N249 delete: Why you should not do it.

[julex]

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.

To illustrate this point, here is a graph showing the boost spike at throttle boot MAP sensor (this would NOT be visible on normal boost gauge hooked up to intake manifold). It shows baseline 32-33psi run then a foot off the pedal with associated instant boost spike to 40psi which takes a good moment to fizzle out as it goes back into the turbo and then the DVs open sometime in the middle of boost fall line. Coincidentally, these boost spikes were popping off my TBB pretty reliably so they were pretty severe.

One sample = 0.1s.

[GramCracker]

Very informative post, thank you!

[Timtheguru]

Thanks!

Also, when the device fails it is then auto deleting itself.

[ilyafil]

Good info, I was wandering about that myself. Why do people delete that valve anyway?

[Delmed83]

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.

To illustrate this point, here is a graph showing the boost spike at throttle boot MAP sensor (this would NOT be visible on normal boost gauge hooked up to intake manifold). It shows baseline 32-33psi run then a foot off the pedal with associated instant boost spike to 40psi which takes a good moment to fizzle out as it goes back into the turbo and then the DVs open sometime in the middle of boost fall line. Coincidentally, these boost spikes were popping off my TBB pretty reliably so they were pretty severe.

One sample = 0.1s.

Thanks!

Also, when the device fails it is then auto deleting itself.

Thanks guys, I finally fully understand exactly what that valve does. It should have been obvious due to the fact that I always wondered how the DV's would open so quickly directly after boosting.

[aysix]

Who wants to sell me the check valve for the N249?

[Timtheguru]

This thing? I've actually never seen it in the car. http://www.ecstuning.com/Audi-B5_S4-...alve/ES292826/



Edit:

I have seen this one though.

[waB5S4]

^ Where the hell is that first one from? It looks just like the 4-way (you posted below) but with two extra posts...

[Timtheguru]

I've got no clue! I've never seen it, I'm guessing it's around the vacuum reservoir if anything. I wonder if ECS's website description is wrong. Maybe someone else can enlighten us.

[rnagy86]

Good info, I was wandering about that myself. Why do people delete that valve anyway?

Because somone came up with the bright idea of deleting it and said it was better. The other half had a broken one and was trying to save a couple of bucks instead of fixing the car for real.

You can always visit the following site and use google translate to get all of the needed part numbers to put this back:
http://vin-online.ru/etka.php?vag=AU...=224&gr=13385#

[Delmed83]

That first one is probably for around the vac reservoir area. It's like a manifold / check valve.

[Delmed83]

Who wants to sell me the check valve for the N249?

All you need is a one way check valve and then two T-pieces in order to make your own. At my house, I have 2 small black in-line check valves that I bought by mistake. I also have tons of T-pieces. Come by and grab them if you want. You can rig some shit.


EDIT:

I have 2 of these but when they came from europaparts they were all black:

http://www.europaparts.com/in-line-c...58905291k.html

I really needed a green check valve at the time......and europaparts didn't have it on their site. They have them now!

[GramCracker]

Because somone came up with the bright idea of deleting it and said it was better. The other half had a broken one and was trying to save a couple of bucks instead of fixing the car for real.

You can always visit the following site and use google translate to get all of the needed part numbers to put this back:
http://vin-online.ru/etka.php?vag=AU...=224&gr=13385#

Where is that thread? I'd like to see who did that so I can know who to never take car advice from in the future.

[AudiSportB5S4]

Great information... Piggie told me to never take it out a few years ago back when it seemed many people were ripping them out for no good reason. He told me basically what you wrote down. Good stuff

How does that bypass part work you mention in the first benefit? Is it off the throttle plate angle that would then send it down through the inlets to the turbos and ICs etc.. Say you're cruising at -10-0 and then go full throttle.. Is that first bit of air not going through the turbos and then when the throttle plate reaches a threshold it goes down to the hairdriers?

[Monty23]

Well my BOV doesn't vent back to my intake so I definitely don't want my BOV open during "N/A mode" lol....

[julex]

Great information... Piggie told me to never take it out a few years ago back when it seemed many people were ripping them out for no good reason. He told me basically what you wrote down. Good stuff

How does that bypass part work you mention in the first benefit? Is it off the throttle plate angle that would then send it down through the inlets to the turbos and ICs etc.. Say you're cruising at -10-0 and then go full throttle.. Is that first bit of air not going through the turbos and then when the throttle plate reaches a threshold it goes down to the hairdriers?

First part has nothing to do with WOT. The two benefits are completely separated in purpose and not tied to each other in any way except for using the same device to accomplish two different things. First is to shorten the intake path for non-boost operating range of engine, second is to send charge into infinite loop until the turbos stop boosting and air gets naturally ingested by engine and/or blown in reverse through maf - depending how quick the turbos de-spool due to their inertia.

1inhg is about 0.5psi of pressure. 29.81inhg is absolute vacuum (or something close to it) and 14.5 is atmospheric pressure, 29.81/14.5 = ~ 2inhg per 1 psi.

710N DVs have about about 7psi spring. Realistically you'd have to be below -14inhg (so my original number is a bit off, I said -10inhg just for illustration purposes) for the DVs to open unassisted as they do when you idle. Once you cross the threshold, above -14inhg they stay closed forever until the manifold pressure drops below -14inhg again. That's without N249.

With N249, the ECU routes vacuum reservoir vacuum into DVs for the -14inhg to about 0psi threshold (it is really based on load instead of pressure I believe). This is to open the direct path from Y-pipe to throttle, omitting turbos. Turbos are not needed until you want positive pressure which is above 0psi.

I have graph somewhere showing N249 activity in relationship to load/boost and that's the conclusion I came to. It showed it pretty much active up until around 70-75 unscaled load.

[julex]

Where is that thread? I'd like to see who did that so I can know who to never take car advice from in the future.

There were quite a few of them, deleting N249 seemed like a FAW thing to do for last couple of years. You know, along with EGT sensors (let it melt!) and N80 (let my tank blow up and/or garage stink of gas!) "mods".

[topquarkpc]

so is deleting N249 same as leaving the N249 plugged in but removing pneumatic hoses that it attach to?

i was given the instruction to leave the N249 plugged into the electrical connector but remove the hoses...by 034...

[jibberjive]

It would be interesting to see some controlled tests done on fuel economy for someone who is going from deleted N249 to re-instated N249. Also, if the N249 part throttle action is based on load, looks like it might be something to look into when tuning for larger turbos, as they obviously have different transition points from vacuum to positive pressure. Off to compare the N249 tables from the RS4 to S4 file!

[phila_dot]

Load is not a factor.

I don't remember 100% exact details, but a basic rundown from memory:

The dynamic path is based on a predicted spike in boost pressure from step changes in target versus actual with a negative gradient pedal.

The stationary path is based on a target boost PR threshold at different RPM points and it's disabled by hysteresis that's basically target boost pressure greater than atmospheric pressure.

[somedude]

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.

This is false information If you wish to test this at your house remove your throttle body boot and feel the massive amount of air the turbos are pumping even at idle. You will have to unplug your MAF or your car will not idle because it will read way more air then the engine is actually consuming and run so rich it will stall. Yet another reason your statement is false. I do understand the theory but it practice the minute amount of energy savings is irrelevant.

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.

This is misleading information information Diesel not having a throttle body is not THE reason they have better economy it is however a fact of their design that they don't need one. I am not going to elaborate on this because if you believe this for even a second you need to educate yourself on basic diesel operation.

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.

Then there is another issue with your car if it stumbles with the N249 removed. My guess would be a vacuum line was not plugged by mistake when doing the mod.

#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.

To illustrate this point, here is a graph showing the boost spike at throttle boot MAP sensor (this would NOT be visible on normal boost gauge hooked up to intake manifold). It shows baseline 32-33psi run then a foot off the pedal with associated instant boost spike to 40psi which takes a good moment to fizzle out as it goes back into the turbo and then the DVs open sometime in the middle of boost fall line. Coincidentally, these boost spikes were popping off my TBB pretty reliably so they were pretty severe.

The theory here is all true but I have a feeling that you may not have correctly supplied manifold vacuum to your diverters if you were popping your TBB over and over or the oil was not cleaned from the surface properly on reinstall . Turbos are designed to handle a spike briefly when the throttle is shut. This is part of their normal operation. If you are hearing compressor surge then you have a issue. You would hear this if the diverters are not opening or if manifold vacuum is not being applied to them when throttle is closed.

I did the bypass on my car as part of the diagnostic process and I never noticed ANY change in driveablilty at all and I am really picky with things like this. Mods like this are fun to do though. It gives you something to tinker with to learn the different systems in your car and how they operate, and it is free to play with.

[somedude]

so is deleting N249 same as leaving the N249 plugged in but removing pneumatic hoses that it attach to?

i was given the instruction to leave the N249 plugged into the electrical connector but remove the hoses...by 034...

leave the connector plugged in. The mod is a bypass. The computer is still actuating the valve even though it cannot control anything

[topquarkpc]

leave the connector plugged in. The mod is a bypass. The computer is still actuating the valve even though it cannot control anything

yes...i did...but...i am wondering if N249, electrically connected, without controlling anything is working against my car properly implementing the intended function of the N249... hope i make sense... can someone chime in please...???

[waB5S4]

yes...i did...but...i am wondering if N249, electrically connected, without controlling anything is working against my car properly implementing the intended function of the N249... hope i make sense... can someone chime in please...???

N249 capped off but still plugged in is THE N249 delete that this thread is referring to and the same one done by many of us. Some tuners also offer the ability to tune it out, which I always assumed meant I could actually UNPLUG the N249 and N80, of course I don't have any actual experience with these options.

So this thread DOES apply to you (and me for that matter). I'll be putting my N249 back in place but I've never had any discernible issues with the bypass/delete in place.

[julex]

To clarify:

Either unplugging the N249 connector with N249 still in place OR by-passing the N249 altogether with it still plugged in are "N249 delete" with the same net outcome - ECU cannot control DVs correctly.

[A62TURBO]

Is deleting the N80 that bad too?

[julex]

Is deleting the N80 that bad too?

N80 is strictly for evacuating fumes out of your gas tank and injecting them into manifold. Without that, your roll over blow off valve in on the tank is venting to atmosphere... On my car, the result of N80 delete resulted in raw fuel smell in my garage all the time...

[topquarkpc]

thanks for the clarification...

so...per somedude's response, the claim OP made is not so valid...??? Julex...not to bash on you but i'm just reading in between the lines here...
i want to make sure the claim is substantial enough for me to justify restoring to original / intended config...

[A62TURBO]

N80 is strictly for evacuating fumes out of your gas tank and injecting them into manifold. Without that, your roll over blow off valve in on the tank is venting to atmosphere... On my car, the result of N80 delete resulted in raw fuel smell in my garage all the time...

Gotcha thanks!

[catbed]

just wanted to chime in here, replaced my N249 in my BT 1.8t after deleting it early on, and noticed less compressor surge sound when coming off throttle. I definitely agree with what julex posted, and anybody who deletes the N249 doesn't fully understand how and why the system works.

thanks for the clarification...

so...per somedude's response, the claim OP made is not so valid...??? Julex...not to bash on you but i'm just reading in between the lines here...
i want to make sure the claim is substantial enough for me to justify restoring to original / intended config...

I'd be more inclined to listen to Julex here. He presents a concrete explanation of how the system works, backed by multiple people AND logs to prove it.

Plus, Bosch/VAG placed the system on every turbo car made from 96-05ish. Isn't that reason enough? The newer cars don't have an N249 because they were replaced by electronically actuated DVs.

[topquarkpc]

just wanted to chime in here, replaced my N249 in my 1.8t after deleting it early on, and noticed less compressor surge sound when coming off throttle. I definitely agree with what julex posted, and anybody who deletes the N249 doesn't fully understand how and why the system works.

well...i don't want to take credit / sound like mr. flyboys4 but your claim sounds more anecdotal rather than basing it on a sample data of some sort...
i hear a lots of things when i am driving but i know nothing...hence i'm asking...

[mrpeterparker]

N80 is strictly for evacuating fumes out of your gas tank and injecting them into manifold. Without that, your roll over blow off valve in on the tank is venting to atmosphere... On my car, the result of N80 delete resulted in raw fuel smell in my garage all the time...

so this might not explain the smell of fuel in cabin sometimes <not all the time> when accel. (and I have checked the fuel line itself into the rail that fails a lot, it's not that)

[waB5S4]

well...i don't want to take credit / sound like mr. flyboys4 but your claim sounds more anecdotal rather than basing it on a sample data of some sort...
i hear a lots of things when i am driving but i know nothing...hence i'm asking...

Julex provided empirical evidence of compressor surge by way of logs. Others are confirming the evidence produced by julex. FWIW, the original N249 delete offered no empirical evidence. We all did it based on anecdotal evidence in the first place so... yea.

so this might not explain the smell of fuel in cabin sometimes <not all the time> when accel. (and I have checked the fuel line itself into the rail that fails a lot, it's not that)

Did you check fuel line by pressure or visually? Still have cats or have a wideband? Intermittent fuel smell in the cabin smells like my 3" dps and/or running a little rich.

[Doug@FrankenTurbo]

I'd like to add my enthusiastic support for this thread's topic. For years I've felt that the "N249 delete" was a disservice to the design intent of these cars. Not every design refinement from the factory runs at cross-purposes to big powah. Keep your N249 -- with its vacuum tank -- and you'll enjoy driving the car more. That's my opinion, anyway.

[Weehe]

Just a though, but is the N249 really needed if you set it up like this: Have a line going from the charcoal canister, with a check valve (probably don't even need that), to the TIP, similar to OEM set up. Then have the vacuum reservoir T-ed into the DV line that goes from the DV to the manifold. In my mind, this would essentially do the same thing as having the N249 and all the lines going to it. You have the extra vacuum to help open the DV when you come off boost and utilize the gas vapors venting out of the gas tank.

I could be missing something here, but seems simple to me.

[catbed]

Just a though, but is the N249 really needed if you set it up like this: Have a line going from the charcoal canister, with a check valve (probably don't even need that), to the TIP, similar to OEM set up. Then have the vacuum reservoir T-ed into the DV line that goes from the DV to the manifold. In my mind, this would essentially do the same thing as having the N249 and all the lines going to it. You have the extra vacuum to help open the DV when you come off boost and utilize the gas vapors venting out of the gas tank.

I could be missing something here, but seems simple to me.

N249 has nothing to do with the charcoal canister? Do you mean N80? You need something to regulate vacuum in the tank/canister, it can't be straight off the manifold.

Regarding your DV set up, that would not work at all. There needs to be a solenoid and check valve inline to keep the vacuum in the tank under boost.

[rtl5009]

I'm reinstalling mine tomorrow. Will report back about fuel economy.

It's easy for me to pick up on it, I track my e85 consumption on the reg.

[Weehe]

OK, for some reason I thought the canister was attached to the the N249, I clearly see it is not. Might have thought that way since most people delete both, if they are deleting stuff. And yes you are right, you would need a check valve inline to the vacuum reservoir, meant to say that. So if there was a check valve in the line going from reservoir to T-ed DV line, what would the problem be there?

I might be getting stuff mixed up with 1.8t's

[catbed]

OK, for some reason I thought the canister was attached to the the N249, I clearly see it is not. Might have thought that way since most people delete both, if they are deleting stuff. And yes you are right, you would need a check valve inline to the vacuum reservoir, meant to say that. So if there was a check valve in the line going from reservoir to T-ed DV line, what would the problem be there?

I might be getting stuff mixed up with 1.8t's

The check valve will only let air out of the reservoir, creating vacuum. There will be nothing actuating the DV. Stock set up is the only way that works.

The N249 functions the same regardless of platform.

[Matt Danger]

hmm. this is probably why I'm getting compressor surge.

I never noticed it with the k04s but I do with the 770s.

Why do other turbo cars that don't have an N249 not experience compressor surge?