blow off valve
26 December 2002, 01:20 PM
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Search is a bit useless, actually! 
Scoobies are fitted with these as standard. There is no need to change it unless you run VERY high levels of boost, or if yours is broken.
The idea of a BOV (or dump valve) is that when you snap the throttle opening closed by lifting your foot off the accelerator, the pressurised air has no-where to go, and so builds up inside the turbo's compressor housing, potentially stalling the turbo (slowing it down) and damaging the blades, shaft and bearings.
Whether or not it slows the turbo is often disputed: None of the WRC cars EVER ran a dump valve, and those who run without have reported better turbo response on gearchange.
However, the dump valve DEFINITELY protects your turbo...noone has ever questioned this.
A dumpvalve is effectively a flap being held closed by a little vacuum hose from the engine. When the throttle snaps shut, a vacuum is created in this hose and the valve is "sucked" open against a spring, letting the charge (pressurised air) in your turbo, intercooler and throttle body be released.
On a standard Scooby, it will be fitted with a "re-circulating" dump valve. This means that the charge released is piped back into the air intake, just before the turbo, in theory helping it to spin up faster. The standard dump valve makes a very very quiet SSSH noise which you MAY hear if you fit an induction kit.
Many people uprate the standard re-circ valve to a more expensive one. These tend to stay closed a bit longer (supposedly giving you better throttle response, but no one really believes this) and that do not pop open at high boost when the car is tuned.
Re-Circ valves tend to make a nicer "chattery" noise, but are still very quiet compared to:
Vent To Atmosphere Valves....the VTA is for those of us who want a TSSSSSH sound that turns heads and makes the car sound like a highly tuned beastie. IT simply dumps all that compressed charge under the bonnet through a very short bit of pipe with little holes in to make a "whoosh" sound. Very nice.
Fitting one to a standard scoobie will cause a few problems, though:
Your engine will run rich on lift off, as you are venting away air that the ECU thinks is trapped in the system. This means it sucks in more air, which means the ECU thinks you need more petrol being injected.
The effect of this is that you end up with unburnt petrol in your exhaust gasses. This can result in flames from the exhaust, pops, bangs, possible failed emissions tests and (in some cases) a nasty black smudge above the exhuast on the paint at the back of the car.
If you can live with all this, then a VTA sounds fantastic. But they really have no benefit other than the nice sound and some flames, and you are potentially introducing problems.
The most popular BOV on here seems to be the HKS or the Forge types. The Forge requires a small modification to work at it's best: It cn use either a blue or a Red sring: forge reccomend the Blue spring, but if asked will provide a couple of "spacers" (read washers) to make it a teeny bit stiffer, so that it works at it's best of Subarus. In standard form with the Blue Spring, it has a tendency to open a little to early, losing some precious boost and detracting from the cars performance. Most highly tuned scoobs run with the red spring as it is much stiffer.
I hope this helps. Anyone want to correct my mistakes?
Edited to add...bl00dy hell that was longer than I thought.
[Edited by Turbo_Steve - 12/26/2002 1:21:39 PM]
Scoobies are fitted with these as standard. There is no need to change it unless you run VERY high levels of boost, or if yours is broken.
The idea of a BOV (or dump valve) is that when you snap the throttle opening closed by lifting your foot off the accelerator, the pressurised air has no-where to go, and so builds up inside the turbo's compressor housing, potentially stalling the turbo (slowing it down) and damaging the blades, shaft and bearings.
Whether or not it slows the turbo is often disputed: None of the WRC cars EVER ran a dump valve, and those who run without have reported better turbo response on gearchange.
However, the dump valve DEFINITELY protects your turbo...noone has ever questioned this.
A dumpvalve is effectively a flap being held closed by a little vacuum hose from the engine. When the throttle snaps shut, a vacuum is created in this hose and the valve is "sucked" open against a spring, letting the charge (pressurised air) in your turbo, intercooler and throttle body be released.
On a standard Scooby, it will be fitted with a "re-circulating" dump valve. This means that the charge released is piped back into the air intake, just before the turbo, in theory helping it to spin up faster. The standard dump valve makes a very very quiet SSSH noise which you MAY hear if you fit an induction kit.
Many people uprate the standard re-circ valve to a more expensive one. These tend to stay closed a bit longer (supposedly giving you better throttle response, but no one really believes this) and that do not pop open at high boost when the car is tuned.
Re-Circ valves tend to make a nicer "chattery" noise, but are still very quiet compared to:
Vent To Atmosphere Valves....the VTA is for those of us who want a TSSSSSH sound that turns heads and makes the car sound like a highly tuned beastie. IT simply dumps all that compressed charge under the bonnet through a very short bit of pipe with little holes in to make a "whoosh" sound. Very nice.
Fitting one to a standard scoobie will cause a few problems, though:
Your engine will run rich on lift off, as you are venting away air that the ECU thinks is trapped in the system. This means it sucks in more air, which means the ECU thinks you need more petrol being injected.
The effect of this is that you end up with unburnt petrol in your exhaust gasses. This can result in flames from the exhaust, pops, bangs, possible failed emissions tests and (in some cases) a nasty black smudge above the exhuast on the paint at the back of the car.
If you can live with all this, then a VTA sounds fantastic. But they really have no benefit other than the nice sound and some flames, and you are potentially introducing problems.
The most popular BOV on here seems to be the HKS or the Forge types. The Forge requires a small modification to work at it's best: It cn use either a blue or a Red sring: forge reccomend the Blue spring, but if asked will provide a couple of "spacers" (read washers) to make it a teeny bit stiffer, so that it works at it's best of Subarus. In standard form with the Blue Spring, it has a tendency to open a little to early, losing some precious boost and detracting from the cars performance. Most highly tuned scoobs run with the red spring as it is much stiffer.
I hope this helps. Anyone want to correct my mistakes?
Edited to add...bl00dy hell that was longer than I thought.
[Edited by Turbo_Steve - 12/26/2002 1:21:39 PM]
26 December 2002, 03:36 PM
#4
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Don't see many mistakes, but would like to add that the OEM dumpvalve also opens on part throttle (pressure difference pre/post throttle butterfly), and I'm not sure you want to lose this with "stiffer" setups.
Another theory from Pete C. wasn't silly either: the recirc system could *possibly* vent air back outside past the MAF, a problem you won't have with a VTA system.
And finally: the OEM I had stopped working correctly when I fitted the FMIC. It worked fine before with the VF23 / sensible boost, but seemed to have a problem dumping the air with the FMIC (more air mass in the pipes/IC ?), resulting in a sort of "jolting" on partial lift off.
A Forge recirc with some custom shims (courtesy of BRD) has solved that (at least on my car).
We really need a FAQ on these things, don't we ?
Another theory from Pete C. wasn't silly either: the recirc system could *possibly* vent air back outside past the MAF, a problem you won't have with a VTA system.
And finally: the OEM I had stopped working correctly when I fitted the FMIC. It worked fine before with the VF23 / sensible boost, but seemed to have a problem dumping the air with the FMIC (more air mass in the pipes/IC ?), resulting in a sort of "jolting" on partial lift off.
A Forge recirc with some custom shims (courtesy of BRD) has solved that (at least on my car).
We really need a FAQ on these things, don't we ?
26 December 2002, 07:21 PM
#5
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the type h is the rfl, which supposed to be the loudest. it a piston designe and is adjustable too. slightly cheeper than HKS and Blitz.
hope this is some help mike
ps it very good too...... if you like noisy things
hope this is some help mike
ps it very good too...... if you like noisy things
27 December 2002, 03:21 AM
#6
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Thanks EvilBevel. Am contemplating starting my own site: something like ScoobyFAQ or something, that people on here can just post links too
Just a load of Cut, copy and paste from threads on here, really.
=Shrug=
Just a load of Cut, copy and paste from threads on here, really.
=Shrug=
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