modifying the N75 boost control

[sambb]

I've noticed that a few of the boost curves that have been posted show boost overshoots and unstable boost plateaus with tunes that use the factory N75 as the boost control. I was also reading on Plautos's build thread (page 1 I think) how he used a series flow control with his N75 to take out boost irregularities, and after reading some of Julian Edgars stuff have a few questions.

AutoSpeed - Shooting the Overboost
AutoSpeed - The Audi's DIY Boost Control - Part 1
AutoSpeed - The Audi's DIY Boost Control - Part 2

Does the factory system run closed loop N75/boost control? ie if you make manual changes to how the N75 works eg with a pressure regulator or flow controller, does the MAP sensor feed the resultant boost changes back to the ECU which will then try to operate the N75 accordingly to pull boost back to where it expects to see it?

OR

is it open loop, and the MAP will see these boost changes and input back to the ECU which might adjust ignition or fuelling within certain parameters ,but not N75 function?

If I do go down the path of further breathing mods, then I expect that I may run into boost spikes, overshoots etc and have always been interested in using pneumatics with boost control. I just first up need to know how the ecu runs the N75 because I wouldn't want a situation where the ECU is just fighting any mechanical changes that have been made to how boost is controlled - which would be interesting!!

thanks

sam

[sambb]

No one knows if the ECU will use feedback from the MAP sensor and try to keep actual boost close to requested??

If the ECU detects knocking, will it operate the N75 to dump boost, or does it only try to protect the engine by pulling timing?

[h100vw]

No one knows if the ECU will use feedback from the MAP sensor and try to keep actual boost close to requested??

If the ECU detects knocking, will it operate the N75 to dump boost, or does it only try to protect the engine by pulling timing?

Boost spikes should be addressed by the ECU by monitoring the MAP sensor. Sometimes I have seen spikes where the ECU hasn't been able to keep boost in line, this is fixed by requesting less boost before the spike occurs, effectively preventing it.

Knock would back off the timing.

I can't see any breathing mods, except a downpipe causing you any dramas.

Gavin

[sambb]

Ok thanks for that. Its good to know that the ECU limits knocking by only retarding timing.

If I was to go down the path of using mechanical controls I'd prefer to retain the N75. I think it'd be interesting to see what effect a pressure regulator set to limit overboost would have when put in series with the N75. That way hopefully you'd retain the tunes' boost ramp and max average boost level but have the pressure regulator in place to clip initial overboosts, yet still allow the boost to tail off approaching max rpm so the K03s won't destroy itself.

sam

[Lito]

Ok thanks for that. Its good to know that the ECU limits knocking by only retarding timing.

If I was to go down the path of using mechanical controls I'd prefer to retain the N75. I think it'd be interesting to see what effect a pressure regulator set to limit overboost would have when put in series with the N75. That way hopefully you'd retain the tunes' boost ramp and max average boost level but have the pressure regulator in place to clip initial overboosts, yet still allow the boost to tail off approaching max rpm so the K03s won't destroy itself.

sam

Adding a Manuel boost controller in parallel with the n75 to limit max boost has been used to address this issue as this setup still allows the n75 to taper boost at higher rpms. I have a g2 boostvalve that I never used, you could borrow it to see if it cures your issue. I never needed to use it because Gavin's tunes are awesome

[h100vw]

Adding a Manuel boost controller in parallel with the n75 to limit max boost has been used to address this issue as this setup still allows the n75 to taper boost at higher rpms. I have a g2 boostvalve that I never used, you could borrow it to see if it cures your issue. I never needed to use it because Gavin's tunes are awesome

Thanks mate. He already has Phase 1 but was concerned that messing with the hardware might make it spikey. I personally don't think it will affect it badly unless there's a downpipe in the mix. Then even my Phase 1 wasn't too chuffed about it.

Gavin

[sambb]

Thanks for the offer Lito - I'm not sure I could use one of those G2's with the N75 though - at least not in series. As far as I could tell they have pressure relief and flow limiting so set your boost ramp and boost level all in one. Have you ever seen anyone rig one up parallel? If I go purely mechanical though then a G2 by itself might definitely be worth a shot.

I think its just going to be a matter of whacking on the dump pipe and seeing how the boost behaves afterwards. If it has big midrange lumps then i'll do something similar to what Lito said with a flow control in parallel with the N75 (see that 'shooting the overboost' story). If the problem is the initial spool up continuing too high then maybe a pressure regulator in series with the N75 may be worth a shot.

Has anyone tried what Plautos did when he used series flow control with his N75? Flow restricting the wastegate line is normally done to raise boost but it worked for him by smoothing spikes - don't doubt him for a second but can't get my head around how that is possible. anyway..

Either way I wouldn't be looking to raise boost at all and I just hope that either approach doesn't dampen the awesome spool up of the CC phase 1.

sam

[Lito]

There's a lot of people over on vortex who rig them up in parallel, that way the only thing the mbc controls is the max boost, the n75 still controls spool up and taper. I don't know why you'd wanna restrict the line going to the wastegate though, won't that just keep it closed longer and give you a bigger spike?? Though I honestly don't think you'll have any spike issues, cc tunes are pretty solid. Why not dp then stage 2

[sambb]

Ok I'll have a squiz at those G2 in parallel threads.
No I don't want to flow restrict the line in series with the N75. As you said that will give more overall boost which is not what I want - the only time I'd use a flow restriction is in parallel with the N75 which does the opposite.
The pressure regulator I mentioned (which would go in series though) doesn't work like a flow restriction - theoretically it would only put a pressure ceiling on what the N75 has to deal with and leave the flow of air relatively unrestricted ie if its overboosting to 24 psi before settling down to 1 bar but I don't want it to go further than 18psi, then setting the pressure regulator to 19psi should allow the top 4psi of overboost to be clipped but let everything else underneath that to work as the N75 intended. The pressure regulator only becomes a restriction once input pressures start trying to exceed its preset.
In all fairness I probably will end up going CC phase 2 but I'm not quite there yet. I won't be doing a front mount (IC water spraying instead) and will only be doing a dump pipe (turbo to cat) so i'll be missing a couple of the pre requisite mods for phase2. Its just that I work with pneumatics a lot and have access to a heap of parts so I'm keen to have a go since it will cost virtually nothing and has every chance of working fine.

[h100vw]

Ok I'll have a squiz at those G2 in parallel threads.
No I don't want to flow restrict the line in series with the N75. As you said that will give more overall boost which is not what I want - the only time I'd use a flow restriction is in parallel with the N75 which does the opposite.
The pressure regulator I mentioned (which would go in series though) doesn't work like a flow restriction - theoretically it would only put a pressure ceiling on what the N75 has to deal with and leave the flow of air relatively unrestricted ie if its overboosting to 24 psi before settling down to 1 bar but I don't want it to go further than 18psi, then setting the pressure regulator to 19psi should allow the top 4psi of overboost to be clipped but let everything else underneath that to work as the N75 intended. The pressure regulator only becomes a restriction once input pressures start trying to exceed its preset.
In all fairness I probably will end up going CC phase 2 but I'm not quite there yet. I won't be doing a front mount (IC water spraying instead) and will only be doing a dump pipe (turbo to cat) so i'll be missing a couple of the pre requisite mods for phase2. Its just that I work with pneumatics a lot and have access to a heap of parts so I'm keen to have a go since it will cost virtually nothing and has every chance of working fine.

You only really need the downpipe for Phase 2 and that's mainly due to the cat efficiency code that will pop up. I haven't tried it but that might not feature if you have a sensor spacer fitted. You could also back off the actuator but that would soften the boost delivery everywhere, which isn't ideal.

Gavin

Essential reading http://forums.vwvortex.com/forumdisp...T-Engine-Forum But don't believe all of it