A portion of Simons post.

Then came the aftermarket they made really big shinny ones, didn't’t quite understand what they were for, thought it was to save the turbo from spinning backwards, they normally got you to plumb them up so all the air just blew out to atmosphere again this was measured air so the ecu added fuel although the engine never saw any of it, and it then ran really rich on gear changes but they did make a dunny flush noise that seems to have attracted a cult following. Funny thing is that Indy cars and world rally cars don’t have them. May the EPA isn't on to them yet. They also don’t blow up turbo’s very often. There goes that theory.

So you asked about the BA XR6T. Guess what it hasn't’t got an AFM. It dose its calculation via the manifold pressure sensor [map] and its mounted after the throttle so it never sees the pressure rise on gear changes. So why do they have a BOV STD? Makes you wonder, may be the after market guys are not the only ones that haven’t quite got a handle on it. The other reason would be they properly didn't’t like the dunny flush sound. That my guess.

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that's all good and well but I think the millions of $$ research and development that ford have done to spend money on adding a BOV to the factory setup has to tell you something. Engineers all around the world have proven that a BOV will reduce the stresses caused to a turbo and extend turbo life... why would they have invented one if it didnt help anything? If you say "to make a funky noise" that is a by-product as the BOVs originally produced were plumb-back and all factory BOVs are plumb-back.

What does he mean its mounted after the throttlebody? If you know how the pressurising in a turbo system works its easy to see how damage can occur to the blades of a turbo, simply saying Indy cars dont have them is a bit irrelevant because we dont drive indy cars do we? In a drag it is a different story, when u have autos pushing high power figures a BOV will not be that necessary as the gearchanges are instant and the boost stays fairly constant. In a manual its a whole different story, as there is time between gearchanges where the throttle is closed. Therefore the back pressure to the turbo will be high.

From the Turbosmart website:

Why do I need a Blow-off Valve (BOV)?

Blow-Off Valves eliminate compressor surge when changing gears. Excessive back pressure is created when the throttle is closed during gear changes or deceleration, causing turbo cavitation. This is detrimental to the life of the turbocharger. Fitting a BOV will substantially improve this problem. Installing a BOV will also significantly reduce turbo spool-up time. A substantial improvement in response from the turbo between gearshifts and a quicker boost delivery will result in an increase in torque at lower RPM.

Flutter - good or bad?

The “flutter” noise reported by many excited enthusiasts may sound good but is actually very unhealthy for the turbo. The “flutter” occurs when upon shutting the throttle, air caught between the free-spinning turbo and the shut throttle. The consequent back pressure forces the air back through the turbine blades. This process, more commonly called “cavitation” places enormous loads on the turbo and can lead to premature wear of the turbo bearings and other load-bearing components. The cavitation or “flutter” can be fixed by either fitting a blow-off valve onto a system that hasn’t got one, or making sure the blow-off valve is matched to the output of the turbo and is not working outside its flow capacity and ensuring the blow-off valve is set up correctly.

From the Turbosmart website:

Why do I need a Blow-off Valve (BOV)?

Because we sell them.

Flutter - good or bad?

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Bad. You should defintitely have a BOV. Did we mention that we sell them?

C'mon people if you're gonna post something as "proof", don't post something from a manufacturer's website!

IMO Indy & WRC cars probably don't have them because they replace turbo's (and whole engine assemblies for that matter) that often that long term wear isn't an issue.

I'd suggest that the damage to SAMSXR6T's turbo is too severe to be caused by a malfunctioning BOV, unless it's a ridiculously modified T, and running "Rigoli spec" boost levels.

From the Turbosmart website:

Why do I need a Blow-off Valve (BOV)?

Blow-Off Valves eliminate compressor surge when changing gears. Excessive back pressure is created when the throttle is closed during gear changes or deceleration, causing turbo cavitation. This is detrimental to the life of the turbocharger. Fitting a BOV will substantially improve this problem. Installing a BOV will also significantly reduce turbo spool-up time. A substantial improvement in response from the turbo between gearshifts and a quicker boost delivery will result in an increase in torque at lower RPM.

Flutter - good or bad?

The “flutter” noise reported by many excited enthusiasts may sound good but is actually very unhealthy for the turbo. The “flutter” occurs when upon shutting the throttle, air caught between the free-spinning turbo and the shut throttle. The consequent back pressure forces the air back through the turbine blades. This process, more commonly called “cavitation” places enormous loads on the turbo and can lead to premature wear of the turbo bearings and other load-bearing components. The cavitation or “flutter” can be fixed by either fitting a blow-off valve onto a system that hasn’t got one, or making sure the blow-off valve is matched to the output of the turbo and is not working outside its flow capacity and ensuring the blow-off valve is set up correctly.

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This is not strictly correct...

Cavitation by definition can only occur in a liquid, and refers to the phenomenon where localised pressures drops below the vapour pressure of the liquid. This creates small bubbles that collapse upon pressure recovery. The collapse of the bubbles generates very high energy, high velocity movement of the liquid in very localised sense - if this occurs near to a solid surface such as a pump casing or impeller, then these can be damaged.

Cavitation is not posible in a turbo charger as it flows a gas, not a liquid.

why would they have invented one if it didnt help anything? If you say "to make a funky noise" that is a by-product as the BOVs originally produced were plumb-back and all factory BOVs are plumb-back.

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If you read Simons post via the link Marcus provided this would be explained... For those that have not read Simon's most excellent post, there are two salient points relavent to this discussion:

1) there are a number of reasons why a BOV might be installed, on the Falcon Simon's guess was that it was installed to make the car quieter, and AVOID the flutter NOISE.

2) BOV's were invented for a reason - Simon has indicated this primarily relates to emissions. Some turbo cars use an air flow meter to gauge the amount of fuel required - flow back through this meter is measured twice resulting in too much fuel and poor emissions, stop the flow going back through the air flow meter and the air flow measurement is correct meaning better emissions...

Also think about this logically, which of the following do you think is more likely to happen - The pressure on the induction side of the motor is so great it a) bends the super high strength fins on the compressor wheel of turbo, or b) you blow off a hose clamp?

Cheers,

Ben.

Also think about this logically, which of the following do you think is more likely to happen - The pressure on the induction side of the motor is so great it a) bends the super high strength fins on the compressor wheel of turbo, or b) you blow off a hose clamp?

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...or c) The air pushes the throttle body back open.

Also, I don't think there wouldn't be time in between gears for this much pressure to build up.

Good post gogo

and dont forget that when the throttle body is closed the volume of air coming out of the exhaust( and driving the turbo) is greatly decreased. Thusly the turbo isnt being driven against the air building up between the outlet and throttle body.....

These posts are interesting reading.

As a guess - maybe a piece of blowoff valve broke off and went into the Turbo?

(sorry if that is impossible - just a guess).

"generally" speaking if a object strikes the turbine blades the fracture and chip. I spose if the material thats hitting the turbine is softer than the turbine it could cause them to bend...?

If you have ever driven the car on the road without the pipe on the turbo, then you could very well have sandblasted them.

Put a new windscreen in your car and drive it on the freeway most days, then look at it properly in a couple of weeks. Little dots all over it.

Well if Simon says you don't need one.. Of course you don't!!

If just about EVERY mass produced factory turbo car comes standard with a BOV then you can be sure it's there for a very good reason.

Your turbo is spinning at thousands of RPM, when you back off your throttle WITHOUT a BOV the turbo is stopped suddenly. Continue with this for a few years and your turbo isn't going to last. I don't care what anyone says.

That bit about the rally cars made me laugh concidering that the turbo's are replaced just about every race.

Short term it's ok, long term = bye bye turbo.