New DVs

[Dippy]

Joshie (and others),

Nige's post has finally got me to look at the technical aspects of this, and I have reasoned as follows. I'd be grateful of anyone's comments.

The boost pressure is controlled by the ECU, by reading the pressure sensor (on the TBB) and then adjusting the wastegates on the turbos via N75 (as described in the AudiWorld article). Since this is a closed loop, as I see it, adjusting the screw on the N75 should ONLY be required if it has been changed for some reason. This might put the control loop out of the ECU's range. This would result in one of:
a) ECU unable to bleed air from N75, such that the wastegates are always partially open (is this possible? - I'm not sure from reading the Audi info). The result would be that max boost could not be obtained (should get a DTC), but this would not be dangerous as the turbos would not be stressed.
b) ECU unable to stop bleed air from the N75 so that the wastegates are not opened enough to restrict boost pressure (should get a DTC). This is clearly dangerous, but should result in limp mode (0.4bar).

The diverter valves are only used to quickly reduce air pressure at the manifold when you lift-off the accelerator. Failure to do this could put a back-pressure on the turbos which is bad for them and the air-induction system. The ECU opens the DVs by opening valve N249 which applies vacuum from the engine to the pistons. However there is a vacuum reservoir such that should this system fail, the valves can still open anyway.

So to the case of DV problems. I think most of us have put non-Bosche DVs in because of the worry of leaks. Now if the DVs are leaking then the turbos have to work harder in order to maintain TB boost pressure whilst some of the air is circulating through the leak. So a DV with a solid valve seems better.

However I can think of the following failure modes too:
a) DV sticks in the open position. If this is permanent it is worse than a leak in the Bosch DV. However this is extremely unlikely. More likely it will be temporary, such that there is a momentary loss of boost, i.e. a lag, after a gearchange.
b) DV won't open (various failures) with the result of potential damage to hoses and turbos, and of course there might be engine surge.

Now the interesting thing about these DVs is the springs. I assume that there must be a balance between max boost and spring tension, as the spring needs to close the valve after use.

There has been a suggestion that the Forge valves are supplied with 'weak' springs. This could make the DVs slow to close after a fast gear change under boost, resulting in lag. Fitting a stronger spring could help, but this must surely increase the risk that the DVs won't open as quickly under certain circumstances, resulting in possible surging.

What confuses me is that you seem to like the new Bosch valves, but surely these are designed for a standard S4, and in fact aren't they used in the RS4? In which case why the need to change spring tension?

I think I need to service my DVs as I have had some hesitation and some marginal surging, although this seems to be only at high speed when I'm running at low boost: After a gear change the boost oscillates for a bit before setting.

Lastly about the limp mode: This can happen in many circumstances when the ECU detects a possible problem. For me it happened when my one of my primary cats was too cold (lowered car and wet weather). The ECU decided that my emissions might therefore be too high and it had better reduce max boost to 0.4 bar.

This was damn scary the first time it happened as I suspected turbo failure. But of course you're used to that now aren't you?

Cheers,
Dave

[Nige_RS4]

Here's a link to an article showing the location of the valve. Unable to upload pics into the gallery at the mo ???
[img]images/graemlins/s4anogaro.gif[/img]

[Joshie]

This was damn scary the first time it happened as I suspected turbo failure. But of course you're used to that now aren't you?

Cheers,
Dave

[img]images/graemlins/roflmao.gif[/img] Hey mate - I have only had a set of turbos fail completely and we both know why that was [img]images/graemlins/wink.gif[/img] The K03's were just a little whiney after 55K miles, half of which was with a split TBB which nobody seemed to be able to be able to diagnose!

I have had intermittent low-boost issues, with only one instance occurring after the Bosch DV's have been fitted and none since the new Forge DV's have been fitted.

Nonetheless, as usual a particularly detailed and considered post - very interesting although I must say that I have come to expect nothing less of you my wise friend [img]images/graemlins/clap.gif[/img] [img]images/graemlins/clap.gif[/img]

[Joules]

Joshie,

So in summary do you choose the Forge (New Units) over the revised Bosch item. OR is there still more lag with the Forge units??

[Joshie]

I was pretty sure that one of the Forge DV's which I returned to Forge Motorsport was not holding the vaccuum. This resulted in a loss of boost from time to time. Forge stated that they tested the valves and found them to be fine but very kindly sent me new uprated Forge DV's free of charge.

The Bosch ones were an interim measure and I would always prefer to run the Forge DV's as I believe that they are more reliable and robust. The only thing to remember is that from time to time they need to be taken out, checked and if necessary serviced.

[Joules]

Thanks for that ...and are you saying there is no increased lag while running the FORGE DV's?

[Joshie]

I have read that the Bosch valves are supposed to be "quicker" than the Forge resulting in a shorter "lag" but I must admit that I have never been able to confirm this as they feel equally responsive. As I said in my previous post what I am after achieving is reliability with no interruptions to boost and my perception is that the Forge DV's appear more robust and therefore, hopefully, more reliable.

One way to try it is to get someone to alternate the valves so you are not aware of which valves are installed. This could be done at a trackday when you'll be able to test them to the limit. That way you could see for yourself! [img]images/graemlins/thumbs.gif[/img]