BLOW OFF (DUMP) VALVES- WHAT DO THEY REALLY DO?

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.
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"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...
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"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?
EXPLAIN YOURSELF!""

Ok, here goes. First, let me introduce you to Mr Compressor Map, with some basic annotations via the amazing medium of MS Paint...

As you may or may not know, the horisontal axis of the map is airflow in lb/min, the vertical is pressure in bar (1.00 being atmo, anything above being 'boost'), and anything within those lines drawn on the map is the 'safe' area that turbo is happy enough to run in constantly, in this case, up to about 2.4bar of boost and 49lb/min.

The three dots I've added show three different situations.

ON BOOST- 1.7bar, 42lb/min airflow, well within the map, it's loving life.

SUDDEN CLOSED THROTTLE WITH A BOV- BOV opens, so pre-throttle pressure rapidly drops to zero, while an open BOV means airflow is less but exists, and therefore the compressor drops to a very low speed point on the map. It's still safe and happy.
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SUDDEN CLOSED THROTTLE WITH NO BOV- Throttle closes, so airflow drops to practically zero, while pre-throttle pressure momentarily spikes, putting the compressor in to somewhere on the top left of the comp map (ie lots of boost pressure, but fuck all airflow), well in the surge zone, which, if there were speed lines shown, would be a shitload of rpm too.

""OK, now I believe you about the turbo speeding up, it sounds like running with no BOV is fucking DISASTROUS and should be avoided at all costs, right?""

So, that'll be a dead Garrett GTX3582R. Expensive billet brokenness...

Ok, this is where the THEORY vs REALITY thing comes in to tuned cars yet again.

There is NO DOUBT the sole use of a vent to atmo BOV is to help prevent turbo damage via compressor surge and overspeed, as that deffo, deffo, does no good for performance.

In THEORY then, running one is a very wise move indeed.

But in REALITY, how many people genuinely have issues when running without one, even with big boost and hard use over long periods? Well, not fucking many at all.

The issue people seem to forget is severe surge and overspeed happens for a TINY amount of time on a turbocharged engine when you close the throttle.
It's not like a belt driven supercharger, or a gas turbine, or when surge happens ON THROTTLE. That is fucking continuous until the power is shut down, as the compressor is being FORCED to operate at that condition- THAT can be bloody disastrous in some applications as there's huge forces turning it regardless, hammering the shit out of the compressor.
But when you close the throttle on a turbo engine you are stopping power to the engine, and in turn stopping practically all power to the turbine wheel, which is what turns your compressor, so once the slight momentum from being off throttle is gone, there's no danger.

In all honesty, in my 18 odd years of messing about with turbo cars, working around and with turbo cars, testing and timing turbo cars, and reading every worthwhile turbo-related thing on the internet, I've personally seen TWO instances I think 'probably' were no-BOV related turbo fails (both on the exact same type of (uncommon) turbo with a known weak shaft, both at over 30psi), and heard of a few others on huge turbo mega boost V8 drag cars where the turbo failed at the end of the run just after they got off the throttle.

All the other times I've seen turbo fails, it's been other issues entirely. And that's thing, though a turbo CAN fail due to this, it's usually broken due to some other reason long beforehand!

On my own personal cars, even running 1.5-2bar boost for long periods on loads of cars over the years with no BOV I've had no issues, and know countless others doing the same. Christ, I've not ran one on anything since the year 2000 now I think back. And by then I'd experienced plenty of split/leaking BOVs, and have since on other cars too.

Ignoring my own experiences, and the fact I love the sound of turbo chatter, let's look at things logically...

Instances of people talking about their turbos failing that's honestly likely attributed to no BOV- VERY FEW.

Instances of people with fuck all 1st hand experience of the subject telling you to run one for whatever reason- FUCKING LOADS.

% of people with turbo fails that do run BOVs and dont run BOVs- About the same, as frankly, turbos tend to fail for various other reasons before the fraction of a second of surge and overspeed would kill it.

% of turbocharged works race cars since the early '90s that run BOVs- Very very few. Even in the 80s and early 90s they were rarely seen on a lot of applications. If they could improve the odds of ending the race, even slightly, and not cause other, much more serious, downsides, would they run them? Of course they would. Having said this, they use some good, strong, expensive, turbos on works race engines...

""So are you saying they're good or bad? I'm fucking lost here now Stav...""

Neither, there's no simple answer- I'm saying read what I've told you and make your own mind up- I'm not taking responsibility for your decisions.

For me, personally, no, I never run one. Ran tons of cars at mega boost, hard used, all kinds of turbos, for years, never and issue. Loads of mates do the same, even on fancy big bucks ball bearing turbos, no issue. 
My old R32 Skyline I built, what, 4+ years ago? 1.5bar+ boost, 470bhp, from a journal bearing Holset-based turbo I bought from Compressor Racing (their RS341 model- Like a slightly improved HY35). No dump valve fitted of course, used hard as fuck by me, and hard as fuck by the owners since, track use, drift use, top speed, all sorts- Turbo is still going strong.

Does this make it gospel though? No.

Would I still not run one if I happened to own a turbo with a KNOWN issue of shaft snapping or wheels exploding with not running one? No, I'm not fucking stupid.

Would I risk not running one on some crazy expensive turbo I couldn't afford to fix? No, as I'd not buy one in the first place. Should you? Well, read the above, do your own research, and make your own choice...

Remember though- Only listen to people with NO motive either way (ie they don't sell turbos or BOVs), only listen to people with proven experience on the matter, and remember, someone saying "Well I've got a BOV and my turbo hasn't ever broken" is like me saying I've got a magic stone that protects me from Gorillas and that's why I was never attacked by Harambe...

Stav