N249 delete: Why you should not do it.

[jibberjive]

So i think these spikes are just because of the map sensor location. It happens on all turbo cars but this is a weird car with the map sensor before the throttle body.

If the MAP sensor was post throttle body, you wouldn't ever 'see' overrun compressor surge via the MAP sensor.

By definition, that spike is what the N249 is designed to mitigate.

[Meow]

If the MAP sensor was post throttle body, you wouldn't ever 'see' overrun compressor surge via the MAP sensor.

By definition, that spike is what the N249 is designed to mitigate.

And I don't think that spike is really that abnormal. I think if you had a sensor on most drive my cable cars you would see the same spike, its not very large and its not enough to cause compressor surge. If the engine isn't firing, nothing is driving the turbo, and all you are seeing is the inertia from the turbo creating more boost against a severly restricted flow path once the throttle has closed. The turbo is slowing down rapidly from the second the throttle is lifted. It would be quite difficult to determine what airflow is actually occurring here as the flow area is severely restricted. The turbo likely will not surge as Rpm's are falling extremely fast and you wouldn't have high shaft speed reverse direction type stuff which really is an issue.

[julex]

you wouldn't have high shaft speed reverse direction type stuff which really is an issue.

I could hear compressor surge when my car was without N249 for a short while. It was a very distinct sound lasting about 1s when blades were chopping through stalled air (due to no flow through either throttle or DVs) until it slowed down enough to stop surging.

[Meow]

I could hear compressor surge when my car was without N249 for a short while. It was a very distinct sound lasting about 1s when blades were chopping through stalled air (due to no flow through either throttle or DVs) until it slowed down enough to stop surging.

Your spike is 8psi and lasts .2 seconds, mine is only 4psi and lasts .2 seconds as well. I do not hear any surge noise at all. I do not think my turbos are surging, however yours might have been. Got any video of the noise?

What diverter valves are you using? I use 710n valves.

[jibberjive]

I think if you had a sensor on most drive my cable cars you would see the same spike,

I agree, most drive-by-cable cars would likely show the same spike, as they don't have an N249 valve and vacuum canister. The N249's sole purpose is to mitigate this occurance.

If the engine isn't firing, nothing is driving the turbo, and all you are seeing is the inertia from the turbo creating more boost against a severly restricted flow path once the throttle has closed. The turbo is slowing down rapidly from the second the throttle is lifted.

You pretty much defined compressor surge/stall. Independent of what causes the pressure in the intake or the lack of pressure in the exhaust, compressor stall/surge happens by a mismatch of intake pressure being too much and exhaust pressure being too little. One symptom is a momentary spike in boost pressure.

If you're looking for someone to justify taking out the N249, I doubt you're going to find it here. If you're not hearing any surge and aren't too worried about the effect it might cumulatively have on your turbos, just keep it off.

[Meow]

You pretty much defined compressor surge/stall. Independent of what causes the pressure in the intake or the lack of pressure in the exhaust, compressor stall/surge happens by a mismatch of intake pressure being too much and exhaust pressure being too little. One symptom is a momentary spike in boost pressure.

If you're looking for someone to justify taking out the N249, I doubt you're going to find it here. If you're not hearing any surge and aren't too worried about the effect it might cumulatively have on your turbos, just keep it off.

There is a difference between a small pressure spike right at the throttle body (where the map sensor on our cars is) and full on surge. How much of a spike is too much? Too hard to answer that right now without a lot of testing, though im sure some engineer somewhere knows the answer. (might have to make some phone calls to find out)


One thing ive seen pop up a lot if running divertervalves backwords for quicker response. Im curious if this is true or not...

[phila_dot]

One thing ive seen pop up a lot if running divertervalves backwords for quicker response. Im curious if this is true or not...

Looks like you're in a position to find out.

Flip your DV's, run those logs again and see what difference, if any, it makes.

[Meow]

Looks like you're in a position to find out.

Flip your DV's, run those logs again and see what difference, if any, it makes.

I missed this response somehow. I will try to do that this weekend and get some logs.

[Meow]

Looks like you're in a position to find out.

Flip your DV's, run those logs again and see what difference, if any, it makes.

So I tested this theory today. I flipped them and did a bunch of logs, this is about the average of what I saw. Somewhere between a 6-10psi spike with them reveres. From this alone I would say they are just as bad, if not worse when flipped. No real difference in sound.

So running them in the normal position is your best bet.