Do I need a BOV?

[mr2magic]

Transient boost is in no way improved without a bov! During surge the compressor wheel decelerates enormously...

[kirk]

BOV's were, and still are, made for the same purpose, to stop "funny noises" and complaints to service and warranty departments from owners wondering what the funny noises were.

[ashley]

What is the advantage of removing a BOV?

Quicker response between shifts. The turbocharger doesn't have to fill the entire outlet to throttle body piping again with compressed air.

On the other hand- by relieving the pressure on the engine side of the compressor blade, you allow it to keep spinning freely as the air can still move through it. So when you then open the throttle again the exhaust has to do less work on the turbine wheel to get the compressor to start making boost again. The inlet tract is still full of air, so the job of recompressing it would be achieved pretty damn fast.


With no BOV, the pressure is never relieved and the compressed gas in the boost pipes has no where to go, hence the "funny" noises as the compressor wheel goes into to surge and is forced to slow down, or worse- stall. Open the throttle again and the air has to start moving again, make it's way through the engine and then speed up the turbine wheel again.


Either way you are going to have some form of transient before full boost is available again, and I guess there is a trade of depending various factors such as compressor wheel size, turbine wheel size, engine capacity, rpm, compression ratio, inlet tract volume, cam profile and timing, etc etc

For what it's worth I've seen directly (not on the 3S engines!) a back to back engine dyno test which illustrated that best transient response was gained through using the biggest BOV possible...but that was a very different engine to ours, and relatively smaller turbo than the after market ones we've tend to see on builds here.

I've also (second hand info from a 3S specialist) heard of a stock turbo (rev 5 maybe?) failing to make any boost, with no apparent damage to the turbo. It was sent for rebuild and the query came back as to whether the customer had removed his BOV. The central spindle hard warped, and the the only apparent explanation was damage from removing the BOV....nothing else seemed to fit. What was more telling were the comments from the rebuild specialists who basically mocked the "internet knowledge" that you don't need a BOV, I'm guessing due to the amount of turbos they have to rebuild because of this!

That said- it's the first one that this 3S specialist has specifically been able to blame on this problem in I don't know how many years of working on twos...so it may well be a very rare problem!

No definitive answer I guess, but I'm keeping my BOV

[C35Rob]

1) Compressor surge is when there is a pressure delta between the compressor housing (lower pressure) and the region after the housing (higher pressure) which causes a pressure wave to impact with the spinning compressor. This is completely irrespective of the cause of the surge.

2) Surge can be induced in two ways:
a) The engine is not able to ingest the mass of air being produced by the turbo causing a pressure build up through the intake system from the valves back to the compressor housing
b) The throttle plate is closed suddenly during spool also causing a pressure build up in the intake system before the throttle plate back to the compressor housing.

Surge never can occur without boost or load, prior or during to it's accurance.

As to the remarks on who is sponsoring what and why - that's pure hearsay and not part of the discussion.

I'll repeat myself - you do not need a bov! Having one will definately and without a doubt reduce the stress on the thrust bearings and the turbine shaft.

this is 100% correct, as is ashley's post above

surge or stall is enough to put relatively huge radial loads on turbo bearings, which is especially bad for journal bearing turbos which rely on an oil wedge to prevent metal-on-metal contact and isn't ideal on ball bearing races either, that's totally ignoring the effect that a throttle plate slamming shut will have on the flow characteristics of the air in the pipework (instant turbulent flow) whereas having a relief path will at least give you a fighting chance of maintaining some semblance of laminar flow.

the only sensible reason you could have for not having a BOV would be if you were running a proper ALS with an air bypass valve. the only other reason would be that you want to drive about making sututututututu noises

[2mad]

Do I need to run a BOV if I have a screamer pipe and TIAL wastegate?

I just don't understand the magic of these damn turbos

If not, is it just a case of blanking the intercooler hardpipe where the recirc would usually go and then re-routing a vacuum line?

It's Martin's old ATS TD06 kit btw if that makes a difference.

If you have a ported compressor housing on your turbo ...NO .. if not ...YES bov is needed ... really it depends on the turbo ... if its designed to take the abuse being bov free courses ... yes one is not needed ... most turbos need a valve .... and for the record i love turbo flutter

[CalMac]

I really opened a can of worms here didn't I

Initially I thought that the screamer pipe replaced a BOV therefore I might not need one (yes I'm a dumbdumb when it comes to turbo mechanics ) Turns out they do two different things.

Didn't intend to start a 3 page discussion on the necessity of a BOV on turbo'd cars

[2mad]

Fit one mate .. recirculate .. but one is needed .. unless your ported

[CalMac]

Right. Back to the wanted section I go then

[JAAASH]

I won't be running one on my new turbo and I didn't run one on my last turbo. I'll let you know how I get on though

[2mad]

Right. Back to the wanted section I go then

Not trying to be a d/ck .. but ct's respond to having a valve ... i would never run one with out .

[2mad]

Would not run a gt with out one ether

[JAAASH]

Right. Back to the wanted section I go then

I've got an apexi twin chamber sat in a box somewhere. Needs a new hose and a clean up but otherwise perfectly working, if you're interested.

[2mad]

Right. Back to the wanted section I go then

I've got an apexi twin chamber sat in a box somewhere. Needs a new hose and a clean up but otherwise perfectly working, if you're interested.

Sounds a plan ... ive never run my car bov free .... ive tryed but the car hated it .... buy a nice bov mate

[masterbateson]

With regards to BOV placement, a so called expert told me the BOV should be as close to the throttle body as possible? so that's where mine is at the mo.

Its post the intercooler and pre the throttle body.

But I'm pondering a little if that's the case why did Toyota put theirs on the hot pipe where it is?

Is said expert talking cake?

[danjama]

I've heard that too ^

I have a question. I had two rev 2 turbos. One had recircc, one had a blitz bov. There was a noticeable difference in performance for me, to the point that I now recommend running recirc to Rev 2 owners.

BUT now I have a rev 3, and it has had a forge BOV on since I bought it. I like the chirp it makes, and I don't notice a drop in performance or any issues between gear changes. Would fitting a stock bov with recirc improve the performance on the rev 3? Or is it less relevant due to the MAF system?

[ashley]

As close to the throttle plate as possible makes sense- that way any flow of gas in the inlet tract out of the BOV is in the same direction the gases are already moving. If you had it right by the compressor, the gas in the pipework upstream would have to stall/ reverse flow to relieve the pressure- not ideal if you want to increase transient response when you open the throttle again...anything you can do to maintain flow from the compressor and in the inlet tract should help.

I think...

[C35Rob]

as it happens I'm actually doing Fluid Mechanics at the minute towards my degree.. I'm not going to claim I'm an expert but my understanding is:

the throttle plate snaps shut, flow in the inlet pipework will stall, the result of that is not a reversal of flow but a pressure wave that will travel backwards, at the same time the turbo is still spinning and whatever is leaving the compressor housing will be stacking up in the inlet (until the point it crosses the surge line), at which point the bov will lift (aided by manifold vac and boost pipe pressure) and some air will be vented off

in the real world, placement doesn't really matter, I doubt even the most skilled racing driver in the world would notice the difference, if there is any at all

[jimGTS]

Quicker response between shifts. The turbocharger doesn't have to fill the entire outlet to throttle body piping again with compressed air.

This is 100% correct.

I have experienced this first hand on a laggy turbo.
It is not as noticeable on a smaller oem turbo, but many folk do notice the gains. There is even a thread on the US forums called the no bov club.
Because many doubters have changed there tune, many are running no bov on oem turbo, ceramic and all.

Gains are only there on quick shifting, ie track/drag. You won't get the advantage with general shifting.

People talking of stall clearly haven't read or seen the evidence (scientific data on the US forums showing actual pressure waves, and high boost kept in the pipework vs a atmso bov). Whilst it may slow down the wheel a minute fraction, the fact boost remains in the system means quicker build up of 'pressure' when throttle is open again. Ie, better boost response!

[CalMac]

Jaaash I'm just after a standard recirc tbh! Cheers though

[orangejunkie]

Although i`ve been registered on here for a few years now, i`ve not posted too often. I`ve tended to sit back and use the forum for help advice and tips.

However this thread has got my interest so i thought i would share my views on it.

Running a small tuning business with a friend, you can imagine we get asked this question a lot!

I have always traditionally been against Bovs. Any car i have bought that had one on, has swiftly had it removed going right back to my Rs Ford days. I also prefer the sound not running one provides and even spent some time `tuning` the sound this makes by altering the shape, size and material of the intake pipe. As mentioned, this sound is the `air wave` or pulse in the inlet system, so an alloy inlet pipe will change the sound from a flutter to a `chirp` depending on boost level.

We get on very well with a well known Turbo company who have been helpful when i have approached them on this topic.

Their general view on it was that it largely depends on the boost pressure that was being run. Taking a 400hp car as an example, to achieve that kind of power, you`re going to be flowing a considerable amount of air (around 1.5 bar + ) . At that point, not running one would add additional stress to the thrust bearing and also surprisingly the Turbine wheel. (This apparently takes more of a beating than the assumed Compressor wheel)

His suggestion was, below this you`re ok, although its not `recommended` As for which BOV, he suggested Re-circ is the preferred method as the inlet is an area of low pressure, meaning the charged air will be drawn in and help continue the Compressor speed.

Who knows. I`ve gone back to an OE re-circ and the car doesnt really feel any different, but then i dont drive mine that hard!