Wastegate spring - Anybody on here?

[prj]

Agreed.
Fitting a 1.9 bar spring on my Stage 1+ results in the boost needle quickly flying to almost 2 bar and then a boost cut.

Messing with springs on Motronic is not plug and play at all, because the base duty needs to be remapped.
Of course with many "cool" chips from stupid tuners, that are being copied by more stupid tuners, the boost and fuel cut limiters are turned off.

And then people wonder how something like this happens:

[A80Avant]

i was gettting the same effect when i tried a 1.9bar spring while using motronic as well. Then i ended up modifying the wastegate cap and using a larger screw i had the opportunity to adjust the spring manually to get the effect i wanted without hitting a boost cut

[prj]

i was gettting the same effect when i tried a 1.9bar spring while using motronic as well. Then i ended up modifying the wastegate cap and using a larger screw i had the opportunity to adjust the spring manually to get the effect i wanted without hitting a boost cut

That means that your chip was not using the most optimal settings regarding wastegate duty cycle.
When you push it closer to the limit it spools better, but altering the spring will result in too much boost.

[A80Avant]

with the screw manipulation of the OEM spring, i managed to overcome the boost cut, but also keep higher boost on high revs, which was dropping too fast. For some reason on my original map in germany, boost was holding on 1.5 to the red line and once i went back to greece, it was dropping to 1.3 , although 2 different boost maps were tried. So since the fuel was there for up to 1.5 to 1.7 bar eventually, i went there with the spring manipulation.

then vems came...

[cooperquattro]

Agreed.
Fitting a 1.9 bar spring on my Stage 1+ results in the boost needle quickly flying to almost 2 bar and then a boost cut.

Messing with springs on Motronic is not plug and play at all, because the base duty needs to be remapped.
Of course with many "cool" chips from stupid tuners, that are being copied by more stupid tuners, the boost and fuel cut limiters are turned off.

And then people wonder how something like this happens:

i had this on a previous s2
standard motronic will hit the boost cut at a much lower rpm....why not just wind the standard spring down a little....

[Thuppu]

I use 1.9bar spring and it works fine, no overboost and boostcut here. Tried also a 2.2bar spring before that, and with it I got minor overboost(1.7bar -> max 1.9bar) but the spool was superquick, liked it though.

[A80Avant]

it is interesting though to say that with vems i didnt get any faster spool when fitting stiffer springs and the spool was improved tremendously when i switched from the motronic.

on the current setup i have tried stiffer springs and DC configuration of the N75 valve, and it didnt make any difference.

i use a 0.4bar custom spring i have made, with a slight preload, because it helped me with the simplifaction of the boost settings on VEMS while making the maps

[prj]

it is interesting though to say that with vems i didnt get any faster spool when fitting stiffer springs and the spool was improved tremendously when i switched from the motronic.

on the current setup i have tried stiffer springs and DC configuration of the N75 valve, and it didnt make any difference.

i use a 0.4bar custom spring i have made, with a slight preload, because it helped me with the simplifaction of the boost settings on VEMS while making the maps

That's because on motronic you had wrong WG duty settings! Too conservative.
Of course it will spool slow then.

[A80Avant]

sure but with the 3071 turbo, as soon as we were giving more DC on the N75 the car was surging like mad. S2central did some tests as well a few years ago, and the engine didnt like it at all. It is very interesting to say that i had no surge at all up to 6th gear once i switched to VEMS without any other hardware change on the engine. NOone could explain that. Same N75, same turbo setup and intercooler, same wastegate spring load. Nothing worked. As soon as the DC of the n75 went a bit higher than what i had on the original map, the needle was bouncing up and down until it settled to a steady boost, after 5500rpm!

[Rusty]

Anything over about 1.7 bar on a stock car doesn't work properly (strock ~1.4), as PRJ says above, you need to remap it to get it to work.

There's little or no reason why you can't just map the car to demand boost earlier, afterall, the WG isn't a failsafe system - if you take off the boost feed then you're on a 1 way street to some serious overboost and the turbo will give all the boost it can, as soon as it can! It's the software that feathers the boost build-up and makes things feel stodgy and flat.

Most people won't have a clue how the WG and spring choice works, here's a short waffle -

Values like 1.9 and 2.2 bar are the WG crack pressure in absolute pressure (so -P_atmosphere to get gauge pressure). The boost profile after that varies.

The spring crack value is with the N75 turned off and thus the boost pressure is fed directly to the WG diaphragm, no boost goes to the inlet. Thus, the boost actually reduces the boost by opening the WG! As a result, if the WG hose is split or removed, then you're in trouble!

The stock spring is capable of ~2.0 bar gauge but does start to get overwhelmed at that point. To do this, the ECU vents most of the boost to the inlet and very little, if any to the WG, to modulate boost. So, the boost created by the turbo is used to press against the spring and so a stiffer spring isn't required on most stock applications. A simple pressure to force calculation vs. the pre-load on the spring will show how high this needs to be.

WG diameter ~34mm , area ~910mm^2

Preload on spring = 46mm x 3.5 N/mm ~160N

Thus P=F/A so in bar (10^5 Pa) (160 / 910e-6) / 10^5 ~ 1.75 bar.

Thus exhaust back pressure has to be exceed approx 1.75 bar gauge to cause WG creep (the other side of the WG I've assumed to be at atmosphere).

The value of 1.9 / 2.2 is only the cracking pressure. The interaction between spring rate, WG size, turbo, EM and a whole host of other things determins what happens after that. In most cases, the boost drops by up to 50% by the redline, thus, a 2.2 bar WG spring will give 1.2 bar gauge @ WG crack, dropping to about 0.7bar @ redline.

Stiffer springs are not the place to start, much better to use software tweeks to demand as much boost as possible early on. Stiffer springs are a cheat's way of getting more boost for a fraction of the cost, however, they remove all the safety net offered by the stock spring and mean that the engine will not go low boost if a problem occurs. This can compound the problem of an over-ambitious home tuner's efforts, resulting in damage like PRJ shows! There's also the mapping issue, mentioned above.

Stiffer springs are required in high boost applications, where the back pressure on the WG causes it to creep and thus lose boost. A bigger WG means more force for the same pressure, and this has to be resisted. It can also improve response on race/rally cars, but at the expense of the safety net that Audi so kindly provided. A little like fitting an overly tight DV and stalling the turbo...

[rs200]

Rusty... perfect explanation .. PERFECT.. so the I want to say this but my English is not so good..
my friend from Zagreb use a stiffer spring and he also put a larger bolt in WG cup.. he said "it was lovely to drive on 1.4 bar, but it was only 3 minutes.."
after that, his rod kick the engine block, and the party was over . new engine must be build.
I will use 1.9bar WGS , before remap , on my project engine. cca 450-470hp

[prj]

I will use 1.9bar WGS , before remap , on my project engine. cca 450-470hp

You can use 1.4 also, will be just fine.

sure but with the 3071 turbo, as soon as we were giving more DC on the N75 the car was surging like mad.

Should learn to map then No offence, just how it is.
Also, as funny as it is, Motronic's boost controller is at least a couple times more sophisticated than VEMS.
There's loads of free space in the boost chip as well, so it's actually possible to write your own boost controller, if you like

[A80Avant]

in VEMS there is an option to tune boost DC on a rpm/kpa table, but there is no point, as it is done much easier without it.

Too late for you to try it our Dmitri, setup is long gone! But it looks like it wasnt just 1 person not being able to tune it right.

[prj]

But it looks like it wasnt just 1 person not being able to tune it right.

Does not surprise me.
It took me a month banging my head against the wall and burning 3 tanks of fuel.

[cooperquattro]

I use 1.9bar spring and it works fine, no overboost and boostcut here. Tried also a 2.2bar spring before that, and with it I got minor overboost(1.7bar -> max 1.9bar) but the spool was superquick, liked it though.

you must have non standard motronic here(re-mapped or chipped)