NefMoto

Hi I’m currently installing a turbo on my 2003 1.8 tt and I’m trying to figure out a range I guess.  So the turbo is a gtx2871r which from the specs can handle up to 30 psi of boost and can be set as low as 9 psi via its own external wastegate.  I’m assuming the 9 psi is a maximum value that it can be set at?  I’m not sure the purpose is in that.  The manifold I have has a flange for a 38mm wastegate.  Looking at the specs of the 38mm ones some can go as low as 4 psi.  But they never specify a max boost of  something like 30. So I’m a little confused.  And I don’t see a range for the n75 valve.  Basically trying to deliberate whether or not to buy a 38mm wastegate because I have to fab a downpipe anyway or should I just use its own and cap off the 38mm flange? 

Wtf

Lol I did post this in the noob questions so I’m not entirely sure your response is valid. 

You want to balance low cracking pressure for part throttle drivability vs high spring pressure to keep the WG closed.

The wg is not open circuit for the most part? 

I have no idea what this means.

The wastegate.  Is it not open most of the time? 

Are you drunk?

It is open when it needs to be open, closed when it needs to be closed. What % of the time is a turbo producing more pressure than requested?

I don’t know.

Nope.  Are you? 

You need a gate, the turbo doesn't have one (according to your original post).  Set the base pressure to whatever spring you fit, and use the N75 to control after that.

The base pressure of?

the 9 psi is the cracking pressure of the internal wastegate ie. that is the lowest amount of boost this turbo can create unless the wastegate actuator is changed to a lower spring pressure. 30 is the max the turbo can produce reliably without blowing the seals. external wastegates can be anywhere from 1 psi to x amount of pressure depending on what spring is in it. cap off your manifold wastegate port and use the gtx's internal gate. the n75 is just a solenoid that lets pressure through to the wastegate at a specific pressure specified by the ecu. you request 25psi and once the ecu sees manifold pressure at 25 it opens the n75 and lets pressure to the wastegate thus opening it and regulating boost pressure.

So does the turbo make any pressure after cracking or what’s the psi value after cracking? I’m assuming by cracking you mean cracking opening of the flap?  I guess this is going to be a lot of trial and error.  How do I make sure overboost will not happen?  Or typically what cracking pressure do I need to be set to to run 300whp @ 5k rpm?

the wastegate regulates boost  pressure by opening and letting the exhaust gas bypass the turbine wheel.
you have a lot of research to do my friend. it isn't as easy as swapping turbos and going. guarantee you are going to make your engine block see-through if you try tuning this yourself if you dont even know how turbos operate.

Literally none of what you said make a whit of sense. They aren't even coherent questions.

The only question that remotely makes sense is this one


And if you don't know the answer I don't know what to say.

Say nothing, because the wtf and are you drunk is just as negative as this one.  This is the noob question forum so let’s not all laugh at people who are not up to parr.  Thanks

Well I am on the noob forum so I think I’m generally aware the research I need to do.  You were the one that mentioned cracking pressure.  So me asking what is cracking pressure seems to be pretty straightforward.....

cracking pressure of the wastegate is as straight forward as can be for the topic. it is the pressure in which the wastegate starts to crack open.

Are you doing this to keep rods in the block?

Thanks starting to make a little more sense as I do more research....Stay tuned.

I rebuilt the engine already with H-Beam rods.  The alternative to buying a gtx was to repair my old K03.  Seems a waste if you'll pardon the pun..  I understand that there are envelopes not to push past and that's all I'm trying to do is reduce the chance of that happening by asking questions. 

the short answer is the wastegate is controlled by the ECU.

it should have a proper operating range such that

1) it is tight enough to hold the maximum desired boost when there is no pressure in the wg control line (N75 WGDC is >95%)
2) the minimum pressure required to open the wastegate is low enough to allow good part throttle behavior (N75 WGDC is <10%)
3) the operating range of useful wg control line pressure results in as linear a response as possible

To elaborate on 2, the ECU controls load via two main methods (torque intervention via ignition angle notwithstanding): 1) throttle plate angle and 2) wgdc.

Below cracking pressure (aka base boost), load can only be controlled via throttle plate angle
Above cracking pressure, load can also be controlled via wgdc

This is all applicable to all kinds of forced induction applications, it is not unique to Audi or even Motronic.

It is a bare requirement to understand all of it before even beginning the tuning process.

Thanks.  I think I know where I’m going wrong.  Thought the n75 was making its own pressure.  Oops.   

That would do it. Yes, there is no other source of pressure that can be used to open the WG.