Quote from Klutch :

Seeing as the BOV has nothing to do with fuel...

It does have a lot to do with it (see post above about engine management). The engine management (lambda-sonde, no idea how you call that in english) will adjust the fuel mixture according to the measured air in the system. Now, this measure includes the air which gets blown off into the atmosphere with a BOV. But because unlike the stock recirc valve, the air is blown into the atmosphere, the measure is now wrong (the blown off air is included in the lambda measurement, but gets blown off instead of "staying in the system"). Results can be damage of your engine/exhaust.

EDIT: Sorry for the double post.

The pressure is still the same while under throttle, it only realeses into atmosphere or back where it came from when you release that pressure (IE: No more boost)

It doesn't vent when under throttle, so the pressure still stays the same.

Quote from Klutch :

The pressure is still the same while under throttle, it only realeses into atmosphere or back where it came from when you release that pressure (IE: No more boost)

It doesn't vent when under throttle, so the pressure still stays the same.

That is true... and you were trying to say...?

If what your saying is true, then it should only really matter while under acceleration/boost.

If you drive normally, theres no pressure build up, so its still reading the same thing as at atmospheric bov.

However, if your on boost, its STILL holding the same pressure as a plumb back bov, the pressure and ammount of air doesn't change, all that changes is WHERE the air vents too.

It vents, because its EXCESS AIR, it doesnt need it, and it has no where to go.

It doesn't control fuel AT ALL.

The air gets measured before the BOV/recirc valve. Now if you let go off the throttle, and the BOV blows that measured air in the atmosphere, the engine still "expects" the before meausred air... which is now missing. Result can be too rich fuel mixture, stuttering engine, fuel getting into the exhaust and damaging your catalysator, etc.

Fuel mixture/engine management does not only happen when you are on full throttle/boost...

Alot of modern cars don't have AFM's, how does that work for them then?

My native language isn't english, so again, excuse my limited knowledge of technical abreviations. Does AFM stand for something like air flow measurement? If yes, sure, not all cars are the same regarding that. However, for those cars/type of AFM that fit my previous post... i guess i was right?

Lambda sensor is in exhaust. It measures the air/fuel ratio what has burned in cylinder. All EFI cars don't have this at all.

MAF sensor (mass air flow) is usually after air cleaner and measures the mass of the air getting in.

MAP sensor (mass air pressure) is in intake manifold, after throttle body and measures the air pressure. Cars with MAP usually don't have MAF I believe.

Getting rich fuel ratio doesn't usually brake engine, lean does. Rich may broke the catalysator tho.
Modern engines are very sensible so I can imagine them suffering from rich ratio, but I can't be sure.

If you change the circulating BOV to atmospheric one in some cars it may have a tiny effect on the air fuel ratio, I'm actually not perfectly sure why, because MAP and MAF are used to measure how much you need fuel.

I have never heard anyone braking catalysator with atmospheric BOV, but it is told as a common fact in some modern cars so I can't deny.

Good thing I'm never going to own a catalysator (I hope).

BTW injection cars stop supplying fuel when the throttle is closed and the car is in engine brake.

this might sound completely stupid, but what's a BOV?

more like this
http://uk.youtube.com/watch?v= ... _scrE&feature=related

Quote from mutt107 :

more like this
http://uk.youtube.com/watch?v= ... _scrE&feature=related

ummm... thanks. but i was asking is what is it really? is it a blow off valve or what?

Quote from BigPeBe :

I have never heard anyone braking catalysator with atmospheric BOV, but it is told as a common fact in some modern cars so I can't deny.

But its not the BOV breaking the catalysor, its the change in air/fuel ratio which is destroying the catalysor usually from the car over-fueling. The extra heat then melts the inside of the catalysor, then goes hard after the car has cooled down, which then cause's the exhaust to get blocked, reducing performance. That why, when fitting any performance part (matters more on slightly more modern cars) (including BOV's), you need to get the car re-mapped to suite the part.

Quote from GrIp DrIvEr :

ummm... thanks. but i was asking is what is it really? is it a blow off valve or what?

google solved my answer. i feel stupid now. and where is that DOH smiley when I need it.

Quote from Nathan_French_14 :

But its not the BOV breaking the catalysor, its the change in air/fuel ratio which is destroying the catalysor usually from the car over-fueling. The extra heat then melts the inside of the catalysor, then goes hard after the car has cooled down, which then cause's the exhaust to get blocked, reducing performance. That why, when fitting any performance part (matters more on slightly more modern cars) (including BOV's), you need to get the car re-mapped to suite the part.

Yes of course it's the rich ratio, but I'm not still perfectly sure how changing the BOV can have that big effect.

Quote from BigPeBe :

Lambda sensor is in exhaust. It measures the air/fuel ratio what has burned in cylinder. All EFI cars don't have this at all.

MAF sensor (mass air flow) is usually after air cleaner and measures the mass of the air getting in.

MAP sensor (mass air pressure) is in intake manifold, after throttle body and measures the air pressure. Cars with MAP usually don't have MAF I believe.

Getting rich fuel ratio doesn't usually brake engine, lean does. Rich may broke the catalysator tho.
Modern engines are very sensible so I can imagine them suffering from rich ratio, but I can't be sure.

If you change the circulating BOV to atmospheric one in some cars it may have a tiny effect on the air fuel ratio, I'm actually not perfectly sure why, because MAP and MAF are used to measure how much you need fuel.

I have never heard anyone braking catalysator with atmospheric BOV, but it is told as a common fact in some modern cars so I can't deny.

Good thing I'm never going to own a catalysator (I hope).

BTW injection cars stop supplying fuel when the throttle is closed and the car is in engine brake.

you meant Manifold Absolute Pressure I take it?

Quote from BigPeBe :

BOVs aren't stupid, but if you want one for only making teh cool noise, it's stupid.

I know a chap who wanted a BOV on an N/A car, just for the cool noise. I didn't want to know him after that.

Quote from Osco :

you meant Manifold Absolute Pressure I take it?

Ah yes, stupid mistake.

Generally, blow off valves which make cool sounds aren't very effective.

With a turbo it's difficult to run a MAP sensor on the intake manifold. The pressures vary widely and the sensors have problems coping from what I understand. My car varies pressure from 30 pounds of vacuum to 15PSI within a second or so. Hence the MAF sensor instead.

@ nathan If you run rich your engine temperatures go down, not up. I think what you mean is the unburnt fuel enters into exhaust manifold, causing an explosion in the exhaust, which may eventually cause problems with the cat.

You would have to go lean to have in increase in engine temps, and then the cat is the least of your worries =)

@ wheel4hummer Can't agree more, that noise comes from restriction of some sort. If there was no restriction than you would have no sound. No restriction = faster pressure bleed-off.

Quote from BigPeBe :

MAF sensor (mass air flow) is usually after air cleaner and measures the mass of the air getting in.

MAP sensor (mass air pressure) is in intake manifold, after throttle body and measures the air pressure. Cars with MAP usually don't have MAF I believe.

Mine has both, (I believe - though I could be wrong), as do most/all VAG Turbo engines.

Quote :

If you change the circulating BOV to atmospheric one in some cars it may have a tiny effect on the air fuel ratio, I'm actually not perfectly sure why, because MAP and MAF are used to measure how much you need fuel.

Not an issue with the mixture so much as the absolute amount of fuel. Swap out a recirculating DV with an atmospheric one and you "loose" the mass of air that should have been redirected back to the inlet, therefore you have less air/fuel mix entering the engine and a subsequent drop in power. You pretty much wont find anyone recommending using an atmospheric DV on a car designed with a recirc one, (except those people that think its more important to sound cool than have performance :shrug. Certainly no reputable tuner of turbocharged VAG engines will anyway.

Why on earth would anyone want their car to sound like a bus/lorry anyway? never mind the mating call of a frog/duck

Well if the blowoff valve is active you want as little power as possible, since the blow off almost always occurs when you want little or no power, hence getting rid of the excess air charge so HP is irrelevant to having recirc vs. open.

You will get a faster spool with a recirc valve than atmo. but some of that pressure makes it back to the engine while deacceling. A/F ratios mean very little during deaccel but it will mess with A/F if you swap to atmo from recirc.

Its rare to see a maf and map sensor on the same car. One can easily calculate the other.

Quote from jibber :

It actually doesn't. If you take a stock car and change the re-serc dump valve to an atmospheric BOV, you can damage your engine/exhaust (without changing something in the engine management). An aftermarket system is usually made to be adjustable to a certain pressure in a certain pressure range. Stock ones usually can't be adjusted. This may be a reason why you'd most likely use an aftermarket one for a tuned car where the turbo pressure has changed (would be my guess). But there is no reason as far as i know (performance wise), why you couldn't achieve the same with a "closed" system. Therefor, i still believe that the only benefit you get from an atmospheric BOV, is the "cool" noise it makes.

@BigPeBe
You are right, it's not the same thing. I always thought of a BOV as a wastegate, while it's not. In german we call it "Umluftventil" i think.

you always map to suit, i thought that was an unspoken rule ?

This rule has yet to be communicated to the majority of people who tune their cars.

Okay, maybe not the majority, but there are lot's of people who think it's just plug and play.

leave them to it i say


let them employ me to fix it when it breaks

Hehe, better yet maybe their car should have an ECU that can cope with it =P But if your going atmospheric your not really thinking with performance in mind...