wetoddid

While I'm in the mood for posting stupid questions, can someone help me understand the true difference from Diverting to atmosphere and recirculating.
Before I begin, I will let you guys know I've read about 25 threads on the topic and didnt see this question asked.
I understand the theory behind the Diverter valve, but is it (stock Bosche)designed to be a 2 way valve(ie, pull vacuum from the pre compressor side)?
Also, as far as venting to atmosphere....I understand that the air coming through the AFM is already "metered" and that recirculation venting is a way to take the 13 psi X 3 ft of compressed air (whatever the volume of the piping from the compressor and the throttle plate)and send it back to the pre compressor side, but what keeps that pressure in the intake....does the Air Flow Meter have a check valve inherent to the design? Or can it back feed through the air filter?
The reason why I ask, is in the event of a nasty back fire, obviously your turbo is in danger, but if the AFM is one way, its going to fail also....or am I misunderstanding.
As far as keeping the compressor turbine spinning with recirculation, I have a hard time believing that since it shares a common shaft with the exhaust, so little exhaust should equal little compression.
Hey I'm learnin here.

Kevin

Metal Guru

When the throttle plate snaps shut, the mixture richens due to a sudden decrease of airflow and more emissions are emitted. On n/a cars, there's a vacuum dashpot that slows the speed of the throttle plate so that there's enough air to burn the fuel that's been injected before the engine goes to idle speed. On our cars, the air required to do this is supplied by the diverter valve (sometimes called the blow off valve or bov). The bov is needed to vent the intake tract of high pressure air, which keeps the compressor from working against the high pressure and going into surge (that chirping sound that World Rally Cars make at braking points is compressor surge), which will damage the compressor. The bov has a valve with a spring to hold it closed against boost presure. On the spring side of the valve is a vacuum port that's connected to a reed valve that that's hooked to the intake, behind the throttle plate. The reed valve is normally closed until the engine is making a certain level of vacuum (hopefully the amount of vacuum generated when the throttle plate is closed). It then sends this vacuum to the bov, which causes it to open, flowing relatively high pressure air into the intake, reducing emissions, muffling the noise of air being vented and slowing the drop to engine speed between shifts.
Problem is that when the reed valve starts to age, that engine speed drop gets real slow, which sucks for fast shifting. Also, as you wait for the engine slow down while shifting, the turbo is slowing down too so that when you finally do shift and get back into the throttle, you have to build turbo speed up to get boost again.
At this point you have three choices: (1) buy a new reed valve (very expensive) (2)"repair" your reed valve by partially collapsing the signal side of the valve by 1-2 mm (3) have the bov vent to atmosphere. Number (3) is the source of endless arguments as to what effect venting to atmosphere will do to your engine. One turbo tuner claimed that the engine would go lean for an instant, which runs contrary to combustion principals. Others worry about the CIS being deprived of this air (since the air in question bypasses the CIS system it's really a moot point). Yet more claim that this air keeps the turbo spinning ( I agree as long as you don't have to wait 3 seconds to shift). One legitimate concern is that raw fuel from the sudden cutoff would make it's way to the turbo where it would be ignited and shorten it's life span. This is possible if your warn-up regulator is going bad.
As to your original question, IMO as long as your warm-up regulator is in good shape it shouldn't make much of a difference except that, you will be able to shift faster and have more boost when you get into the throttle again, you will hear the air hissing out to atmosphere and you'll get a puff of dark exhaust out the tailpipe when you let off of the throttle.
I'm sure others will disagree with this.......

wetoddid

This possibly explaines the engines tendency to decelerate slowly while shifting gears.
Good stuff....
If the reed valve is found to be questionable, and you chose to vent to atmosphere, just plug the reed valve?
Time to start learning about the warm up regulator. The Bosche Jetronic manual doesnt offer a bunch of info (theory and troublshooting) on the regulator itself so I will browse the forums.
With regards to the diverter valve, what do you run Paul? Or anyone else that wants to chime in?

Boeing 717

Do you think your deceleration valve is going bad?? Mine went bad and led to excrutiating times for the RPM's to come down.

Metal Guru

I run a Forge "hybrid" valve. It can vent both ways (atmosphere and engine), vent like a stock valve or vent to atmosphere. I'm venting both ways since I disconnected the output side of the reed valve (actually it's a diaphragm valve; my bad) and plugged the intake port. That way I'm venting the intake tract between the throttle butterfly and compressor at every deceleration (I'm assuming), not just after boost.
Type "930 WUR" into Google and the first site up will be Brian Leask's site. This is the guy that you want to rebuild your wur. If you intend to run higher than stock boost, have his install the heavy duty diaphragm.

Gandara

Metal guru, can you please answer why when I am on boost and I let go of the gas pedal or while shifting I hear a little "ding" sound, a little little small bell ringing kind of sound... Thank you

Metal Guru

Id the noise continuous while you are off the gas?
Does your car have the original bov?

Gandara

Probably not, but it sounds like it is coming from the dash, everytime I go into boost and I suddenly let go of the accel pedal as if I were to shift halfway through the tach or brake while in boost, I hear a "ding".

Metal Guru

Well Gandara, I'm stumped. Maybe an exorcism is required.
I'd have someone ride along with their head under the dash and try to isolate the area that the noise is coming from.

Gandara

THank you