Welcome to my world, my world of turbos, tyre smoke, and tuning...
Tuning cars, driving cars, testing parts, and complaining about everything. It's my job, and a the majority of my non-work life too...
RWhen it comes to modifications of turbocharged engines, one of the first things that come to mind with a lot of people are blow off valves, or dump valves, depending what you like to call them (same thing!).
They've been around since at least the 1970s on race cars (though pretty much disappeared on turbocharged works race cars after the early/mid 90s...), factory fit parts on most turbocharged petrol engines from the mid 80s onwards, and from around the mid-90s onwards aftermarket ones became THE thing to have- It was, and to some extent still is, the first engine mod people do to a turbo car.
But what DO they do? What are they REALLY for? Well this is where the confusion/bullshit comes in, not only thanks to the internet, but thanks to the fact most the things said about them to sell them, in the past at least, were lies too.
Christ, the "Blowoff Valve" Wikipedia page even talks total and utter shit about what they do, and why, and how, and whatnot...
THINGS THAT PEOPLE SAY DUMP VALVES DO...
"Reduce turbo lag"
No. They increase it if anything. Turbo shaft speed drops far more with a BOV fitted than without. That's their true purpose, to prevent surge and overspeeding when you shut the throttle. Honestly, anyone with a turbo speed sensor will have seen this, and I've done tests with timing equipment a few times now, that shows, even if it's not noticeable seat of the pants, it makes it worse, not better. This IS fractional though, don't expect a big, or even noticeable difference on most engines- Really depends on the application.
"Prevent your compressor wheel from slowing/stalling (or going backwards)"
No, they actually SLOW a turbo down, that's their actual job, they're a safety thing, to prevent overspeed and surge on a closed throttle, which in some extreme situations can damage the turbo. Honestly, watch a turbo with a turbo speed sensor fitted, the RPM drops far far more with a BOV than without. The 'spinning backward' thing people say is fucking retarded btw.
It's the AIRFLOW that stalls, not the turbo, something I'll explain further down...
"Help prevent compressor surge"
Yep, they do this, but only off-throttle compressor surge, which isn't that damaging or sustained to turbos, as there's no load on the turbine off-throttle. It's on-throttle surge that can be hugely damaging, and it does nothing to help that. "Certain" turbo manufacturers and their badly mis-matched compressor/turbine combos are the main cause of on-throttle surge...
"Help stop turbo damage due to (whatever)"
Well yes, this is the real reason they're fitted, and can be useful for this, but their need is often hugely over-stated, and how they help prevent it is ass-backwards too. The usual bullshit myth is they help prevent turbos stalling, and the sudden slowing of the turbo is what damages them. NO. They help SLOW the turbo, as without them turbos can overspeed when the throttle is shut, and surge all over the shop, and in certain applications that can damage the turbo.
"Help MAF sensor equipped cars run right (recirc ones ONLY)"
While it's indeed true many engines with MAF sensors run like shit off-throttle if you fit a vent to atmo dump valve, the common info that you HAVE to run a recirc with a MAF is bollocks too in everything I've ever experienced.
Running no BOVs at all is absolutely fine almost always.
It's possible there's a car where the MAF sensor does shit itself, but so far I've not found it. I've heard a whole lot of rumours, I was told by countless people without a shadow of a doubt it'd cause the 2.7TT (ie S4) Audi engine to run like shit and so on if removed, but I tried it, and it's fine- Turned out, as ever, none of these people had ever tried, just 'heard' it did.
EDIT- Now I think back, I do recall a time when it made the MAF go crazy for a second, but that was a BIG aftermarket turbo on BIG boost on a small engine, and that was the only time it happened. Generally, no issue at all.
"Makes that cool chatter noise"
I know most of you know better, but it's rare to see a video of a car with decent turbo chatter without at least one comment saying it's the BOV making that noise, or asking what BOV it is as they want one that makes that noise.
IT'S NOT! HAVING NO BOV AT ALL IS WHAT HELPS MAKES THE NOISE!
Fuck, when I was 18 (1998!) even I at first thought the same, as MAGAZINES TOLD ME SO. In fact these "expert magazines" told me that it was HKS SQV valves did that noise, and though they were about £350 back then, and I wanted that noise SO BAD I would've paid that.
In fact, after a tuned MR2 Turbo came past and chattered like fuck one day I rang Torque Developments (they were the only HKS dealer back then) to order one. THANKFULLY, the sales persons tone of voice and choice of words made me very suspicious, while also accidentally giving me some hints that helped me research what REALLY made the noise (No internet back then!), so I thankfully didn't get skanked out of £350...
THINGS DUMP VALVES DO THAT PEOPLE DON'T TALK ABOUT...
"Help suppress noise"
The main reason they're fitted to OEM vehicles is as noise suppression devices, in fact that's the exact name they're given in many official workshop manuals. The noise I mean is turbo chatter, which is a lot harder to silence with an airbox than the ptschhh of a dump valve. We might like chatter noises and induction noises, but your average Joe new car buyer doesn't. In fact back in the day I worked at a dealership and an Impreza STi we had in on a P/X that we then sold was returned to us by the customer due to "Funny noises" it was making, as it had slight chatter as it had an induction kit on it.
"Help slow the car when in limp home mode"
A lot of new cars also use a system that holds the BOV(s) open, preventing boost/power being made in case of an issue that puts the car in limp home mode. Useful as an OEM.
"Leak like bastards, causing underboosting and turbo wear"
I know they're not meant to do this, but this is a major and common issue, and often impossible to detect unless looking for it. A dump valve is another potential leak point, and split diaphragms and/or weak springs means they leak surprisingly often, especially OEM and cheapo ones, and even if your turbo is still making full boost, a leak can cause a slower boost rise, and a turbo (and therefore entire engine) working far harder.
Confused? Don't be. I'll clear your mind (a little) in this next bit...
""SO! I was always led to believe that turbos stall with no BOV, and now you're telling me turbos SPEED UP?
Hi, I'm Stav...
You may or may not have heard of me, but I've spent the last 15 years working full-time in the tuning scene, and the last decade or so writing for various car magazines.