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2004 GLS 1.8T
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And so it begins... A BOV isn't compatible with the engine management system in your Passat. Use of one will invariably throw off the fuel mixture, typically making it run quite rich. If you MUST have the whooosh! sound, there are hybrid valves, like the Forge, which vent a little air for sound effects, but don't mess up the engine operation too badly. Buy one of those, if you must, and follow the installation instructions in the box.
 

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784 Posts
in our cars (your cars, i have the V6) it does not have a blow off valve, it has a diverter valve.

first off, what does the DV do?

When you hit the gas your throttle plate opens allowing air into the engine. The turbo is in the intake stream and compresses the air and forces it in to the engine at a certain pressure. When you need to shift and let off the throttle, throttle plate closes, and the turbo is still spinning. The turbo still spinning is a good thing because it will have less work to catch back up when you reapply the throttle, but... a spinning turbo means it is still compressing air and the throttle plate is closed so there is only one way for the compressed air to get out, back the way it came in. Well this is where the Diverter Valve comes into play. The diverter valve diverts the air that would have gone back through the turbo causing it to stop spinning in one direction and spin backwards, to go back into the intake stream pre-turbo so you recycle that air. Now since your Volkswagen is tuned to run the air back into the system after the MAF sensor the MAF is calculating based on a certain charge still being in circulation when the throttle plate re-opens. This is why using a DV is most efficent on vw's motors, rather then a BOV which vents to the atmosphere. If you were to put an aftermarket Blow-Off Valve on the system that air that is being calculated for just blew out into the atmosphere. This causes the computer to still inject fuel metered for the air that was in the system already as well as the new air coming through the intake. With an aftermarket blow off valve you will get a fun turbo sound but your engine will run a little richer. So basically you need to pick performance or sound?

the above paragraph stolen from
VW Jetta: Diverter valve vs Blow-off valve on a 1.8T|Forum Volkswagen Bora

basically VW has it set up for power and efficiency. the computer knows what its doing. if you want the sound, go buy a Honda, but you are a man of substance, so you own a VW.
 

· PassatWorld Elder
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27,992 Posts
I have a BOV on my engine when I swapped it over. Surprisingly, I did not get a CEL from it. I just did not like the sound at 1:30 am when I am driving to work. I still have it in the garage and need to sell it.
 
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