So in furthering my (not too) mild obsession with increased efficiency from the car, I've been looking into improving the turbo design next.
Audrey is on 172,xxx miles and is on the original turbo. I think most in the know would agree, it's time to think about a precautionary replacement. The KKK turbo used on the A2 TDi90 is unique to the model and very scarce, so in line with my policy of collecting rare spares to keep the car going when I can, when a brand new genuine unit popped up on eBay 18 months ago I snapped it up and it's been sat on my shelf since.
Now the thought of modifying it before fitting was prompted when an advert popped up recently for a modified turbo. I must thank Tom
@timmus for
this post in his excellent TDI120 modified engine thread for laying out the dynamics of how a turbo works and what can be done - even if he was referring to the TDI75 turbo as a base.
Investigatory conversations with the specialist I spoke to (Adam at Pioneering Performance near Bournemouth), were fruitful:
This little 3 cylinder motor turbocharger is not actually all that well supported in the aftermarket as the A2 is the only vehicle which uses that VNT turbo - other VAG cars like the 4 pot big brothers share a plethora of parts between them so the market is much broader, whereas this ATL motor is the only 3 cylinder one to wear that turbo.
As a result of this, there actually aren't all the parts we need to rebuild it or do a billet compressor wheel upgrade available. In the past, we have machined in a bigger compressor wheel but the supplier doesn't make one as a "drop in" for this particular application unfortunately. That being said - we could commission one using the OEM original in your turbo to send off and have optimised: You'd have the first and only one in the world with a drop-in billet compressor wheel. (Drop in meaning OEM size but optimised for performance).
The real world result of this would mean your turbo being about as peppy as it could ever be - the billet compressor wheels are made to help increase spool wherever possible. It wont be a hybrid and it wont be a rocket ship - it would still have the same size inducer and exducer, but it would spool better. We could also rebuild your unit with a strengthened 360° thrust bearing along with the new compressor wheel and this should see you good for many '000s of miles of happy motoring.
If that isn't for you, then the only other option would be to make a hybrid where we install, with machining to the compressor housing, a larger than stock compressor wheel. This is what I have done before on this turbo, but be aware that this little 3 cylinder motor isn't the most responsive and larger than stock turbos can take away some of the drivability even with a fairly modest increase in compressor wheel size over stock. If you weren't looking to get a new custom map put on the car then this isn't the better option, the first one is.
Finally, the performance upgrade which I have seen done a few times is taking the 4 cylinder 150hp turbo, deleting the 4th cylinder from the manifold and blocking it up, and using that as a 3 cylinder. This would be an interesting project, not one Ive done before but something Id like to have a play with if you wanted to go down that route. (That one would probably be done when seeking a lot more performance)
Now, in terms of current state of tune, Audrey is rolling road tuned to 117 BHP and already fitted with
Tom's all-alloy intercooler, so that, along with Paul
@depronman's lower turbo pipe bracket, eliminates the other weak spots in the ATL boost system, from a reliability point of view.
As my aims are enhanced longevity, efficiency and usability, whilst the last two paragraphs look very interesting, I'm not looking for an increase in max power and torque because I don't want to further stress components such as the clutch assembly - and the car is pretty quick already! Therefore, in the spirit of trying to eliminate mass production trade-offs and getting the exisiting design as good as possible to achieve my aims, I'm going with his first suggested route.
So, as the cost isn't TOO outrageous (and I have some bits and bobs from my old BMW days in particular I can sell to raise the funds), the turbo is on it's way for a makeover - Scope is to strip, supply and install the uprated thrust kit, one-off billet compressor wheel, then balance the whole assembly, re-assemble and flow set back to the OEM flow settings. I will then have the manifold heat coated with Zircotec or similar ceramic coating (if possible, given the combined nature of the manifold and turbo), to help reduce heat soak into the intake charge (I can see from CDIS that when the engine is working hard the charge temperature rises fairly dramatically at the moment). I am thinking of purchasing a good used inlet manifold to have treated in the same way at the same time - just as Tom did.
If anyone has further suggestions on how to further optimise the turbo, I'm all ears and can ask PP.
I appreciate there could be an increased risk of stressing the DMF in particular at low RPMs with a turbo that now spools quicker and at lower RPM than standard, however I have trained myself to keep the revs above 1550-1600 RPM before applying much throttle in all gears beyond 1st (and applying it progressively, rather than ramming my foot to the floor) anyway, so I am willing to take the risk. The gains may be fairly marginal, but I'm interested in both seeing how far I can go to get this little car to the peak of what's possible without harming usability; and also improving the fuel consumption over shorter journeys. Currently Audrey can easily top 70mpg on long runs but struggles to achieve much over 50mpg on shorter ones around the Blackdown Hills, where I live - I wonder whether this will help.
I would appreciate any thoughts.
In other news - I've been getting the occasional front suspension clonk again, so as one of the few easier-to-change components left in my mission to permanently eliminate this is the ARB bushes, I've managed to snag a pair of new genuine VW ones for £15 posted which will go on in due course ahead of the winter. I've already checked the springs for cracks and they're fine, so I am hoping the top mounts or struts are not worn - everything else will have been replaced! That said, I did get a twang just now when turning the steering wheel with the car barely rolling....
