99 T5 - questions about boost control: boost maps, why the adjuster Topic is solved

[j-dawg]

I recently turned my attention to my wastegate actuator for the first time since I bought the car 3+ years ago. I set it yesterday with a garden sprayer and kind of a crappy gauge to an indicated 4psi (4.4 is the spec for ME7 cars, according to IPD). I haven't noticed a very big difference in performance, but I was curious to know when I should be getting full boost. My car builds a confident and reliable 12psi, according to my boost gauge, but having tweaked the wastegate I want to make sure that's not coming on too soon.

I'm not sure I 100% understand the function of the actuator adjustment, either. As I understand it, the ECU controls boost by toggling the wastegate via the TCV/BCS. Adjusting the wastegate actuator higher "locks out" the ECU's access to boost control, giving me up to 4psi of boost at all times, assuming the turbo can generate that. It's the functional equivalent of locking out the TCV until reaching 4psi.

Why would I want to do that? Why isn't the wastegate just set to 0psi so that the ECU always has control of boost? If 4.4psi were a safe limit for all engine speeds, the ECU could keep boost there using the TCV. That seems like a much smarter setup that would require less hardware, so I feel like I'm misunderstanding something about boost management.

[sleddriver]

I wrote up my experience on adjusting mine earlier this year. Made quite a bit of difference as mine was severely "de-tuned" and had been most of its life. I'd never bothered to check it, as with many things on this car. But all was not as I thought, as with many things....MVS may have a loaner gauge.

[RigsPGT]

I thought my boost gauge was lying to me being locked at 12psi on full boost but looks like I'm not alone. I messed with the rod too and when I tightened it, the boost was coming on way to soon meaning even at light throttle you would be in boost (gauge is a must on turbo cars). I then went back a bit and left it there. In the mean time I've been looking for a low psi gauge to make the tool for stock recommendation but I'm out of luck on that one.

[tryingbe]

Why isn't the wastegate just set to 0psi so that the ECU always has control of boost?

Think about this. The only way you can adjust actuator enough to have 0 psi is when the punk at the turbine housing is always open. At 0psi setting, you may not generate any boost the entire RPM range at all. One more thing to remember, the wastegate actuator depends on BOOST to work. No boost, no control.

[j-dawg]

Think about this. The only way you can adjust actuator enough to have 0 psi is when the punk at the turbine housing is always open. At 0psi setting, you may not generate any boost the entire RPM range at all. One more thing to remember, the wastegate actuator depends on BOOST to work. No boost, no control.

I have no intention of setting the actuator to 0psi myself; the manufacturer recommends 4.4, so 4.4 it is.

I'm just curious about why the system was designed, if I understand it correctly, not to allow ECU control of boost until it reaches 4.4psi. If the ECU should desire to open the wastegate at 2psi, why not let it? Seems like it'd be a useful feature for economy, limp-home modes, etc. I assume there is some reason we lock the ECU out with the wastegate adjuster, rather than letting it control minimum boost pressure with the TCV alone. I just don't know what it is.

Maybe it's what you mentioned - perhaps 4.4psi is the minimum boost pressure at which the wastegate can reliably be opened. But if that's the case, why doesn't the ECU just keep the TCV shut until 4.4psi?

[polskamafia mjl]

You might be right, I honestly don't know, but what makes you think that the ECU can't control boost below ~4psi?

[j-dawg]

The way I understand it, the wastegate is opened using boost pressure, via the TCV. The actuator adjustment rod sets the minimum pressure at which the wastegate may open (4.4psi in my car). When the ECU wants to limit boost, it opens the TCV, feeding boost pressure to the wastegate actuator and opening it. The wastegate adjuster sets the minimum pressure to which the wastegate will respond: if I apply less than 4.4psi to the actuator, a correctly-set wastegate will stay shut. This is how I understand the system; someone correct me if I'm wrong.

If I'm right, setting the wastegate to 4.4psi prevents ECU boost control below 4.4psi. I don't understand why. If the wastegate were permanently set to, say, 1psi, the ECU could just leave the TCV shut until boost pressure reached 4.4psi and achieve the same effect as setting the adjuster. As a bonus, it would also be able to limit boost below 4.4psi if it wanted to. Instead, we have the adjuster, an extra mechanical device that ambitious owners can use to bend rods.

I think I understand how each piece works mechanically, but I don't understand why they're all there. Something is missing from my understanding of the system - the adjuster adds a lot of needless parts and complexity if you could get the same result just by leaving the TCV shut.

[sleddriver]

On years up to 98, there is no "boost gauge" per sae. Instead it's a calculated value. On 99+, there IS a boost gauge to directly monitor the amount of boost.

My understanding of mechanical WGs is that pressure alone will open them if sufficient. Mine was so loose, I'm surprised I had any boost at all. The TCV allows boost to be shunted around the turbo if necessary or not, if not. Depends upon the ECU based upon all the sensor readings & its programming.

Suggest you read up on the "Fast Friday Performance Blog" in the repair database. You may find an answer to your Q.

[tryingbe]

If the wastegate were permanently set to, say, 1psi, the ECU could just leave the TCV shut until boost pressure reached 4.4psi

Try it, see what happens.

I predict your boost response will be super slow, you might get 5psi by 4000rpm while WOT.

[sleddriver]

OP: Read this: http://www.ardideas.com/Wastegate-setti ... _b_37.html