Spring boost

[Donnie]

Just a quick question, total turbo boost pressure is governed by the ecu, does increasing the wastegate spring tension increase the total boost applied? I think what I'm asking is is it spring pressure plus ecu, or would the ecu stop at the same pressure regardless of spring tension? (obviously spring tension boost is still below requested boost pressure in this case).
Before I induce a torrent of "don't remove safety systems" or "why interfere with motronic" remarks, this is just for my information.

Cheers

[newsh]

Spring + ECU. Unless the ECU sees too much then it will reduce by a lot.

[driftin jo]

The ECU will adjust the duty cycle to meat target boost for given rpm, etc, as long as the spring is not too strong.

[UrS4boy]

Just a quick question, total turbo boost pressure is governed by the ecu, does increasing the wastegate spring tension increase the total boost applied?

No. The spring provides a base for the upper limit of mechanical boost, i.e. if you disconnect the N75 Waste Gate Frequency Valve, but it does not add to the total. That is done by the ECU controlling the N75. That said, increasing WG spring "tension" does seem to increase the rate of boost pressure rise when you stick your foot down. If you have screwed the WG spring down too tight or you are running a 1.9 Bar spring with more than zero turns, the rapid initial rise of boost can freak the ECU out, leading to a cyclic bucking (BTDT).

N75 WGFV info: http://forums.quattroworld.com/s4s6/msgs/20987.phtml

[Donnie]

Ahhh...
OK - so if your spring degrades - boost will be reduced

I was assuming the ecu would be able to detect lower or higher (than expected) manifold pressure and compensate for that

[Donnie]

you guys posted the other answers before i finished typing!!

[driftin jo]

Ahhh...
OK - so if your spring degrades - boost will be reduced

I was assuming the ecu would be able to detect lower or higher (than expected) manifold pressure and compensate for that

It can, within a given %, I think the N75 map has 80% duty cycle maximium, thus
if there is a boost leak it does not kill your turbo by over spooling it trying to make
target boost.

[merlin's S4]

With the map on my car I should see 1.9bar peak boost (2.9 absolute). I fitted a 1.9bar spring to test the other day and my peak boost soared to well over 2bar. I promptly lifted off and replaced it with the standard one. In the early days when I first had the car, a tuner who shall remain nameless fitted said spring to my car on a standard map. The peak boost increased from 1.1bar (2.1 absolute) to 1.6 which the ecu didn't like at all. I promptly removed the spring and had the S4 mapped properly by Mihnea. So in my experience the increased spring rate does increase peak boost if not mapped accordingly.

[newsh]

Correct.

[driftin jo]

With the map on my car I should see 1.9bar peak boost (2.9 absolute). I fitted a 1.9bar spring to test the other day and my peak boost soared to well over 2bar. I promptly lifted off and replaced it with the standard one. In the early days when I first had the car, a tuner who shall remain nameless fitted said spring to my car on a standard map. The peak boost increased from 1.1bar (2.1 absolute) to 1.6 which the ecu didn't like at all. I promptly removed the spring and had the S4 mapped properly by Mihnea. So in my experience the increased spring rate does increase peak boost if not mapped accordingly.

That's kind of my point, you increased the spring boost by 125% !
There is no way the ECU can correct for that, all it can do then is see overboost
and switch off boost control.

[UrS4boy]

That's kind of my point, you increased the spring boost by 125% !There is no way the ECU can correct for that, all it can do then is see overboost and switch off boost control.

This is the cyclic "bucking" that I mentioned.

[Rusty]

Even the stock spring can see ~2 bar of boost if you run the lower WG chamber to atmosphere. It's the ECU leaking boost into the lower WG chamber that reduces the boost over this peak amount. When you unplug the WGFV, it allows all of the IM pressure into the lower chamber and opens the WG. It's the resultant force of the boost pressure acting against the spring pre-load that leads to the quoted WG crack pressure. This is not necessarily the amount of boost that you will have all the way through the rev range either. e.g. the stock spring cracks at 0.4 bar and tail off to around 0.3.

Simply:-

Force exerted by the spring, when compressed by a given amount (WG spring pre-load)

F = k.x

and for the WG diaphragm

F = p.A where p (pressure) is boost pressure above atmosphere, as that's what acts on the upper face of the diaphragm and A (area) is the WG diaphragm effective area.

There is also the pressure acting on the WG valve itself, although this is a relatively small area and small pressure difference.

When the 2 are equal, the WG starts to open.

I have mathematically modelled this and tested it against a range of springs with good results

[simons2]

Does cyclic "bucking" show up as boost "flutter" ?

[UrS4boy]

Does cyclic "bucking" show up as boost "flutter" ?

Flutter is like a mild vibration. Bucking is like facemash, whiplash, facemash, whiplash.

[simons2]

It shows up on my boost gauge