wastegate theory.

B5 S4/ RS4
#1

I’m new to Audi and to forums, I normally just Google my problem till I find an answer. I need help understanding the wastegate.
If a stock s4 pushes about 8psi and the wastegate are a failsafe and open at 9psi, how do people chip their car to 16 psi with no issue? Wouldn’t the car build boost till it matched the spring tension and manually open the wg? If people chip the car for 16psi shouldn’t they also adjust the wg to a different tension?

Sorry if this sounds kinda newbish, I tried searching and you guys seem a bit more logical that the rest of the Audi community.

#2

A stock s4 has its k03 wastegates set to about 5psig. The boost controller (n75 valve) bleeds off 3 to psi so that the wastegates see 5 psi when the intake manifold is at 8-10psi (stock s4 tune boost pressure).

#3

But doesn’t the wg have a manual override incase the n75 goes out? So an example would be, n75 fails and doesn’t tell wg to open, so at X psi the pressure pushes the wg open once it is strong enough to beat the spring tension?

#4

The N75 valve should be of normally open design, so if the N75 fails all the pressure go straight through the N75 and the wastegates will see full pressure. This will cause the wastegates to open their preset pressure of 5psi in the case of OEM BW K03s. Look up how a boost controller works.

#5

see page 25 for more info on the N75:

http://www.vaglinks.com/Docs/Audi/B5/S4/Audi_S4_engine_english.pdf

#6

I started to look that up but got caught up in what might be a misunderstanding. OK let’s forget about the n75 for a second, if somebody runs 16 psi on stock k03’s wouldn’t the pressure from the turbo just force the wg to open? It was my understanding that if the turbo hits a certain amount of boost the pressure alone would force the wg open as a safety net to prevent overboost. Am I wrong on this?

Thank you for your help by the way, I’ll read that of once I get off work.

#7

Your not quite getting the purpose of the wastegate. The wastegate is not a safety mechanism. The wastegate is a regulator. The boost pressure a healthy car will make with the boost pressure line going straight to the wastegate will be what ever the wastegates are set to. You can not make 16psi of boost with a k03 wastegate set to 5psi and no boost controller, you will only make 5psi.

When you add a boost controller such as the N75, then the wastegate pressure goes from being the only amount of boost the car will make —> to the minimum amount of boost the car will make. This is because the boost controller adjust the pressure signal going to the wastegate to something lower than what the IM manifold (and engine) will see. So with the boost controller you can raise your boost pressure from wastegate pressure.

See this article: http://www.turbosmartusa.com/technical-articles/how-does-a-wastegate-work/

#8

Quoted from your link.
“as running high boost through a factory internal wastegate can overpower the actuator spring, limiting maximum boost level. Aftermarket external wastegates feature bigger inlet and outlet ports, higher pressure springs and bigger actuator diaphragms to effectively control high boost pressure.”

This is what I mean, by safety net.
Thank you guys so much, I’ve only ever posted about 3-4 topics in my life of forums and they just get skipped.

#9

It is generally accepted that you should not run boost pressures greater than about 2X the wastegate spring pressure due to the wastegate spring getting overpowered. This will cause boost to fall off in the upper rpms. This has nothing to so with the wastegate acting as a “safety device”.

The wastegate’s roll is to regulate the turbocharger so that a certain intake manifold pressure is maintained under WOT conditions. Nothing more, nothing less.

Safety devices that are built into charge pressure control systems generally have a means for detecting boost pressure that is too high, and the corrective action is to limit boost to wastegate pressure by overriding the boost controller in order to keep the motor from killing itself. This corrective action is usually referred to as “Limp mode”.

#10

So what is the stock spring pressure, and is it recommend to upgrade the spring if I chip the car?

#11

The K03’s internal wastegates are adjustable (look up how to adjust an internal wastegate for more info), but you should just leave them where they are. They will be fine for any stage 1 or 2 car since the limit is the K03’s flow rate and not the wastegate’s spring pressure.

#12

I looked some stuff up since I got home, looks like I was not fully understanding the wastegates function.
I appreciate all of the help this forum has offered.
One last question, one of my wastegate diaphragms are bad, and both turbos leak. Once I hit 0 psi it flat lines regaurless of rpm. Should I get the turbos rebuilt or just buy used k03’s? I don’t have the cash as of now to get k04’s

#13

I think this explanation will help.

While the external waste gate is pressure actuated you have a valve that keeps pressure away from the waste gate (n75) till needed. Once the n75 valve sees enough pressure it introduces that pressure to the waste gate. The waste gate is responding to what the n75 feeds it. If you closed the n75 all the way and never let it feed the waste gate any pressure your turbos would spin as fast as they could and make as much boost as they could.

#14

I understand where I was getting mixed up and I have a much better understanding of the system now.

I’ve decided to buy a used or rebuilt turbo for my car. The dealer gave me my model code (701sx) but aren’t all 01 s4 turbos the same?

#15

Other then the presumed need for a specific driver and passenger side turbo yes. I would just remove your turbos and have them rebuilt. Don’t buy a used turbo.

#16

Well a guy is selling 2 turbos that have been rebuilt for $450, just seems like a good deal to me.

#17

Buyer beware of rebuilt turbos. That is all I will say

#18

Is there something I should look for? It was rebuilt by a shop, and he has the recipt

#19

I agree.

You’d be better off to see how much it costs to rebuild your turbos first. In my experience; you look for a good deal when you’re repairing the car to sell it. You look for good quality when you’re fixing the car and keeping it.

#20

If you can pull the motor and do the turbos yourself that you can gamble with the used turbos… but if you are paying a shop $2000 to install turbos for you do you really want to risk that labor charge on a questionably set of turbos. Whether you can do the labor yourself or not, your gonna be really pissed “if” those rebuilt turbos start howling/blow in less than 50,000 miles…