what does the BOV do ?
08 August 2003, 04:02 PM
#1
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I've heard many stories about the BOV some say it increases performance others say no. surely if the turbo looses lag then it must increases performance. Your views please !!!!
08 August 2003, 04:07 PM
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In certain circumstances, throttle response can be improved by holding the dumpvalve shut (or omitting it), but this is far from a universal scenario.
To be honest the design of most dump valves is not optimal as far as performance is concerned, as most of them open too early/close too late and let too much air out. They're designed for safety/turbo lifetime/pose value rather than outright throttle response.
However, what is really needed is a better design of dump valve, rather than not having it at all!
What happens in the "no valve" situation, as you probably know, is that the air is forced back through the compressor, causing stall, which, over time, will damage the turbo, and can, in extreme circumstances, damage it extremely quickly.
In performance terms though, the induction can sometimes recover from this stalled situation quicker than it can from a "dump valve let all my boost out!". Even if the compressor's stalled, the air is still in the induction tract, and is still under some pressure...
However, while crudely blanking off your dumpvalve may improve the engine's performance up to a point, all you need is a tiny amount of damage to the turbo's compressor, before it all goes completely pear-shaped very quickly indeed...
[Edited by greasemonkey - 8/8/2003 4:19:24 PM]
To be honest the design of most dump valves is not optimal as far as performance is concerned, as most of them open too early/close too late and let too much air out. They're designed for safety/turbo lifetime/pose value rather than outright throttle response.
However, what is really needed is a better design of dump valve, rather than not having it at all!
What happens in the "no valve" situation, as you probably know, is that the air is forced back through the compressor, causing stall, which, over time, will damage the turbo, and can, in extreme circumstances, damage it extremely quickly.
In performance terms though, the induction can sometimes recover from this stalled situation quicker than it can from a "dump valve let all my boost out!". Even if the compressor's stalled, the air is still in the induction tract, and is still under some pressure...
However, while crudely blanking off your dumpvalve may improve the engine's performance up to a point, all you need is a tiny amount of damage to the turbo's compressor, before it all goes completely pear-shaped very quickly indeed...
[Edited by greasemonkey - 8/8/2003 4:19:24 PM]
08 August 2003, 04:09 PM
#4
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The BOV attatches to the gribble nuts and stops the donkey flange from causing excess grunching on the flippleflimflamflomdrive, I THINK
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08 August 2003, 05:14 PM
#8
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grease... as much as you know about rollcages 
Nope. This is not what happens. Sure, there is a shock to the bearings (the real reason for a DV), but it is almost impossible to "stall" the compressor which at that point was turning say 150.000 RPM
If you ever have a knackered turbo and you shut down the engine, you will hear how long it acutally takes for the turbine to "stall". Even at idle...
Don't believe me though, just run without a DV for say 5 minutes and report back if you think you have better or worse gearchanges (not taking the "jumpyness" into account). The pressure is there on tap, not bled away (back out the air filter past the MAF if you don't run a non circulating one BTW)
With a decent turbo, you don't need a DV, simple. But you do need a solid 360° bearing, otherwise turbo life will be shortened. Ball bearing turbo's don't seem to take it kindly either (VF22/23 etc).
[Edited by EvilBevel - 8/8/2003 5:20:08 PM]
What happens in the "no valve" situation, as you probably know, is that the air is forced back through the compressor, causing stall
If you ever have a knackered turbo and you shut down the engine, you will hear how long it acutally takes for the turbine to "stall". Even at idle...
Don't believe me though, just run without a DV for say 5 minutes and report back if you think you have better or worse gearchanges (not taking the "jumpyness" into account). The pressure is there on tap, not bled away (back out the air filter past the MAF if you don't run a non circulating one BTW)
With a decent turbo, you don't need a DV, simple. But you do need a solid 360° bearing, otherwise turbo life will be shortened. Ball bearing turbo's don't seem to take it kindly either (VF22/23 etc).
[Edited by EvilBevel - 8/8/2003 5:20:08 PM]
08 August 2003, 07:40 PM
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It's probably not relevant here, but in theory, air passing through any compressor can achieve a state of "stall" with the compressor wheel still spinning (remember what happened to Tom Cruise's mate in Top Gun? Well, that was nothing to do with it, but Top Gun's a pretty cool film :aviator: 
).
The load on the turbo bearings goes up suddenly (the "shock load") which, as described above, can result in damage to the turbo internals (I think it increases somewhat exponentially the more you wind up boost).
Hence the reason for a BOV/dump valve, but as mentioned, the majority of existing aftermarket designs aren't 100% suited to their jobs and adversely affect the character of the engine.
Umm... well, what I just wrote certainly looks pretty. I'm not sure if it's relevant or even correct though, so I think I'll refill the pint glass and shuddup
Al
).The load on the turbo bearings goes up suddenly (the "shock load") which, as described above, can result in damage to the turbo internals (I think it increases somewhat exponentially the more you wind up boost).
Hence the reason for a BOV/dump valve, but as mentioned, the majority of existing aftermarket designs aren't 100% suited to their jobs and adversely affect the character of the engine.
Umm... well, what I just wrote certainly looks pretty. I'm not sure if it's relevant or even correct though, so I think I'll refill the pint glass and shuddup
Al
15 October 2003, 01:10 PM
#11
So how does a dump valve / BOV work - the mechanics of it with all the springs, shims, hoses to inlet manifold?
Trying to get my head round what happens to the std recirc alve when I shut the throttle .. what opens the valve to dump the air into the induction tract - is it vacuum from the inlet manifold?
Unconfuse me please!
Trying to get my head round what happens to the std recirc alve when I shut the throttle .. what opens the valve to dump the air into the induction tract - is it vacuum from the inlet manifold?
Unconfuse me please!
15 October 2003, 01:25 PM
#12
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What does a BOV do ?
PTSSSSSSSSSSSHHHHHHHH...........Squeak.
Or mine does anyway, sounds nice and looks nice, as for performance,who cares.
PTSSSSSSSSSSSHHHHHHHH...........Squeak.
Or mine does anyway, sounds nice and looks nice, as for performance,who cares.
15 October 2003, 01:33 PM
#14
those of u that cant be bothered to click:
Blow Off Valves (aka Dump Valves)
This is not to control boost nor does it affect it, directly. When you change gears or slow down the turbo is still spinning, when you take your foot off the throttle it closes a flap stopping the flow of air into the engine. this is commonly called the "butterfly" The turbo (still spinning) tries to keep pumping air in and can't. So it rapidly slows down with the restricted flow. When you put your foot down again, opening the butterfly and reducing the restriction the turbo has to "spool " up or gain rpm's again to develop boost. (This is called turbo lag)
The difference in time from foot down to full boost (at a suitable RPM) is the lag time.
NOTE remember full boost can not be gained when load is not correct as turbo RPM is based on a "thermal load" NOT rpm as some people think.
Back to the blow off valve....
What this does is detect a vacuum on the engine side of the butterfly and opens a valve to "dump" excess pressure, (back to the inlet filter) and hence reduces the restriction on the turbo allowing it to almost free spin, and NOT slow down as quickly. As a result when you put your foot down again its likely to be still spinning fast and takes LESS time to get to full boost and this creates LESS lag.
Obviously the valve is CLOSED on full boost.
remember it does not control boost, BUT it can leak causing lack of boost and also "creep" this is it can theoretically open at low boost levels if there is enough vacuum on the engine side of the butterfly to start it to open.
This venting of pressure is the whoosh you hear on gear change especially noticeable on early model WRX's where the vent pipe was close to the air filter assembly.
The whirr noise problem on current model WRX's like a blowing across a bottle is the vibration of the valve beginning to open.
Advice
If you wish to fit an externally venting "BOV" then read below:
Spend your money wisely, fit one last if you want power, (most BOV's especially Subaru's are ok up to 16 PSI) if you want that great noise then sure, fit one now!
A blow off valve does NOT control the boost LEVEL
The main benefits of an after market one is adjustability to remedy low boost leakage.
Capacity (as in early models) [the late model one is simply huge!
Few sales people will take the time to explain that am externally venting (as opposed to fully plumbed ) valve confuses the A/F meter, as a result the ECU has no idea that air has been vented and thus causes a huge change in fuel mixture ( as the ECU thinks there is more air than there really is) resulting in a very rich mixture.
That's why the standard one's outlet is after the A/F meter. This end result is funny misfires and flat spots.
Plus you will consume a lot more fuel
Most people argue that why then are they available?! Because "most" people want the whoosh noise, its as simple as that! Market forces!
Blow Off Valves (aka Dump Valves)
This is not to control boost nor does it affect it, directly. When you change gears or slow down the turbo is still spinning, when you take your foot off the throttle it closes a flap stopping the flow of air into the engine. this is commonly called the "butterfly" The turbo (still spinning) tries to keep pumping air in and can't. So it rapidly slows down with the restricted flow. When you put your foot down again, opening the butterfly and reducing the restriction the turbo has to "spool " up or gain rpm's again to develop boost. (This is called turbo lag)
The difference in time from foot down to full boost (at a suitable RPM) is the lag time.
NOTE remember full boost can not be gained when load is not correct as turbo RPM is based on a "thermal load" NOT rpm as some people think.
Back to the blow off valve....
What this does is detect a vacuum on the engine side of the butterfly and opens a valve to "dump" excess pressure, (back to the inlet filter) and hence reduces the restriction on the turbo allowing it to almost free spin, and NOT slow down as quickly. As a result when you put your foot down again its likely to be still spinning fast and takes LESS time to get to full boost and this creates LESS lag.
Obviously the valve is CLOSED on full boost.
remember it does not control boost, BUT it can leak causing lack of boost and also "creep" this is it can theoretically open at low boost levels if there is enough vacuum on the engine side of the butterfly to start it to open.
This venting of pressure is the whoosh you hear on gear change especially noticeable on early model WRX's where the vent pipe was close to the air filter assembly.
The whirr noise problem on current model WRX's like a blowing across a bottle is the vibration of the valve beginning to open.
Advice
If you wish to fit an externally venting "BOV" then read below:
Spend your money wisely, fit one last if you want power, (most BOV's especially Subaru's are ok up to 16 PSI) if you want that great noise then sure, fit one now!
A blow off valve does NOT control the boost LEVEL
The main benefits of an after market one is adjustability to remedy low boost leakage.
Capacity (as in early models) [the late model one is simply huge!
Few sales people will take the time to explain that am externally venting (as opposed to fully plumbed ) valve confuses the A/F meter, as a result the ECU has no idea that air has been vented and thus causes a huge change in fuel mixture ( as the ECU thinks there is more air than there really is) resulting in a very rich mixture.
That's why the standard one's outlet is after the A/F meter. This end result is funny misfires and flat spots.
Plus you will consume a lot more fuel
Most people argue that why then are they available?! Because "most" people want the whoosh noise, its as simple as that! Market forces!
15 October 2003, 04:33 PM
#16
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Upon fitting a BOV to my STi8 I achieved the following: -
1. pssstchhh brmrbrmrbrmbrmrbrmmr pssstch brmrbrm...etc.
2. Poping from the exhaust when coasting in gear.
3. Less responsive car from low revs.
Needless to say its back in its box in the garage!
1. pssstchhh brmrbrmrbrmbrmrbrmmr pssstch brmrbrm...etc.
2. Poping from the exhaust when coasting in gear.
3. Less responsive car from low revs.
Needless to say its back in its box in the garage!
15 October 2003, 04:36 PM
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Live fast, die young (from a turbo POV then) eh Richard
rb5, appreciate the FAQ, but it is still wrong.
You will have less lag without a BOV, just try it for once.
A BOV bleeds away pressure, which otherwise would stay on tap.
A 5 minute experiment will prove that conclusively
rb5, appreciate the FAQ, but it is still wrong.
You will have less lag without a BOV, just try it for once.
A BOV bleeds away pressure, which otherwise would stay on tap.
A 5 minute experiment will prove that conclusively
15 October 2003, 04:49 PM
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rb5,
That write up fails slightly with lag description.
Yes the BOV dumps the air pressure and allows the turbo to keep spinning but when you come back on throttle the turbo has to re-pressurise the entire intake.
A turbo car without a BOV keeps the intake system charged when you lift off, yes the air stalls across the compressor blades causing shock loading on the bearings but the turbo does not stop dead, its just slowing down quickly under load. Under a fast gear change a Turbo car without a BOV will come back on power with the turbo still spinning quickly and the intake system still pressurised.
As we can see a BOV can actually increase lag.
WRC and competition turbo cars are not fitted with BOV's and go one step further by keeping the turbo spinning at speed with a pressurised intake (Anti Lag).
A BOV is fitted for only one reason and that's to prolong the life of the Turbo. Fit a Turbo with 360 bearings that can withstand shock loading then a BOV is not required and lag should be reduced in most circumstances.
Lee
That write up fails slightly with lag description.
Yes the BOV dumps the air pressure and allows the turbo to keep spinning but when you come back on throttle the turbo has to re-pressurise the entire intake.
A turbo car without a BOV keeps the intake system charged when you lift off, yes the air stalls across the compressor blades causing shock loading on the bearings but the turbo does not stop dead, its just slowing down quickly under load. Under a fast gear change a Turbo car without a BOV will come back on power with the turbo still spinning quickly and the intake system still pressurised.
As we can see a BOV can actually increase lag.
WRC and competition turbo cars are not fitted with BOV's and go one step further by keeping the turbo spinning at speed with a pressurised intake (Anti Lag).
A BOV is fitted for only one reason and that's to prolong the life of the Turbo. Fit a Turbo with 360 bearings that can withstand shock loading then a BOV is not required and lag should be reduced in most circumstances.
Lee
15 October 2003, 06:09 PM
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Under a fast gear change
) but that is of course the key issue. A "slowish" gear change without a BOV would still do the "shock loading", but would not benefit from the absence of the afore mentioned device Rich, what ya up to ?
Bucket sized TD06 for starters ?
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