Blow Off Questions

porschepilot62 · 01-07-2006, 09:43 PM

Howdy Folks!

This may seem like a goofy question, but I am trying to figure out what guys are doing to get the familiar "woosh/ppfftt" sound from their cars.

First off, is this caused by the wastegate or the BOV? ...Or a combination of both? My car is dead quiet, and frankly, I think the "woosh" sound is pretty cool.

My car has a has been chipped, and I'm installing an Accuboost and boost gauge (I hate the idea of not knowing what my boost really is). I'd like to get that cool sound, and I plan on upgrading the wastegate and exhaust.

If it is the BOV, do you vent to the atmosphere by disconnecting the connection going to the J-boot? ...Assuming that I would have to plug the hole in the J-boot. Is an upgraded billet BOV necessary for this? (I plan on upgrading this anyway).

Any info would be greatly appreciated, and I hope everybody is having a great night....

Lawrence Coppari · 01-08-2006, 03:25 AM

The noise is due to the BOV venting to atmosphere. I would think that one would soon tire of the woosh/ppfftt sound.....

porschepilot62 · 01-08-2006, 07:33 AM

Hi Lawrence,

So how exactly is "venting to the atmosphere" accomplished? Is it the connection from the BOV to the J-boot?

Lawrence Coppari · 01-08-2006, 07:58 AM

I never tried it. Mine has a cone filter behind the driver's headlight and makes noise even without venting. It sounds like a vacuum cleaner with a shhhh sound when you let off the throttle. Mine is a track car so I do not want to lose turbo rpms which is what happens if you vent to atmosphere. I imagine you would open the connection on the down stream side of the BOV so the compressed air does not get into the compressor intake but instead vents to atmosphere.

There was a thread on this earlier this year. Do a search. I think some people tried it and their cars did not run properly when they did it.

porschepilot62 · 01-08-2006, 09:31 AM

Thanks! I think the venting to atmosphere thing is just something I wanted to explore, but I can already tell (after doing reading) that it's not the correct or best thing for the car. Thank you for your help.

nize · nize's Garage

fyi, if you vent to atmosphere while keeping the stock airbox, you're essentially venting metered air, which will confuse the car's computer and is generally not good.

this is more like a ricer mod.

AaronM · 01-20-2006, 06:27 PM

Mine loops back into the intake, but it does make that "woosh" noise when I shift.

nize · nize's Garage

was this before or after the bov popped off?

AaronM · 01-21-2006, 09:18 AM

Quote:

Originally posted by nize
was this before or after the bov popped off?

With everything connected as it is supposed to be. When you think about it though, even when the air is being recycled back into the manifold it still has to make a hard 90 degree turn inside the diverter valve. When the valve opens, it's going to make some noise even if it's venting back into the J-boot.

FSAEracer03 · 01-21-2006, 12:19 PM

You can't vent with the stock Bosch diverter valve. It operates under two conditions: under any manifold vacuum, it's open-port; anything from atmosphere up it's closed. Basically, if you take it off the J-boot and plug the remaining hole, you will have to drive around with your foot on the gas at all times or it will be a large vacuum leak.

andye · 02-05-2006, 09:57 AM

Quote:

Originally posted by nize
fyi, if you vent to atmosphere while keeping the stock airbox, you're essentially venting metered air, which will confuse the car's computer and is generally not good.

this is more like a ricer mod.

absolutely, I tried that with just that result, I then fitted a BOV which re circulates back into the j pipe and also blows off part to atmosphere, you hear the pssst at high boost, but it sounds a bit like a wet fish!!

Andy

ronin · 02-08-2006, 07:41 AM

now there's a sound I can't readily imagine

nize · nize's Garage

i think he meant a wet fart ?

geoffbaltz · geoffbaltz's Garage

On my track car, I have a Forge BOV, MAP sensor, and a hard pipe with cone filter. Mine vents to atmosphere since my pipe has no place to recirculate into the intake side of the turbo.

It makes that cool racecar sound when I shift. All is good when under pressure (throttle/boost). However, I do not like it when having to sit on the line (no vacuum/no boost). It causes the car to buck a bit and can unsettle the car going into some turns.

As stated earlier, the pressure vents back to the intake side of the turbo to keep the spindle spun up. This keeps the system pressurized. It doesn't hurt the car to run vented to atmosphere. I would think you would have some driveability issues for a street car since you are not driving like it a track car where you are either on full boost or on the brakes and not very often in-between.

If you want the cool sound, that's how you do it. I don't think you want to attempt this with a stock airbox setup since you will be screwing up the computer as stated earlier though.

-g

Bill A · 02-15-2006, 06:16 PM

Hi guys. Funny this topic.. I bought my 951 7or 8 years ago and it has that swosh sound between shifts as well. My car has been a rare weekend driver without many problems except for what going on now,(cam sprocket). I may have that solved soon however. At the time my car gets fixed I will ask and get more details about it what causes the noise. There is this valve thing on (sorry if I dont know the part name)the large pipe between the intercooler and the throdle body which I suspect is the thing everyone is talking about. As far as I know it has not effected the way my car operates. But I will learn more soon and relay back to everyone.

JustinL · 02-16-2006, 08:52 AM

I vent mine to the atmosphere. I use a MAP system so I don't have to worry about mismeasuring the air on shifts. I also have a custom intake pipe and it's just easier to not have to weld on another port.

The noise is quite noticeable at high boost shifts, but subtle just driving around town. The advantages to venting to the atmosphere is primarily getting rid of hot air and allowing fresh air into the intake pipe. Also there is probably less resistance to venting the air outside the pipe so the valve might be able to flow more.

Recirculating is supposed to keep the turbo spooling, but I'm suspicious that blowing air into a pipe with a turbo on one end and an open side on the other may not actually do a heck of a lot.

Bill A · 02-18-2006, 06:49 AM

Mind is in the same place. And is welded to the pipe. Also my part is not so shiney. I have what looks like a first generation unit and blows out directly below it.

MOVOVR951 · 02-27-2006, 10:13 PM

Quote:

Originally posted by nize
fyi, if you vent to atmosphere while keeping the stock airbox, you're essentially venting metered air, which will confuse the car's computer and is generally not good.

this is more like a ricer mod.

I have a Duke video of 956s on tracks in Europe, you can here the bov there. Definately not "a ricer mod".

I have had my Turbo XS RFL for 4 years and experianced zero problems. Due to the number of vac lines these cars have it may be a number of things that cause problems and the Bov may just exagerate the symptoms. Mine has a spring for upto 20psi and only goes off on upshifts or sudden lifts. Maybe a wimpy greddy or something meant for 12psi or 8 psi like the "rice" bolt on turbo kits would leak and also vent too often. Like I said, as for my car, I have had no problems. Vent one to atmosp. once and set how often you actually here. If you car sound like the Fast and Furious the setting is way too soft. My friends STI can be set so soft that at the stop light you think all hell is breaking loose just by revs to 3000 rpm max. How slow does everyone think the metered air is actually moving? WOT, LIFT , SHIFT WOT, wouldn't it seem logical that air is already through the car? I don't know for sure.

nize · nize's Garage

do the track cars use metered air? my guess is they don't.

this seems to be a recurring debate that pops up every few months/years, especially on rennlist. the general consensus is that on a car with the STOCK AIRBOX it is not a good idea.

instead of using the argument 'well i've never had any problems' i prefer to use the argument 'porsche's genius engineers who are much smarter than me must have designed it that way for a reason'.

if i'm unsure of the reason, i won't modify it until i am sure.

nize · nize's Garage

here's a related thread;
http://forums.rennlist.com/rennforums/showthread.php?t=191518&highlight=bov+vent+atmosph ere

and quotes;

Basically they say that recirculating is better because the air is reused (as oppossed to being lost when dumped)
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I did some data logging with my W/B with the stock bypass valve VS " dump to atmosphere"
valve and it was very interesting. TONS of oscillations with the VAF output when using the dump valve that is properly adjusted.
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Just a waste to throw away that metered air.

Seems to me that since the function of the BOV is to protect the Turbo from the pressure wave of closing the throttle, having boost bleed back to the turbo inlet would be just that much more protection. It seems it would also keep the available boost just a bit higher and the turbo spinning a bit more for when the throttle re-opens.

Or how about this one...
Bleeding boost pressure back to the compressor inlet when closing the throttle unloads the compressor side of the turbo so the turbo keeps spun up a bit even though there is reduced exhaust flow.
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My understanding is that on a mass air flow system, recirculating is best because the mass flow sensor has accounted for the volume of air that has entered the system, when you dump it, the car will run rich for a short period of time.

The idea of prespooling the turbo with the dump charge seems feasible, and that is why even with a map system (TEC3R), we still run a return on our racecar.
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From what I found, recirculating is better. The air is cooler, as it already went through an inter-cooler, when you get back on the throttle there is boost right away, and as Andrew Wojteczko said, maf thinks there is a X amount of air in the intake and will give equal amount of fuel.
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My take on it is that I wouldn't even know why you would use the dump version.
Like others have said, here, all the advantages come from the recirculation type.

The ricers like the dumping 'cause it makes that sound.
Personally, I think it sounds stupid.
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Ventingt to atmosher also gives lag between gears. As previously stated the maf will allow for the sound turst me on that. Very high boost is some times good to vent to atmosphere so that you don't blow lines off. That is my understanding.
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