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View Full Version : N249 bypass? Pros and cons?



Davdraco1
07-13-2014, 03:53 PM
Hey all. I've been reading up on this sensor/valve and what it does and how it does it. I've been having some issues and I'm thinking about doing the bypass mod and see if it works for my car. I just bought a 710N DV and will be installing it soon. May as well do the mod at the same time. My only thing is what is the spring in the 710N holding point(psi)? Does that matter or am I just over thinking it. Looking for some more info so I an understand how all these parts work and what the mods actually do. Thanks!!!!

Corrado_Guy
07-13-2014, 09:09 PM
http://www.audizine.com/forum/showthread.php/554859-N249-delete-Why-you-should-not-do-it

Davdraco1
07-13-2014, 09:32 PM
So I guess it's staying plugged in..... I read so much about deleting it and it seemed like a great mod..... Huh...

Corrado_Guy
07-13-2014, 09:49 PM
Sad thing is mine is bypassed and I have never read that thread. A buddy sent it and I haven't taken the time to read it.

Davdraco1
07-14-2014, 05:25 AM
Do you like yours being bypassed? My car will hit boost fine but then all of a sudden, it sounds like the WG opens up, closes, then starts to boot again. It's done it up to 3 times when I'm just flooring it to get up to speed. I was reading people reviews of doing the bypass and I think it will solve it

walky_talky20
07-14-2014, 05:59 AM
I think you should keep it in place *unless* you have a Forge splitter valve, or some other type of dump valve that vents the charge pipe to atmosphere when vacuum is pulled on it.

The issue: The N249 will activate and engage during pre-spool/off boost. This is fine if you have a regular diverter valve, but if you have a splitter valve, it creates a gaping hole in your charge pipe at part throttle (off boost). This throws your MAF for a loop, and fuel trims go wacky because you have a leak that creates itself, then fixes itself randomly. In my experience, this can affect fuel trims by 5% or more (and make the car run like poop).

Ideally one would never use a splitter valve anyway, but if you must, then results are better if you bypass the n249.

And for the readership: This entire discussion only applies to 2000+ 1.8T and 2.7T B5's (for North America). V6 and AEB cars need not apply.

Davdraco1
07-14-2014, 06:19 AM
I jut have the 710N that I will be installing

Davdraco1
07-14-2014, 06:20 AM
So were all the people raving over doing this bypass mod?

UCFQuattroguy
07-14-2014, 12:17 PM
From what I gather..the "mod" is nothing more than backdating to the way the early 1.8t plumbs in the Diverter Valve (i.e. Off direct manifold pressure/vacuum).

Being that my B5 is nothing more than a track/autocross monkey...I'm considering his for no other reason than to reduce the amount of vacuum hoses I have to maintain. ;-p

Davdraco1
07-14-2014, 02:44 PM
I've read that the 710N DV has a 7psi spring in it, if it were plugged right into the mani, wouldn't it open as soon as you hit that 7 psi and you would never boost past that?

fed0ra
07-14-2014, 03:14 PM
I've read that the 710N DV has a 7psi spring in it, if it were plugged right into the mani, wouldn't it open as soon as you hit that 7 psi and you would never boost past that?

You're thinking about this like a wastegate, where a wastegate hooked up directly to the turbo/intake would open at spring pressure. Here is a quote from an evolutionm post (http://forums.evolutionm.net/evo-engine-turbo-drivetrain/209640-diverter-valve-tech.html):

"During normal operation under idle conditions there is a vacuum applied to the top of the stock diverter valve, and the valve is partially open. Also during idle conditions the upper IC pipe has very slightly positive pressure in it (much less than 1 psi of pressure). If you take the hose that connects the stock diverter valve to the intake off you will feel a small amount of air coming out of the hose. This is perfectly normal.

Under heavy load conditions there is boost applied to the top of the diaphragm in the stock diverter valve helping to push it closed along with the pressure from the pre-load spring. When the throttle plate shuts you now have a vacuum on the top of the diaphragm and this pulls the valve open along with the boost pressure in the upper IC pipe pushing the valve open."

Basically, the more force the spring inside of the DV exerts, the easier it is to hold boost, but it isn't limited by the spring's rate. So a 7lb spring wouldn't stop holding at 7psi, it would just have a harder time at say 20+psi where a heavier spring/larger diaphragmed unit would have no problem.

Davdraco1
07-14-2014, 04:17 PM
Ahhhh, okay. Makes much more sence. I was thinking literally.... So would I benifit any by doing this mod? Everywhere I read people say mid range is much smoother, no hesitation anymore....

Corrado_Guy
07-14-2014, 05:16 PM
This is mine with Maestro on an AWP tune and no N249 but I am pretty sure I had the N75 hooked up at the time, You can see I get a small amount of drop-off as the engine revs across the range and then a spike when I shift. I also have a MBC which was set to about 10 - 12 PSI at the time on a GT28RS. I have no problems with trims although it takes a long time for them to increase so it is running pretty nicely. The motor is out right now to do the rods so I may connect the N249 back up and see how it runs with it and maybe disconnect it and see the difference.

http://members.shaw.ca/SyncroJetta1/Audi-Jan_4_Run.jpg

Davdraco1
07-14-2014, 05:18 PM
I'm sorry but I'm not sure if that's good or bad to have that small spike when you shift

SquishyPanda
07-14-2014, 08:21 PM
I'm sorry but I'm not sure if that's good or bad to have that small spike when you shift

The spike is pressurized air trapped between the throttle plate and the compressor wheel. It can't go through the throttle so a shockwave sort of bounces back toward the compressor wheel and acts as a brake. I'm not sure how much damage this does to the turbo, but it also means a little more turbo lag when you get back on the gas because you need to spin the compressor back up. This happens at roughly the speed of sound so the shockwave hits the compressor before the DV opens unless you have an N249 to open the DV immediately when the throttle closes, so the DV is open when the pressure wave comes (which I suspect is why Audi puts it near the turbo and not near the throttle body).

Davdraco1
07-14-2014, 08:28 PM
It sounds like a horrible idea after learning all this, but why are people raving over this mod?

MetalMan
07-15-2014, 08:11 AM
It sounds like a horrible idea after learning all this, but why are people raving over this mod?

Who are these people?
UFCQuattroguy explained it pretty well, it's mostly a reduction in the number of vacuum hoses to deal with, and its also the elimination of another electrically-controlled valve (which like anything has the capacity to fail).

fed0ra
07-15-2014, 08:38 AM
It sounds like a horrible idea after learning all this, but why are people raving over this mod?


Who are these people?
UFCQuattroguy explained it pretty well, it's mostly a reduction in the number of vacuum hoses to deal with, and its also the elimination of another electrically-controlled valve (which like anything has the capacity to fail).

I performed the mod after reading a thread about a year ago that claimed the N249 could cause part throttle issues. It wasn't the source of my issues (which was a combination of many problems) but the biggest factor for me at the time (and the reason I never plan on reinstalling it) is the simplification of my vacuum system, like MetalMan said. Its one less solenoid to worry about and allows me to remove a nest of vacuum hoses when performed alongside the SAI/SJP/constant vacuum system deletes. I like having fewer hoses to tear or check when I have an air leak; fewer points of failure with a negligible/nonexistent change in drivability/performance is enough reason for me to recommend it.

Some people complain about hits to part throttle, response, etc. but I didn't notice any change (positive or negative). The only thing I noticed was that my engine bay was a little cleaner.

Davdraco1
07-15-2014, 02:59 PM
Huh. My part throttle is fine. It when I'm WOT or close to it that it sounds like my WG is opening and power disappears for a second then comes back.... Idk what it could be.

fed0ra
07-15-2014, 03:06 PM
Huh. My part throttle is fine. It when I'm WOT or close to it that it sounds like my WG is opening and power disappears for a second then comes back.... Idk what it could be.

Have you performed a boost leak test and fixed all of the leaks? Have you logged anything in VAG-COM?

Davdraco1
07-15-2014, 03:27 PM
Boost leak yes, fixed a leak, didn't do anything. I will need to pressurize the system again and re check. Vagcom no. Don't know how to do that

Corrado_Guy
07-15-2014, 06:46 PM
My car still has all of the solenoids in place but the outputs are capped off. When I looked at cleaning things up I would only get rid of the 'F' hose going to the solenoid for the air pump solenoid and the N249. I don't have an air pump because I don't have a cat and I left all of that stuff in place so you don't clean up much. When I checked my PCV system I replaced all of the check valves and any hoses that looked questionable. Once I get the car back together I will connect it back up and then log again with the same boost setting on the MBC and see if there are any differences.

melomandn
07-16-2014, 10:49 AM
So I bypassed mine awhile ago thinking it was no big deal and it would make life easier.

After reading this I'm putting it back on but trying to figure out what goes where, looking at walkys diagram I can easily tell where the green and red lines go to but I'm confused as to where I should connect the pink line since I deleted all the check valves/suction jet pump and all that crap.



http://i19.photobucket.com/albums/b154/walky_talky20/DV-stock.png

MetalMan
07-16-2014, 11:24 AM
So I bypassed mine awhile ago thinking it was no big deal and it would make life easier.

After reading this I'm putting it back on but trying to figure out what goes where, looking at walkys diagram I can easily tell where the green and red lines go to but I'm confused as to where I should connect the pink line since I deleted all the check valves/suction jet pump and all that crap.



http://i19.photobucket.com/albums/b154/walky_talky20/DV-stock.png

I did EXACTLY this. My car has no SAI or SJP, but has PCV, EVAP, brake booster, and now N249 and vacuum reservoir.
Oh yeah, you need to reincorporate the vacuum reservoir to install the N249, which I'm guessing you deleted. This means another check valve.
Here's a diagram I came up with for my setup, basically stock except deleted SAI/N112 and SJP:
http://img.photobucket.com/albums/v132/4x4pede/Audi%20A4/AWM_vacuum_simplify_N249_zps9a92a4b0.png

walky_talky20
07-16-2014, 01:01 PM
^I agree with above method for re-incorporating the N249. My only note would be that I probably wouldn't tie into the PCV line for a vacuum source. That one has lots of oily crap going through it. I'd tie into a "fresher" vacuum source if possible. But either will work.

MetalMan
07-16-2014, 01:05 PM
^I agree with above method for re-incorporating the N249. My only note would be that I probably wouldn't tie into the PCV line for a vacuum source. That one has lots of oily crap going through it. I'd tie into a "fresher" vacuum source if possible. But either will work.

Had the same thought myself. Could use the brake booster line or add yet another tee to the EVAP/leak detection pump line. Will check it out soon and see how much crap has accumulated due to PCV.

Using the brake booster line the way I have it configured would require a tee and a reducer to fit the line going to vacuum reservoir (about 3/8" to 1/4"). Normally the stock "E" hose (for lack of a better name) performs this reduction in diameter.
Arguably my configuration also requires a reducer (about 1/4" to 1/8") to fit the N249's nipple but I cannot recall exactly what I did there.

rodgertherabit
07-17-2014, 06:34 AM
Does aftermarket software (and BT software) keep this function/feature active? or just stock/factoryish software?

MetalMan
07-17-2014, 07:12 AM
Considering how many options there are for tunes, nobody could say.
My guess: since it's a stock feature, and there's a chance customers will have stock features left in their BT engine bays, most tunes will keep it unless the customer specifies otherwise.

melomandn
07-17-2014, 07:18 AM
^I agree with above method for re-incorporating the N249. My only note would be that I probably wouldn't tie into the PCV line for a vacuum source. That one has lots of oily crap going through it. I'd tie into a "fresher" vacuum source if possible. But either will work.

This is starting to seem much more difficult than I expected.

What other vacuum source could I tap into? My pcv system consists of 2 hoses ran to a catch can so that options out for me.

fed0ra
07-17-2014, 07:19 AM
Does aftermarket software (and BT software) keep this function/feature active? or just stock/factoryish software?

I would see no reason for tuners to remove it. If you unhook the hoses then the N249 just does nothing and shouldn't have any bearing on performance, there isn't even a code. It would be more work for them to remove it than to just have people run the DV off of the manifold.

rodgertherabit
07-17-2014, 08:22 AM
Well mainly the concern would be for larger than K frame turbos... They have completely different spool characteristics. Its probably all the same for most, but if correlates with spool then I feel it might need to be adjusted to maximize the system performance as the Engineers did with the Stock set up.

MetalMan
07-17-2014, 08:29 AM
This is starting to seem much more difficult than I expected.

What other vacuum source could I tap into? My pcv system consists of 2 hoses ran to a catch can so that options out for me.

PCV is irrelevant to reincorporating the N249. I happened to tee off of my PCV's vacuum line, and that is the only relevance.

You just need a line from the intake manifold, a check valve on that line, and a line from the check valve to a tee. One end of the tee would have a line going to the vacuum reservoir, and the other end of the tee goes to the N249.
Did you delete the vacuum reservoir from the fender well?

SquishyPanda
07-17-2014, 08:33 AM
I don't think spool characteristics would play too much of a part on the N249's functioning since it takes its cues from throttle input, not RPM (in addition to manifold pressure). Unless there is also a pressure sensor in the charge plumbing, somewhere between the compressor and TB that the ECU also uses to determine whether to signal the N249 to open the DV.

rodgertherabit
07-18-2014, 05:00 AM
Unless there is also a pressure sensor in the charge plumbing, somewhere between the compressor and TB that the ECU also uses to determine whether to signal the N249 to open the DV.

There is...Called a "MAP" Sensor. Came on the later 1.8ts

MetalMan
07-25-2014, 12:57 PM
^I agree with above method for re-incorporating the N249. My only note would be that I probably wouldn't tie into the PCV line for a vacuum source. That one has lots of oily crap going through it. I'd tie into a "fresher" vacuum source if possible. But either will work.


Had the same thought myself. Could use the brake booster line or add yet another tee to the EVAP/leak detection pump line. Will check it out soon and see how much crap has accumulated due to PCV.


As promised I checked the tee to the PCV line... there was some oil making its way toward the semi-hard line (plastic one) for the vacuum reservoir, but no oil had reached that semi-hard plastic line yet.
So I rerouted the vacuum reservoir/N249 line away from the PCV system, and now the PCV has its own dedicated nipple on the intake manifold.
This is how my vacuum lines are routed now: (compare to a prior post in this thread to see the differences)

http://img.photobucket.com/albums/v132/4x4pede/Audi%20A4/AWM_vacuum_simplify_N249_2_zps9ece09d5.png

B5nDisciple
03-19-2016, 03:44 PM
Can this be added to the FAQ/DIY? The original PCV simplification thread is missing some of this newer info.

And this is great and should be easy to find.

Sent from my VS990 using Tapatalk

GrapeBandit
03-19-2016, 04:06 PM
N249, whats that?

walky_talky20
03-19-2016, 08:27 PM
Can this be added to the FAQ/DIY? The original PCV simplification thread is missing some of this newer info.

And this is great and should be easy to find.



If the "original PCV simplification thread" you are talking about is my DIY for "B5 1.8T Vacuum/Check Valve/SAI/PCV - Delete & Simplification", then your wish is hereby granted. A link to this thread has been added to the post regarding the N249 delete.

Davdraco1
03-19-2016, 08:59 PM
I feel so dumb reading over this and realizing how little I knew about engines and then knowing what I know now. Crazy what a few years does for ya.

B5nDisciple
03-19-2016, 09:02 PM
If the "original PCV simplification thread" you are talking about is my DIY for "B5 1.8T Vacuum/Check Valve/SAI/PCV - Delete & Simplification", then your wish is hereby granted. A link to this thread has been added to the post regarding the N249 delete.
Thanks Walky, makes your awesome DIY better.

Sent from my VS990 using Tapatalk

GrapeBandit
03-20-2016, 06:49 AM
I feel so dumb reading over this and realizing how little I knew about engines and then knowing what I know now. Crazy what a few years does for ya.
how about them days you wanted to rally jump your audi[evilsmile]

good thing for the forums, a lot of useful information and DIY's here. [az]

GrapeBandit
03-20-2016, 07:02 AM
N249 Delete

Application Notes:
Only 2000+ has the N249 valve.

A little discussion:
On 2000+ 1.8T's,the diverter valve is not directly connected to the intake manifold like it was on the earlier 1.8T's. It is connected through a solenoid called the N249, which is able to connect it to the manifold (in it's passive state), or connect it to a constant vacuum source (in it's active state). Thus, the ECU is able to open the DV in situations where it would otherwise be closed. It is somewhat controversial to modify this system as solenoid control of the DV does have its advantages. It is able to help with pre-spooling the turbo (opening the DV at part throttle when boost request is low), and opening the DV sooner upon throttle lift. Many owners who have "deleted" the n249 have noted more "crisp" boost and more predictable DV response. In any case, the 97-99 cars did just fine without it, so we can delete it too.

**Here is a thread which discusses the pros and cons of deleting the N249: http://www.audizine.com/forum/showthread.php/605776-N249-bypass-Pros-and-cons
so. . .[confused]conflicting statement[confused]
not that it matters to me, but I know it will for others.

Davdraco1
03-20-2016, 07:16 AM
how about them days you wanted to rally jump your audi[evilsmile]

good thing for the forums, a lot of useful information and DIY's here. [az]

Lol. I forgot about that too. Damn kids....

walky_talky20
03-20-2016, 02:56 PM
so. . .[confused]conflicting statement[confused]
not that it matters to me, but I know it will for others.

I'm not sure what is conflicting there.

- It is true that there are benefits to what the N249 does.
- It is also true that the 97-99 cars did not have an N249. Those cars still worked fine.
- In some cases the benefits of the N249 no longer apply and it actually is a disadvantage and makes the car run worse.**


**For example if you install a Forge Splitter valve (or other vent-to-atmosphere type diverter valve), when the N249 activates for pre-spool (or for compressor bypass during light cruise), you get a gaping hole in your charge pipe. This causes moderate to major problems with fuel mixture. Air metering doesn't work properly in that scenario and the N249 is to blame. Deleting the N249 fixes that issue. Vent to atmosphere then only occurs momentarily when fuel injectors are off (or almost off).

It can also affect the sound of the blow-off/recirculation. By having the valve open slightly later, the boost spikes slightly and makes for higher pressure when the valve cracks. This can result in a different sound and may be desirable for some owners.