Purpose of Blow off valves and dump valves....

[mrbeige 0]

Can someone confirm the purpose of the above on turboed engines please, I'm still a little unsure??

[Jim 2]

I thought I had a good idea but went and hit up Wikipedia.. explains the difference between the two nicely:

 

Dump valves are fitted to the engines of (usually older) turbo charged cars and sit between the turbo outlet and the throttle body. When transitioning from a boosted state to a closed throttle state (as in between shifts), due to inertia, the turbo continues to pressurize air, but the closed throttle prevents the compressed air from entering the engine. In this case the pressure exceeds the preset spring pressure in the dump valve and the excess pressure is bled off to atmosphere.

 

Even with a dump valve the compressed air acts as a brake on the turbo (slowing it down), because the pressure on the backside of the turbo is at a higher pressure than on the front side (and the air actually wants to flow through the turbo backwards).

 

A blowoff valve is a more elegant solution to this problem by allowing the turbo to "freewheel" when the throttle is closed (equalizing the pressure on both sides of the turbo). Unlike a dump valve a blowoff valve can be used at multiple boost settings without reconfiguration.

 

Blowoff valves are sometimes incorrectly called dump valves because they serve a similar function, however they are very different solutions to the same problem.

 

Even more lovely knowledge at the expanded section on Wastegates: http://en.wikipedia.org/wiki/Wastegate

[vwdeviant 0]

If it's a Chavvy motor then its sole purpose is to sound like a toilet flushing!

 

Something to do with recirculating gases and spool up speeds.. but I guess "Sir Cheesy-of the-turbo" will be along soon to full explain in fine technical detail! :lol:

 

Or Like Jim says...

[RS VR6 0]

You either have a "Diverter" valve which recirculates the air back ino the intake or a "Blowoff" valve that vents into the atmosphere.

 

They both basically help to prevent compressor surge, which is a back pressure of air after the throttle plate closes. It also helps the turbo or SC to spool faster between shifts.

[Mave 0]

That was like trying to read Japanese lol.

[CrazyDave 0]

Imagine you've got the throttle wide open and accelerating down the road, turbo whisttling away, then back off the throttle (to change gear). The throttle body closes, the turbo / supercharger is still spinning putting pressure into the pipework (and stalling the turbo), it has to get out somewhere or the pipes will all start blowing off!

 

So.. when the throttle closes it creates a vacuum in the inlet manifold, this sucks the diverter / blow off valve open and the excess pressure is released out of the system (and allowing the turbo to spin without load).

 

When you open the throttle again, the vacuum disapears from the inlet manifold, the diverter / blow off valve closes (cos it's got a spring in it), and the system pressurises again and off we go!

 

The only real difference between the two is blow off valves release the air out completley (and make a 'pssst' noise) and diverters (strangely enough) divert back into the system near the air filter somewhere (still make a 'psst' noise but not so load as it's still closed into the pipework).

 

Hope that makes sense.

[Toad 0]

Imagine you've got the throttle wide open and accelerating down the road, turbo whisttling away, then back off the throttle (to change gear). The throttle body closes, the turbo / supercharger is still spinning putting pressure into the pipework (and stalling the turbo), it has to get out somewhere or the pipes will all start blowing off!

 

So.. when the throttle closes it creates a vacuum in the inlet manifold, this sucks the diverter / blow off valve open and the excess pressure is released out of the system (and allowing the turbo to spin without load).

 

When you open the throttle again, the vacuum disapears from the inlet manifold, the diverter / blow off valve closes (cos it's got a spring in it), and the system pressurises again and off we go!

 

The only real difference between the two is blow off valves release the air out completley (and make a 'pssst' noise) and diverters (strangely engough) divert back into the system near the air filter somewhere (still make a 'psst' noise but not so load as it's still closed into the pipework).

 

Hope that makes sense.

 

That's a very good explaination, kind of what I would strive to, but fail to deliver.

 

It's worth noting that the chuffing kind of noise aka "turbo chatter" on some cars without correctly set up wastegates is caused by the air being forced back past the compressor stage, which can lead to the shaft snapping.

[ninoG60 0]

The old cossies and the Audi sport Quattros used to do the turbo chatter, sounds mental! :D

[CrazyDave 0]

It's worth noting that the chuffing kind of noise aka "turbo chatter" on some cars without correctly set up wastegates is caused by the air being forced back past the compressor stage, which can lead to the shaft snapping.

 

World Rally cars make a new kind of noise, like a 'chirp chirp chirp' I believe this is caused by the boost control system that trys to balance the blow off together with the wastegate. It actually doesn't release all the charge air when the throttle closes, so the inlet is still pressurised, so when you open the throttle again the pipework already has pressure in it so minimal lag. That and igniting fuel down the exhaust to keep the turbine spinning (chirp.. chirp.. bang bang bang!). 8)

 

I just like writing threads with wordy explanations of car noises :lol:

[junkie 0]

It's worth noting that the chuffing kind of noise aka "turbo chatter" on some cars without correctly set up wastegates is caused by the air being forced back past the compressor stage, which can lead to the shaft snapping.

 

World Rally cars make a new kind of noise, like a 'chirp chirp chirp' I believe this is caused by the boost control system that trys to balance the blow off together with the wastegate. It actually doesn't release all the charge air when the throttle closes, so the inlet is still pressurised, so when you open the throttle again the pipework already has pressure in it so minimal lag. That and igniting fuel down the exhaust to keep the turbine spinning (chirp.. chirp.. bang bang bang!). 8)

 

I just like writing threads with wordy explanations of car noises :lol:

 

Also on rally cars with the sequential shifts it only takes a nano second to change gear so not really much use in bleeding it all off anyway.

[CrazyDave 0]

It's worth noting that the chuffing kind of noise aka "turbo chatter" on some cars without correctly set up wastegates is caused by the air being forced back past the compressor stage, which can lead to the shaft snapping.

 

World Rally cars make a new kind of noise, like a 'chirp chirp chirp' I believe this is caused by the boost control system that trys to balance the blow off together with the wastegate. It actually doesn't release all the charge air when the throttle closes, so the inlet is still pressurised, so when you open the throttle again the pipework already has pressure in it so minimal lag. That and igniting fuel down the exhaust to keep the turbine spinning (chirp.. chirp.. bang bang bang!). 8)

 

I just like writing threads with wordy explanations of car noises :lol:

 

Also on rally cars with the sequential shifts it only takes a nano second to change gear so not really much use in bleeding it all off anyway.

 

Oh yes, very fast change! The electronic throttle lifts very slightly to reduce torque whilst the car is mid gear, but thats not so much of a problem these days because the gear shafts are syncronised to prevent mashing the drive dogs between gears. All good stuff!

[mrbeige 0]

Some good info there. See, why can't I be developing software to do that sort of schitt! I'd love to be doing that.

 

So where in the system does the BOV go then?

[CrazyDave 0]

http://www.tialmedia.com/documents/w3_tial_bovinstall.pdf

 

Here :) .

 

Mr Beige, well I don't get to write the code for the actual car system, just the systems that are involved in testing the car system on a rig. But I bet you get up to some interesting stuff anyway, just different stuff. 8)

[mrbeige 0]

http://www.tialmedia.com/documents/w3_tial_bovinstall.pdf

 

Here :) .

 

Mr Beige, well I don't get to write the code for the actual car system, just the systems that are involved in testing the car system on a rig. But I bet you get up to some interesting stuff anyway, just different stuff. 8)

 

Ah cheers dude!

 

Not really been that interesting to be fair, was promised loads and haven't really seen any of it! Maybe the risk of being made redundant might force my hand and look for a new job!!

[Kevin Bacon 5]

Some good explanations there!

 

Blowoff is generally what the Americans call dump valves, but it's the same principal.

 

It's preferable to have the BOV as close to the turbo as possible and also the largest available. The Tial BOV Dave and I use has a 50mm valve and it really shifts air. The quicker the turbo can vent unwanted pressure, the better, as it prolongs compressor blade and bearing life. Having the BOV near the turbo means fewer pressure waves bounce back off the throttle plate. You'd think a big BOV would be louder than say, a 25mm Forge valve, but the opposite is true. Big valves are quiet 8)

 

It's worth noting that MAF based engines can't vent to atmosphere as it dumps metered air and also with turbos, it's not nice venting 70+ deg air back into the turbo intake either, so love or hate the pshhht noise, it's quite essential.

[mrbeige 0]

It's worth noting that MAF based engines can't vent to atmosphere as it dumps metered air and also with turbos, it's not nice venting 70+ deg air back into the turbo intake either, so love or hate the pshhht noise, it's quite essential.

Does the same apply for MAP based engines? I'm assuming not as the reading is post throttle?

[Dec 1]

Nope, MAP sensors will work fine!

[jonnyboyo 0]

I miss my Blitz Super Sound blow off valve :( :(