above , fit the NADS then book in to get it DYNOED.

[WogsRus]

One thing to note, the stock ECU on a CRD allows for fuel mixture lean out when cruising, up to 18psi or more, with the valve installed, you no longer have any lean out of cruising mixtures. I recently installed a 3" pipe on my CRD and all i did was to tweak the VNT stop screw in about 1/2 to 3/4 turn and shes juts about back to normal seting, a bit more never hurt anyone.

[the evil twin]

Oookaaaay (E.T. says very hesitantly) after trolling thru heaps of sites trying to sort shyte from clay I believe I have the adjustments sorted in my head. My understandings below were gleaned mainly from the Factory Manual, Garret and other VNT manuf's, several forums and Chaz's website

Boost Pressure
Easy bit first. Ex factory the ZD30 Boost pressure is controlled solely by the ECU electrically sensing the boost pressure at the intercooler outlet plenum and modulating a vacuum signal via the boost control solenoid to the turbo actuator which then mechanically increases/reduces the angle of the vanes directing gases into the turbine.

This engine control process is what the Dawes Valve partially replaces by limiting max boost pressure and, hopefully for those who fit them, eliminating overboost. Note that as Chaz says in his site, the Dawes alone will not fully control proportional boost settings just maximum.

Turbo response (often also referred to in forums as 'spool up'. I'm not going to argue other than say the manuf's of VNT's seem to distinctly differentiate the two terms)

Adjusting the actuator rod length (if your game). Adjusting the rod length doesn't change the max boost pressure and it also doesn't change the max spool up rate as such. What it does do is make the actuator more sensitive to the modulated vacuum signal from the boost control valve.
Shorter rod length means the same actuator movement for the same ECU commands will move the vanes over a larger angle earlier up until you hit the grub screw stop.

Now the important bit... it appears that this ONLY affects the turbo when the ECU is modulating below max boost. IE if you 'metal the sucker' everything goes max and the vane angle is once again limited by the stop screw. Intermediate settings with an incorrect rod length however and the turbo could possibly hunt like a bitch
It is the turbo response from the ECU via the bost control solenoid that the Needle Valve will affect and here is the interesting thing.
The needle valve adjustment can only make the turbo less responsive not more. Again, using Chaz's illustrations the Boost at a given RPM is often significantly lower with a needle than when controlled from the ECU.

If you look at typical VNT curves they show much higher boost available at lower engine RPM which is exactly why VNT turbo's are used in almost every modern small diesel. Also I discovered that the fluctuations observed in VNT curves are usually attributed to EGR control modulations not the VNT turbo operation itself.

Adjusting the grub screw. This adjustment adjusts the maximum vane angle available to the turbo nozzle. This affects the maximum spool up rate as it allows more exhaust gas to be directed away from the bypass plenum and into the turbine by the variable vanes as they can open further. Larger vane angle, more gas, faster spool.
Now the important bit again... neither the Dawes nor Needle valves can affect this setting as it is 100% mechanical.

Bottom line... no wonder Dawes and Needle mods are so popular rather than dicking with the actuator on the Di's.
CRD's however with their higher factory boost pressures (arguably up to 22 PSI manifold), different EGR plumbing and different ECU maps maybe not needed so much.

In no way am I saying "Do" or "Don't Do" any mods etc. I am simply saying I like to understand what is happening and previous posters caused me to rethink a few things and very rightly so on their part.
Also like any subject I could still be rrr... rrrrrrr..... rrroooonnn... incorrect

[Gunner93]

Evil Twin you are a legend!

So what we have established thus far is that the dawes valve is a good idea to prevent overboosting and that the needle valve does control spool up time, but only as much as the grub screw will let it if that makes sense (i.e. the grub screw still controls max spool up time, the needle valve just lets you vary it without getting under the hood )

[boots]

Thankyou for setting me staight evil twin . This last post of yours should be a sticky on its own outright on this topic , cheers .

[Chaz]

Evil Twin, very well explained!

Just if I may add - people often ask why we would want to slow down the spool rate. There are several good reasons. The factory spool rate is quite fast and VNT’s are generally faster than a conventional turbo, but the ECU will pull boost back fairly quickly after it peaks and usually before it goes into limp or fuel cut. It becomes a bigger problem after fitting an aftermarket free flowing exhaust.

Early on there were quite a few turbo failures and this wasn’t only with ZD30 Patrols. When VNT turbo’s became more popular we started to see diesel turbo failures more often due to some working very close to the point of surge. Turbo surge is often a result of very fast spooling at lower rpm.

Slowing the spool rate can have negative effects on turbo lag, but there should be a happy medium and this is what people need to try to achieve if they go with a manual TDi controller. The objective is simply improved reliability and drivability without a negative effect on performance, but the main reason to bypass the ECU solenoid is to overcome the drop in boost for EGR after EGR is disabled.

Good work and I hope that helps.

[the evil twin]

Hi Chaz,

Excellent points and very relevant. Indeed you just gave me a light bulb moment about some fuel flow fluctuatiuons I occasionally observe on the Freeway with my vehicle 'light' IE not towing etc and cruise control engaged. Might also figure in some discussions I am having elsewhere over the last 12 months or so on fuel economy and do you baby a ZD or drive 'em up in max boost.

As you (and many others) would know the EGR is closed at higher loads by the ECU. In my case, like a fair amount of us, I have the EGR blocked but the ECU doesn't know that of course.
Now, I like to get my truck on boost and keep it there till I get to whatever the speed limit is or my cruising speed. This means that the ECU will tell the EGR to close anyway and maps the fuel injection, boost etc accordingly so blocked or closed there is now no difference just the same end result.

I mean to say I don't "metal it" 'cept on rare occasions nor am I (IMHO anyway) a lead foot... what I do is get the accelerator down far enough to keep the boost on and steady and there is still a fair amount of pedal left.

Perhaps the reason I don't see other vehicles doing what mine does and vice versa is because of the ECU and EGR being managed differently due to different driving styles. I forget the EGR settings now but I think it was 50% load or more and the ECU closes the valve. I'll go check the manual

[the evil twin]

Oh Oh... my bad...

After consulting the all knowing manual I discovered that the EGR on the CRD's is a totally different animal to the Di's.

Humblest apologies if I confused anyone... E.T. has been a very naughty boy