Dump Valves? WTF
04/01/2006
Dump Valves
‘Dump’ or ‘blow-off’ valves are one of the greyest of grey areas of turbocharged engine tuning. They are an area of confusion and one where much hearsay, rumour and mythology have been allowed to mix with the facts. So hopefully this will clear some of the un-true’s
BOV Basics
Point number one is, of course, that a BOV (blow-off valve) is a component that’s only used on a forced-aspiration car, be it either a turbocharged or supercharged. Given that factory-produced supercharged Japanese cars are few and far between, this means were concentrating here on turbocharged applications.
That done, lets take a detailed look at the component in question. What exactly is a BOV – and why do you need one? To understand that, it’s worth getting back to basics on how a turbocharged engine works because, if you can understand that, then you can understand where the BOV fits into the bigger picture.
The turbocharger – for all its complexities and technical jargon – is just and exhaust – gas driven pump. A pump that has certain amount of inertia that needs to be overcome by the pressure of the exhaust gasses before it starts providing boost. And this inertia – the delay before the turbo starts to provide boost – is commonly known as ‘lag’.
Turbo inertia issues
But inertia isn’t a one-way street, because the opposite of inertia is momentum. So, while there are forces to overcome to start the turbine turning (when you want to accelerate), once its going there are forces to overcome to stop it (when you want to brake, or lift-off to change gear). During wide throttle openings, the turbocharger overcomes its inertia, spins up onto boost and continues spinning for a while after throttle is closed.
So what? You might think. Well, this phenomenon has quite serious effects on the engine’s performance and the potential durability of the turbocharger. How? Imagine for a second that the turbo is spinning away, pumping boost through a nice, wide-open throttle body as the accelerator pedal is fully depressed by the driver. As the driver lifts off the throttle for a gear change, or to slow for a bend, the throttle butterfly closes like a slamming door.
Meanwhile, momentum means the turbo is still spinning at very high rpm. Spinning and producing boost (positive pressure), in fact, boost that was intended to go through an open throttle body. Its now known as excess boost, meaning its not actually required by the engine, but it has nowhere to go because its hit the brick wall of the closed throttle butterfly. And its this excess boost that can cause some nasty problems, longevity, if nothing is done about it.
Flow reversal
That’s because, having hit the shut throttle body, the excess boost bounces back down the inlet pipe towards the turbocharger unit, in a process of flow reversal. The turbocharger objects to the boost travelling the wrong way around the inlet system. Such are the pressures involved that theres enough force to actually ‘stall’ the turbo, which means the turbine suddenly slows in a violent, uncontrolled manner.
This has two effects. Stalling the turbo means that it has to then spool up once more, before it can start boosting again, which takes time. From a drivers point of view this would be felt as a hesitation between gear changes, or when he came on or off the throttle, around a bend – which would make for a frustrating, surging, jerky drive. That’s unpleasant enough for driver enjoyment, but mechanically its bad news, too, as it’s a real turbo killer. Continuous stalling will overload the thrust bearings and the seals, eventually causing the turbo to fail – and its an expensive component to replace.
The OEM factory solution
Naturally, car manufacturers are well aware of this flow reversal phenomenon so, to overcome it, they developed a blow-off valve (BOV) which vents this excess boost during the on-boost transition from open to closed throttle, recirculating it back into the inlet system (downstream of the turbo) out of harms way. In this guise, the more accurate description is a recirculation valve.
This means that, when the throttle is wide open, the valve is sealed shut, so that it maintains boost pressure levels within the engine. When the throttle is closed, the recirc valve senses the excessive pressure build-up from the still pumping turbo and it opens, venting off the excess boost. That way, turbo stalling is avoided, drivability is improved and reliability is enhanced. With one eye on costs, factory recirc valves are usually made from plastic and rubber, featuring a boost-sensitive diaphragm and associated plumbing, and are fitted to most popular turbocharged Jap cars. So, when people say they’ve put a ‘dump valve’ on their wish list of modifications, it should be pointed out that they’ve already got a very effective item fitted straight from factory. Which begs the question: if its that good, why change it for an aftermarket item?
Aftermarket BOVs
Lets make one thing clear: fitting and aftermarket BOV will not increase your engines horsepower one jot. Fact. Anyone who says otherwise is talking rubbish, albeit with one slight proviso for, should the factory BOV malfunction so that it didn’t fully close, it could possibly cause a leak that robbed the engine of seeing full boost. That aside, its not a power tweek, it’s a drivability enhancer. Or, for some a aural delight!. And on many tuned engines, fitting an uprated BOV is vital in maximising drivability and turbocharger longevity.
So what are the choices? Well, some of it comes down to finances (expect to pay less than £200 for a good BOV) – and they come in two main designs: recirculating and atmospheric.
Recirculating BOVs are much like their factory cousins except designed for tuned applications, for higher boost motors, where the volume of boost is much greater within the turbo system. In these situations, the factory BOV, complete with its small vent holes, may struggle to vent all the excess boost effectively.
Switching to an aftermarket recirculating BOV, with larger exit ports and higher flow rate, cam dump this excess boost more effectively than a factory unit.
Rubber or plastic factory recirculating BOVs go through many, many thousands of boost cycles throughout their lives. This can eventually weaken the spring mechanism inside, allowing boost leakage, especially if your boost is over 1bar.So a switch to an aftermarket item is a good idea.
Venting to atmosphere
Ok, that’s the quite recirculating BOVs delt with. What about the noisy variety, the BOVs that breath to the atmosphere? These are the units that provide the ‘psshheuu’ noise beloved by some and hated by others. But is there any arguments to switch to one of these units? Well, there are arguments for and against. On the plus side, venting to atmosphere is definitely the quickest way of dumping excess boost, as it doesn’t then have to find its way back into the inlet system. Arguably, that more rapid operation makes the design more effective than the recirculating version. On the minus side, because the BOV is exposed to atmosphere, any sealing issues it has can lead to gasses being drawn into the inlet system through the BOV. This plays havoc with off-boost running and idle running a standard ECU, so it’s a factor to bear in mind.
Hope this helps!!
