# N75 bypass



## Hawwy (Sep 22, 2013)

So basically i want to do n75 bypass on my bam
What is correct way is it this?
http://www.seatcupra.net/forums/showthread.php?t=215866

I searched forum and getting different answers e.g some say just connect hose from dv to inlet? But it diesnt seem possible as theres a few hoses orignally coming off dv going into valve box etc
So which is right way


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## TT Tom TT (Oct 9, 2015)

Don't do it :?... This along with the N249 delete isn't good for performance. It actually gives you crap MPG and essentially deletes NA mode of your car.


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## CollecTTor (Jan 17, 2014)

TT Tom TT said:


> Don't do it :?... This along with the N249 delete isn't good for performance. It actually gives you crap MPG and essentially deletes NA mode of your car.


What? Can you explain that in more detail? I don't use the n75 or n249 and have no issues like you're describing. ..


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## TT Tom TT (Oct 9, 2015)

CollecTTor said:


> TT Tom TT said:
> 
> 
> > Don't do it :?... This along with the N249 delete isn't good for performance. It actually gives you crap MPG and essentially deletes NA mode of your car.
> ...


________________________

After few recent posts about N249 delete I decided to write this quick explanation of what N249 is for and why anybody doing "N249 delete" is simply having no idea what he/she is doing beyond anecdotal "my car is so much smoother now" and "no more leaks" you hear here and there.

N249 serves two purposes:

1) In light and medium part throttle, where intake manifold pressure is -10inhg to 0psi which is pretty much where normal DD car lives 90% of its life, the N249 enables DVs (diverter valves) and allows air to come unobstructed directly from inlets to up-pipes skipping turbos, ICs and all that piping there.

2) When coming out of heavy load/WOT pull, where there was positive boost developed by turbos, the N249 pre-opens DVs just before throttle closes to route compressed air back into inlets and letting the turbos to freely spin down.

With N249 delete you get these side effects with no benefits other than cosmetic changes to top of engine:

#1:
During normal operating conditions in -10inhg (this is dependent on what spring you have in your DVs though, 710N DVs have 7psi for example) to 0psi with non-functional N249, the DVs stay closed forcing the intake air to go through turbos, ICs, and piping AT ALL TIMES. At this point, there is not enough exhaust flow to positively spin the turbos so the intake air is actually propeling the turbos in kind of reverse fashion and experiences additional pressure drop in ICs. This kills efficiency of engine directly affecting economy. Basically what you do with N249 is you're "deleting" NA mode of operation our cars have with intact N249.

Guess why diesel engines get their stellar economy? Due to simple intake tract. Diesels have no throttles so they always take all the air they can, very little obstruction here. Their output is regulated via amount of fuel injected only.

My own car with N249 delete felt very weak in that area with actual bucking when cold started and immediately light accelerated. With N249 is a peppy happy NA like car in the light load situations.

#2:
When coming off-WOT where there is large amount of pressurized air between turbos and throttle, the car without N249 is about to experience what is technically called a turbo surge phenomenon. As throttle closes to practically nothing, the air has nowhere to go as DVs are still closed. It takes about 0.2 - 0.3s for the manifold to develop deep vaccum, evacuate the air from DVs and then for DVs to meanigfully open. That air goes the only route it can, which is back into the turbo which at this point has no forced propelling it anymore. The compressor wheel, shaft and turbine then experiences severe g forces as the assembly brakes from about 150k rpms to nothing within fraction of second.

________________________


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## Hawwy (Sep 22, 2013)

Well niky at r tech has told me it needs to be done before i go for stage 2 map


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## TT Tom TT (Oct 9, 2015)

Hawwy said:


> Well niky at r tech has told me it needs to be done before i go for stage 2 map


Weird, they say that has to be done yet they say that changing DV spring and cold-side re circulation and Forge IPCV are useless. I guess they pick and choose which theories to subscribe to.

Niki also says that you can't get more than 330-350hp out of k04 hybird. I phoned Rtech about getting a remap after my car is up to scratch and they was like yeah K04 hybrids don't get past 330hp sometimes 350 max if you have large port. That's evidently not the case.


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## Beunhaas (May 14, 2014)

CollecTTor said:


> TT Tom TT said:
> 
> 
> > Don't do it :?... This along with the N249 delete isn't good for performance. It actually gives you crap MPG and essentially deletes NA mode of your car.
> ...


Without n75 you are probably running an electronic/manual boostcontroller right? Dont think the OP is in for something like that.

The n249, yeah do what you like. I think it doesnt make that much differece running wise. Only pros are to avoid a lot of old hoses that can leak or Cleaning the enginebay if you dont run the top engine cover.


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## firediamonduk (Dec 24, 2013)

Beunhaas said:


> CollecTTor said:
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> 
> > TT Tom TT said:
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If all the N249 did was create a mess in the engine bay and use up rubber pipe then i dont think Audi would have wasted their money fitting them to every one of the 1.8t engines... It must have a purpose otherwise it wouldnt be there in the first place...


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## Beunhaas (May 14, 2014)

firediamonduk said:


> If all the N249 did was create a mess in the engine bay and use up rubber pipe then i dont think Audi would have wasted their money fitting them to every one of the 1.8t engines... It must have a purpose otherwise it wouldnt be there in the first place...


Thats a bit short.
Are you stil running the catalytic converter, PCV or SAI?  
Keep in mind there is a lot of things in the car just for I.E. environmental purposes but doesnt add value to the running caracteristics of the engine.


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## Wak (May 6, 2002)

N249 Improves DV response time by using the vacuum chamber to open the DV on throttle lift. 
N75 is standard boost control and if you go manual or electronic you remove the capability for the ECU to cut boost if it finds something going wrong elsewhere.

I wouldn't recommend removing either of these.


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## Madmax199 (Jun 14, 2015)

Beunhaas said:


> Without n75 you are probably running an electronic/manual boostcontroller right? Dont think the OP is in for something like that.


You guys need to stop making a big deal about simple things on this car. The N75 is nothing but an ECU controlled electronic boost solenoid and works just like any other boost solenoids within its design parameters. The N75 was designed to control factory boost in a tame manner, once operating beyond factory boost (remapped and tuned) it is proven to be very limited in reaction. It was also not designed with performance in mind, just a factory solenoid for factory-type boost control... so plenty of room for improvements.

The big tuners in the US have spent countless time attempting to fine tune the PID control of the N75, but it can never be as accurate as other methods due to its design limitations. Bloody thing will always overshoot or undershoot to some degree (especially at higher boost levels). Ever ask yourselves why the boost request curve is never 100% matched by the boost actual in our cars? The answer is the design limitations of the N75 valve.

As a result, there are many other more efficient ways of controlling boost, ranging from mechanical to electronic. It is nothing new or major, MBC and EBC have been in use in every turbo platform for decades, the 1.8t is no different. A good aftermarket EBC will run laps around the N75 in terms of control, safety, reaction, and PID tuning capability. Turbo boost control technology have been around and in use since turbos started being used in cars, let's stop pretending that Audi has reinvented the the wheel and that the systems in the 1.8t is some kind of high-tech stuff that shouldn't be tempered with... because honestly half of it subpar and could use some upgrading.


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## Madmax199 (Jun 14, 2015)

As for the N249 (Diverter Valve Control Solenoid). It is german overengineering at its best, and as it often happens backfires on the design intent.

Pros: 
- Allow the ECU to have a say in when the DV can operate (regardless of being in positive/negative pressure)
- Provides a failsafe method when to the ECU to step-in if needed

Cons:
- Adds complexity to something very simple
- Adds failure points with all the extra hardware (a valve, vacuum tank, hoses, clamps etc.)
- Interfere with having a clean vacuum source to the DV

DV operation is simple => lift off the throttle => engine goes into negative pressure (vacuum) => valve operates hundredths of a second later. Clean, simple, straightforward! That's how it has worked for every other turbo car for decades, and will continue to do so. A N249 works fine when it works, but due to the complexity and the multiple failure points, it can also be hazard when it doesn't.

How many of us have chased vacuum leaks in these cars? Well think about what happens when one develops in the N249 system. IMO (not that it's worth much), a single and short dedicated vacuum line from manifold to DV is better and safer. Two joints, two clamps, crisp vacuum source, easy to check and diagnose, no complications, works every time!

PS: Two things you always want to have dedicated vacuum source without disruption: The DV, and the fuel pressure regulator. These are too vital to an engine's operation to be messed with. It's a known thing with turbo cars, leave these two lines clean and alone!


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## CollecTTor (Jan 17, 2014)

Madmax199 said:


> DV operation is simple => lift off the throttle => engine goes into negative pressure (vacuum) => valve operates *hundreds of a second* later. Clean, simple, straightforward! That's how it has works of every other turbo car for decades, and will continue to do so. A N249 works fine when it works, but due to the complexity and the multiple failure points, it can also be hazard when it doesn't.


Thanks for explaining so I didn't have to. :lol: And I'm guessing you meant *hundredths* of seconds. :wink:


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## Madmax199 (Jun 14, 2015)

CollecTTor said:


> Madmax199 said:
> 
> 
> > DV operation is simple => lift off the throttle => engine goes into negative pressure (vacuum) => valve operates *hundreds of a second* later. Clean, simple, straightforward! That's how it has works of every other turbo car for decades, and will continue to do so. A N249 works fine when it works, but due to the complexity and the multiple failure points, it can also be hazard when it doesn't.
> ...


Edited for you Sir! :wink:


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## CollecTTor (Jan 17, 2014)

TT Tom TT said:


> CollecTTor said:
> 
> 
> > TT Tom TT said:
> ...


There's so much wrong here, I don't know where to begin. The N249 does not enable inlet air to use the DV to bypass the turbocharger. Even if it did, unless you had a cold side DV relocation, you wouldn't be bypassing the "turbos, IC, and piping," but literally only the compressor as the OEM DV connects the TIP post MAF to the charge piping post compressor outlet. The N249 does not allow the DV to begin opening in anticipation of the throttle closing either. The ECU isn't able to read the driver's mind. Diesel engines get their efficiency from high compression ratios, small bore to stroke ratios, and the lack of pumping losses from not having a throttle as you mentioned, not from "simple intake tracts" like you claim, as they often have IC's just like spark ignition motors. It doesn't take anywhere near 0.2 to 0.3 seconds to develop manifold vacuum when the throttle closes. In 0.2 seconds = 0.00333 minutes * 5000 revs/minute (approximate RPMS after WOT shift) = 16.667 revs or rotations. Being a 4 stroke 4 cylinder engine, that means each cylinder has completed 8.333 cycles, so the engine has completedd a total of 33 intake strokes, but only half of these created vacuum due to intake stroke happening one every two crank rotations. I guarantee that a 1.8/4= 0.45 liters completing 16.5 intake stroke is enough to create vacuum in an approximate 1L intake plenum to open the DV. Therefore it's pretty easy to see that it doesn't take 0.2 seconds to open the DV at WOT.

PS Is that more like the old me, Max? [smiley=cheers.gif]


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## Beunhaas (May 14, 2014)

Madmax199 said:


> Beunhaas said:
> 
> 
> > Without n75 you are probably running an electronic/manual boostcontroller right? Dont think the OP is in for something like that.
> ...


I do know what it is, does, how it works and that it isn't the most sophisticated system on earth BUT the OP asked if he could totally remove/bypass it or something. And that isn't that good of an idea without an MBC or EBC to control the boost. I don't want to pick a side on keep the N75 and N249 or remove it.


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## Beunhaas (May 14, 2014)

CollecTTor said:


> Madmax199 said:
> 
> 
> > DV operation is simple => lift off the throttle => engine goes into negative pressure (vacuum) => valve operates *hundreds of a second* later. Clean, simple, straightforward! That's how it has works of every other turbo car for decades, and will continue to do so. A N249 works fine when it works, but due to the complexity and the multiple failure points, it can also be hazard when it doesn't.
> ...


Now whats wrong with Hundreds? wear it all time [smiley=smoking.gif]


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## CollecTTor (Jan 17, 2014)

Beunhaas said:


> I do know what it is, does, how it works and that it isn't the most sophisticated system on earth BUT the OP asked if he could totally remove/bypass it or something. And that isn't that good of an idea without an MBC or EBC to control the boost.


Guilty as charged of not reading the provided link DIY, but yes, SOMETHING has to control the boost if the N75 isn't used. And yes, I use an EBC for it's smoother part throttle response, but an MBC run in parallel with the N75 valve is a nice option as well.


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## Beunhaas (May 14, 2014)

CollecTTor said:


> Beunhaas said:
> 
> 
> > I do know what it is, does, how it works and that it isn't the most sophisticated system on earth BUT the OP asked if he could totally remove/bypass it or something. And that isn't that good of an idea without an MBC or EBC to control the boost.
> ...


What size turbo, pressure, or flow is the N75 going to struggle? Going efr 6758 so interested to see if the N75 will stay in control of things :lol:


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## Madmax199 (Jun 14, 2015)

CollecTTor said:


> Beunhaas said:
> 
> 
> > I do know what it is, does, how it works and that it isn't the most sophisticated system on earth BUT the OP asked if he could totally remove/bypass it or something. And that isn't that good of an idea without an MBC or EBC to control the boost.
> ...


I mean, that's obviously obvious! If we have to spell everything and mention that a form of boost control is needed to replace the N75, we're in serious trouble.


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## Madmax199 (Jun 14, 2015)

Beunhaas said:


> What size turbo, pressure, or flow is the N75 going to struggle? Going efr 6758 so interested to see if the N75 will stay in control of things :lol:


Question not directed at me, but they struggle even on a stock turbo at moderate boost levels. I will not be rude and let Adam answer however!


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## CollecTTor (Jan 17, 2014)

Beunhaas said:


> CollecTTor said:
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> > Beunhaas said:
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I wouldn't know as I've never used on on anything but a K03/K04, and that was just until I could get a replacement solution in place. :lol: I'd assume flow and pressure don't really matter, especially flow. Like Max said, the N75 is no different than other EBC's except that the logic is in the ECU and unadjustable (by the end user anyways) and the valve has a slower response, and you're capped at control to 21.5 psi since that's the limit of the OEM pressure sensor that feeds back to the ECU. Greddy Type S EBC is what I run in my daily 225 with OEM K04.


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## Beunhaas (May 14, 2014)

Madmax199 said:


> Beunhaas said:
> 
> 
> > What size turbo, pressure, or flow is the N75 going to struggle? Going efr 6758 so interested to see if the N75 will stay in control of things :lol:
> ...


Nah was not only at Adam. I'm like a sponge. Gathering info everywhere :lol:

So if you have a nice addition then share it!


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## CollecTTor (Jan 17, 2014)

Beunhaas said:


> Madmax199 said:
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> > Beunhaas said:
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Exactly, not rude. Especially when Max has way more experience than me across multiple platforms.


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## Madmax199 (Jun 14, 2015)

Ok, let's have at it!

1) Design limitation
The N75 is a basic bleed valve actuated by a solenoid. Remember, this pre-milenium technology, solenoids have specs and reaction speed. Let's just say that fast-reacting modern solenoids are years ahead in what they can achieve in terms of response (and sealing property). The second major design restriction is that it's a bleed-type system.

2) Software limitation 
The TT ECU is limited in control resolution for the obvious reasons (limited range pressure sensor feedback). A standalone full configurable PID will give much more user control.


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## Hawwy (Sep 22, 2013)

All thag time i meant n249

Is this the correct way of doing it

http://www.seatcupra.net/forums/showthread.php?t=215866


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