N249 delete: why you shouldn't do it.

[mynameismud76]

Hi all, came across this and interested to see what everyone thinks. It's from another forum (http://www.audizine.com/forum/showthread.php/554859-N249-delete-Why-you-should-not-do-it)

 

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.

[sammyhale30]

Rubbish.  I had mine deleted when it was setup on the dyno.  it's been my daily car for years making 260bhp and 280ib/ft torque and I never have any driving issues or problems with fuel economy.  Maybe disabling it without a proper setup on a rolling road would cause issues yes.  To be fair any change to an engines original factory setup should be followed by a proper setup on a rolling road.  Anyone with any common sense should realise this.

Edited 21 March, 2018 by sammyhale30

[SuperbTWM]

I kept mine in operation when I had my TT, I don't know what the supposed benefits are but I figured they wouldn't have designed it that way unless it was better.

 

My general feeling was that 'some' people only deleted it because they had problems with he solenoid itself and were either too tight to buy another or couldn't be bothered fault finding when they could just remove it and alter the vac lines.

 

Either that or it was just a cool mod to do and tidied up the rocker cover

[sammyhale30]

It may serve a purpose with the standard factory engine.  But after a lot of modifications and mapping it will hinder rather than help, this is why tuners remove it, my tuner CC Tuning deleted it from my map.  It is designed to work with the stock engine air flow.  Anyone going for big power figures will naturally have a better flowing engine are advised to delete it because it can have an impact on high airflow. 

Edited 22 March, 2018 by sammyhale30