BOV - Why have you fitted one, apart from the 'look at me' noise?

old-git

Moderator
Points
657
Location
Essex
Car
Elan & Robin Hood
Car manufacturers don't fit them, so why are they popular with modders? Is it just for the look and noise or is there a performance advantage?
 
I think they have a value in highly tuned throttled engines where the throttle plate is operated my a mechanical cable. Possibly even more so with fixed geometry turbochargers. With full ECU management of throttle plate(s) opening, fuelling and turbo geometry then excess boost can be curtailed using programming instead of a BOV to vent it to the atmosphere.

They are often retro-fitted for their distinctive noise. This, maybe, is the exactly same reason car makers don't fit them?
 
Imho bovs should never be fitted for the noise.
but........... Fixed vane turbos produce boost which is rev related so when the revs drop (changing gear on a lot of cars) the boost needs to be "lost" .
With pretty much all stock turbo engines ( using basic ECUs) this is done using a recirc valve which deflects it into the system on the induction side (hence no noise).
These recirc valve are designed to operate at the stock boost level with a margin. On my engine stock this would be 0.7 bar with the recircs good untill say +50% which is around 1 bar.
But my engines turbos are designed to run at around 1.6 bar and recircs just cant handle that boost but bovs can. Hence their use.
Remember the kid are copying high tuned cars often on the track which only have them because they are deemed an advantage. No serious track car ever had things it didnt need BOVs included.

re controlling boost - It is physical and for best performance you need full boost up to gearchange and once it is there if the vanes are fixed an ECU cant make it dissapear without using a valve of some kind
 
So I am on the right lines then? Variable geometry turbos are less prone to transient overboost than fixed ones. I am guessing that fixed geometry turbochargers suit ultra high performance engines better than the increasingly common VGT.
 
Last edited:
My understanding is that when the throttle is slammed shut it sends a shockwave back that can over time damage the turbo and a BOV will reduce this by releasing boost pressure and by doing so the turbo will not slow down as much as the back pressure is way less and will spool up faster when the throttle is opened again .

I liken it to running flat out at an open door and just as you are about to go thru it is suddenly shut and you would bounce back in the opposite direction.
 
Yes, that's exactly what they do. But if the throttle plate is under ECU management then the driver cannot slam it shut by releasing the pedal if the ECU dictates otherwise. The ECU can still cut fuelling thus alleviating acceleration when none is wanted. A BOV seems to be a crude way of working around the issue. Issues which are more readily dealt with by current engine management systems.

And if the BOV's release pressure is triggered by pressure build up in the intake then the pressure hits the BOV, throttle plate and turbo anyway.

This is why I think that turbocharging suits un-throttled engines (diesels) very nicely.

It is especially good with steady speed engines (air, rail, and marine). Turbocharging of cars is a relatively new application of turbocharging.
 
Last edited:
I was chatting with Graham Hatherway this afternoon and he thinks that BOVs are for idiots who know FA about what they are doing, and that is putting it mildly as this is a family friendly forum :-)

When the BOV releases the intake pressure it removes the air resistance from the compressor thereby allowing the turbine to suddenly speed up. Sudden changes in RPM is what does for turbos, snapping shafts. A properly set up engine with a decent ECU doesn't need a BOV, a waste of good money and BAR, or so I have been informed.

There will be no BOV anywhere near my engine -) When racing I will be full throttle grear changing so the last thing I need is a sudden loss of intake compression!
 
So is GH saying that Subaru,Mitsubishi and Nissan that they knew FA seeing that recirculating BOV's were fitted to Evos,200sx and WRX's:confused:

Logic tells me that when the throttle is closed there is less exhaust gas to drive the turbine so it seems illogical to me that the compressor which is driven by exhaust gasses to speed up but am always willing to learn why this is not the case.

PS am told that the current Golf GTI has a recirculating valve
 
Last edited:
A recirculating vent seems more logical than an atmospheric one. Less noise; less confusion for the ECU trying to account for air that's been metered by the MAF and then disappeared.

I can't see the point of fitting one just to make a noise. It's as silly as playing synthetic engine noises through the audio system in a Tesla Model S.
 
When young, we used to fit playing cards to our bike frames with clothes begs so they flicked against the spokes, same thing just with bigger toys :-)
 
A recirculating vent seems more logical than an atmospheric one. Less noise; less confusion for the ECU trying to account for air that's been metered by the MAF and then disappeared.

I have MAP sensor computer controlled injection so there are no problems with fueling so when the BOV opens only releases air that is cleaner than it was before it passed thru the air cleaner :)

IF you still have MAF then I would recommend a recirculating/plumb back BOV .
 
Last edited:
[QUOTE="old-git, post:

There will be no BOV anywhere near my engine -) When racing I will be full throttle grear changing so the last thing I need is a sudden loss of intake compression![/QUOTE]

Agreed but seeing that you will be flat changing a BOV will remain closed
 
I was chatting with Graham Hatherway this afternoon and he thinks that BOVs are for idiots who know FA about what they are doing, and that is putting it mildly as this is a family friendly forum :)

When the BOV releases the intake pressure it removes the air resistance from the compressor thereby allowing the turbine to suddenly speed up. Sudden changes in RPM is what does for turbos, snapping shafts. A properly set up engine with a decent ECU doesn't need a BOV, a waste of good money and BAR, or so I have been informed.

There will be no BOV anywhere near my engine -) When racing I will be full throttle grear changing so the last thing I need is a sudden loss of intake compression!

Then GH ( whoever he is ) also knows more about turbos than Garrett :confused:
http://www.turbobygarrett.com/turbobygarrett/surge_line

Not trying to start a war just trying to understand where he is coming from and his reasons for his point of view/opinion:confused::)
 
He has an impressive CV |B

What I understand from Garrett is that snapping shafts is not a result of sudden over speed but resulting from sudden increase loading due to the comp wheel having to slow suddenly due to the increased back pressure/stress/loading bought on by shutting the throttle and the compressed air having no way to be reduced due to the lack of a bypass BOV in the inlet tract.
 
So electronic management of the throttle(s) plate(s) is likely advantageous in civvy street. I agree that in racing situations it's necessary to have direct mechanical control of the throttle so atmospheric BOVs might have a purpose. But on a road car it's starting to look like glam/bling.

Not so much look at me, but listen to me :D
 
one thing we all agree on bovs for noise is just plain daft

back to the discussion
On a drag car with flat changes they may well be pointless but on high power road cars they have some point .

My source for that

most of the japanese and uk tuners who all use them (on road & track cars)
 
Just sneaking my pov in here. I will never be able to drive or own a racing car but if I can just get that sensation and thrill from my car when popping down to the shops to get a pint of milk then why deny that one pleasure, even if it is at only 30mph, by labelling me as someone who's just showing off and doesn't know the first thing about engines. I like the experience and a set of BOVs will be going on the car if I ever get a car like my Skyline or 944t again.
.
.
.
.
.
.
Oooh don't know what came over me, need a lie down now ?-/
 
I fitted mine purely for the gains not the noise. My turbo conversion is a mafless, cable throttle operated engine and without venting the excess pressure somehow, the turbo would go a hell of a lot sooner than not having one with the extra tuning it has. Also, the Forge is a lot safer item than the stupid baileys that's was fitted lol.
 
A bov is one of those mods that has a 'proper' function to aid performance but is also used in setups where it adds nothing but the illusion of performance.
 
Just come back from an interesting chat with my engine builder, Geoff Page:

http://www.geoffpageracing.com/about.php

And here is what is in his workshop today, plus an Aston Martin, Renault and two old Ferraris:





At the high end of the tuning industry BOVs are called lag valves, as they remove rather than increase performance by throwing away hard earned boost. Car manufacturers and turbo companies fit and recommend them as they reduce their warranty liability : -)

Yes, BOV's increase a turbo's lifespan but what do you want, performance or longevity? No one can tell me how much longer a turbo will last with a BOV fitted as it depends on the car's usage and design of the turbo.

Pros:
Increases the life of the turbo by an indeterminate amount.
Childish noise.

Cons:
Reduction in power and performance.
Childish noise.

Of course, what is a childish noise to me is a wonderful noise to others :-)
 
Then I guess all turbos he installs will have surge ported shrouds to reduce the loadings to the shaft and trust bearings due to increased stress that happens by suddenly going from WOT to closed throttle butterfly and air flow reversal??

I would hazard a guess that all "his high end" and other builders race cars would have a dog or sequential gearbox that does not require lifting the throttle to change up so that will/would negate the need for a BOV/bypass valve.

PS another impressive CV to bolster your opinion/decision but in racing circles didn't Colin Chapman say something along the lines of "components only have to last long enough to finish and win the race and make it back to the pits"??

PPS am enjoying this discussion and furthering my knowledge|B:)
PPPS an too deaf to hear that "childish noise";)
 
Last edited:
Yes, he did but the two white RS200s are road cars and there isn't a BOV between them :)

All this is way above my head so I have to trust someone. I never wanted a BOV (even nitrous, if fitted, will purge on to the intercooler and not outside the car as I am not into this 'look at me, ain't I cool? stuff) and both my gurus don't like them either so all is good :)
 
My inquiries to Porsche Australia reveals that all of their turbo powered cars have a DV so that leads me to ask how would removing one from say a 911s Turbo improve it's performance and can your engine builder enlighten me/us as to the result/data I am sure Porsche would appreciate his input on how to improve their cars?

Just playing the devils advocate here ;) :)

PS had a look at his workshop and engine building area and all I can say is amazing|B

PPS waaaay above my head as well but I have an inquiring mind:confused:|B
 
The point he made was that manufacturers have to look at reliability, longevity and warranties. Fitting a BOV/DV/RV or whatever reduces th erisk of damage and extends turbo life. I guess that the small loss in performance is a price worth paying.

It 's like remapping. The manufacturer has to make compromises to take into consider different fuel qualities, lack of maintenance, longevity (again), variable climatic conditions, etc. , so has to install a map that takes these into account. This allows tuners to remap in order to take advantage of the latent performance in the engine. Same with BOVs I guess.
 
Cheers Steve ,appreciate your answers and wish you nothing but the best outcome with your Lotus Cosworth |B
 
perhaps he can also explain how much powers bov/ Recirc actually loses
 
Last edited:
Same thing, aren't they? Romove boost and shove it back into the intake. Or have I got that wrong?
 
I
Cheers Steve ,appreciate your answers and wish you nothing but the best outcome with your Lotus Cosworth |B

I am only passing on their opinions. The more I learn the more confused I get :-) Every question seems to have more than one answer. Sifting through to find the correct answer is the difficult bit.
 
You're not wrong there ;)

One has to sort out the wheat from the chaff :)

Now get back to work on the Lotus as I/we all want to see it on the track along with a new class record :)
 
I am not aware of a class record for my car, all I am trying to do is build the world's fastest UK Road legal Elan :-)
 
Same thing, aren't they? Romove boost and shove it back into the intake. Or have I got that wrong?

Bovs and recircs are different.
A bov vents direct to the atmosphere hence the whoosh which gets louder with higher boost
Recircs vent back to the intake. So virtually no whoosh at any boost.

Mine has two blitz bovs . Im told the reason is that Nissan recircs are not designed to hold high boost as the stock engines run .0.7 bar . Mine need to be reliable at over double that .
The blitz bovs on mine will hold 2 bar + boost without leaking.
Obviously leaking valves of any type are worse that useless but there is also a potential problem with using launch control how can you build up boost if the valves can't hold the boost -

Before anyone asked I don't know why they don't make aftermarket recircs that will hold high boost , but logic dictates that if they worked they would exist and I've never heard of them.

But If I ever fit a flat shift holinger gearbox my bovs would be in the bin .
 
It appears that the manufacturers fit a recirculating plastic DV that has a reputation for leaking at higher than factory settings so that's why owners upgrade to a better quality metal unit that will hold higher pressure without leaking.

Manufacturers are ruled by the mighty $ and if the accountants can save a buck or three by installing a cheap plastic part they will.:(
 
Manufacturers are ruled by the mighty $ and if the accountants can save a buck or three by installing a cheap plastic part they will.:(


Hahahahaha yeh look at MG/R lmao!!! I've replaced a lot of plastic bits to proper metal stuff on my T16 lol.
 
Nothing wrong with plastics that are engineered to do a particular job. The problem is that it's generally more costly than a similar metal component.
 
Similar threads
Thread starter Title Forum Replies Date
D Just fitted a turbo charger into my stock engine Superchargers & Turbos 15

Similar threads


Please watch this on my YouTube channel & Subscribe.


Back
Top