Aftermarket BOV, performance or wank factor

[Steve]

eg my stock R31 turbo passage was brought out with no bov at all, not even a plumb back so it makes a cool flutter

some one here believes it does make a difference to NVH.

Emissions - once again, this is just theory - if no BOV, the compressed air reverts out of the turbo, and keeps going - back out to atmoshpere, this would have a similar effect to an atmo BOV.

Turbo servicing: if it was the same turbo, but the servicing schedule changed once it was fitted to a car with BOV, then yes, good point. If its a different turbo, different story - how many cars do you run nowadays with the sierra turbos? didnt you seem to think that alot of the problem with the sierra turbos was the type of turbo, being old school bush bearing? Did you fit a bov to a cosworth and notice any improvement in the life of the turbo?

The jap D1 driver I am talking about doesnt rebuild his turbo after every meet, certainly not 3 times in a weekend of driving - which, by your reasoning, would be the case - after all he uses a bush bearing T78 and runs 2 bar boost. He also uses no bov on his street car, which is in a pretty wild state of tune, and doesnt have dramas.

I know others too, James Vahoumis, when he was running a T518Z with no bov on his drift car, never had to rebuild his turbo, and argueably he runs that alot harder than most people would run a road car. When he pulled the turbo off to upgrade, it was in perfect condition after many, many drift sessions and competitions where it spends most if its time on full boost, with lots of on/off throttle, more so than a circuit car would experience.

If you are rebuilding your turbos every 2,000kms (track car) and D1 cars can make many meets without turbo failure problems - the point is mute. you could even say (using the comparison of different cars and different turbos, as you did above) that using NO bov is better than using one!

Once you start pushing any limits, the longevity of parts is effected - the brighter it burns, the quicker it burns out. If longevity is the ultimate goal, then OEM is probably the best way to leave your car. If you want performance, it costs more than just the parts you fit.

You must finish to finsih first, but you must have a performance advantage to finish first.

Out of interest, what aftermarket bovs do you use? do they increase performance over stock parts - or just longevity?

[Steve]

Simple question, do aftermarket BOV actually improve performance or do they just make a cool sound

depends on if the stock one is working ok or not. refer to elbee111's post

Are they better than the stock ones at reducing lag or do they do the same job just soundin better??

refer to above

I ask cause a few ppl ive talked to think they just sound cool and some ppl have told me they actually reduce performance cause they let more air out to get the sound and thus it takes more spool time once the clutch is let out again?

This is what I have found, and what I was advised by a D1 driver amongst others - once again, refer to previous posts.

Sydneykid, your turn

[Roy]

Not really related, but kinda:)

PPL say they get better throttle response with no BOV. My own experience iisnt in line with that...different cars etc etc i suppose it isnt surprising.

But the way i think of it, is Steves post is kinda right, all a turbo does is move air. You want to maximise the cfm coming out of the turbo to your engine, so what if there is a huge spike in boost, its not the elevated pressure that is going to give you good throttle response when you get back on the power...you want that turbo spinning as quick as possible, and with reduced exhaust gases driving it you dont want the compressor side of the turbo trying to pressurise anything, you want it to be flowings air to the least restrictive tract as possible, that would be an open system, ie with a BOV

So just as your about to jump back on the throttle you can have either high inlet pipe pressures and little cfm (no BOV) or you can have next to no pressure but high cfm (with BOV). When i crack the throttle open adn the engines pressure signal drives that BOV closed i want all the cfm ready and available to me:)

Thats the way i look at it anyway...

[pentae]

If its of any consolidation, and I have limited mechanical knowledge on the topic, I've always felt that cars with stock bov's feel faster than those with aftermarket ones.

Whats wrong with the stock BOVs?

[Rolls]

Whats wrong with the stock BOVs?

They dont sound cool : )

[Sydneykid]

I think Roy hit the mark with his post, airflow in the wrong direction and a stalled compressor logically must display poorer throttle response than airflow in the right direction and a free spinning compressor. To that I would also add that a stalled turbine is going to have more resistance to flow than a free spinning turbine.

As for reliability, the reverse direction of rotation (backwards) changes the thrust. Instead of the compressor being forced back towards the core, it is being forced forwards out the compressor cover. This has an interesting effect on the thrust loadings, particularly in a plain bearing turbo.

I have had this discussion many, many times since 1989, when GTR's came out with twin BOV's. We always end up back at turbo reliability and performance. The other items, such as emissions and noise, can be explained away with other factors.

PS; Personally I prefer the standard GTR BOV's, they don't leak and are free flowing, all that I ask from a BOV.

[netrati]

Wow you guys really get into it! Personally I don't run a BOV, check out my sig. Most tuners in Japan advise against them. I have alot of friends running fairly big "drift" turbos without BOVs for 4+ years without any problems. I figure I won't even have the same turbo a year for now because I'm constantly changing parts.

[xRHETTx]

some one here believes it does make a difference to NVH.

Emissions - once again, this is just theory - if no BOV, the compressed air reverts out of the turbo, and keeps going - back out to atmoshpere, this would have a similar effect to an atmo BOV.

Sorry but IMHO i think your statement is not quite correct. Since the "reverted" air goes out via the AFM (or just slows the air coming in through the AFM), emissions are not affected, as the engine is reading the correct amount of air flow. That is, no "extra" air is being lost due to the atmo BOV.

This leads on to another idea of mine...

Could you make an amto BOV legal/work better by having an AFM measuring air coming out of the BOV? Then subtract the voltage of the BOV AFM from the air inlet AFM. The ECU could then use this to work out everything. No more stalling etc when coming to a stop, and better fuel economy... or am I just dreaming?

[netrati]

Wow two MAFs, one for the engine the other for the BOV, I have trouble trying to keep one MAF from malfunctioning. There are already plenty of ways to correct stalling with a atmo BOV.

Could you make an amto BOV legal/work better by having an AFM measuring air coming out of the BOV? Then subtract the voltage of the BOV AFM from the air inlet AFM. The ECU could then use this to work out everything. No more stalling etc when coming to a stop, and better fuel economy... or am I just dreaming?

[JNR24]

First issue:

Drag cars and BOVS??

Does somthing look weird there?

Why would a "Drag" car be closing its throttle? (not to mention running off boost?)

And please dont tell me that "Pro drag" cars run bog stock 5 speed boxes, as most are either running Glides (off-street boys), or air assistated clutch setups, or Holinger boxes (Pros)

If a Drag Racer is depending on a BOV for a time diffrence, he shouldnt be drag racing. Drag cars should be on Boost for the entire 1/4mile.

Sounds a bit bizzare to me, even if they only release at the end of a run...

Second point:

Guys think outside the square, nissan designed this car from a balance of the following:

economy, reliability, & performance. Not JUST performance. (Refer to Sydneykids post above) These are "Production Cars" produced for the Masses! Not D1 Drivers.

Nissan has spent Millions of Dollars on R&D, more than ANY aftermarket garage in the world would spend on R&D. This is because they are not performanced focused, but a balance between all feilds.

BOVs are there for a reliability reason, & perhapes a small performance benifit....

And they play such a small role in in turbo reliability anyhow, there need is questionable. Cooling, and Oil flow/quality would play a more major part in turbo reliabilty than a BOV.

Cheers,

Trev

[Tonba]

++++

Greetings All.

Correct me if im wrong, im not exactly a turbo person, as in i know more about off turbo responce and N/A, BUT....

If you are running NO BOV, let off the throttle, the compressd air hits the T/b, and begins the return trip to the turbo...im right so far...?

wont the wastegate reliese some exhaust pressure so the turbo slows down enough for no damage...the air will hit the compressor wheel and exit through the intake making a cool fluttery noise...but because there is no power spinning the compressor, it will slow down... and you will have little if no shaft play???

Cheers,

--Tonba

++++

[Motorman]

There are already plenty of ways to correct stalling with a atmo BOV.

Netrati, what ways are there?

I have a 34 with power fc and hks atmo bov; after installing the bov (but before the pfc) i had the following issues:

1/ backfiring between gears (obviously unburnt fuel in the exhaust due to confusion of the afm)

2/ on idle it was like the engine was 'missing a beat', almost like it was dropping a cylinder.

3/ stalling on decelleration.

I mentioned these problems to the tuner before the pfc was installed and he said to some degree he could 'iron' them out - I am assuming he would lean out the fuel mixture at idle rpm to stop the idle problem, i'm not sure exactly what he would have adjusted to stop the backfiring. When I got the car back 1/ and 2/ were completely fixed, but 3/ was hardly any better.

Any suggestions?

P.S I would change back to the standard bov but I can't stand the sound of the skyline spool with the trade mark stock bov, also it only really stalls if you try and make it do it..

[Steve]

Wow you guys really get into it! Personally I don't run a BOV, check out my sig. Most tuners in Japan advise against them. I have alot of friends running fairly big "drift" turbos without BOVs for 4+ years without any problems. I figure I won't even have the same turbo a year for now because I'm constantly changing parts.

did any of them say why not to use one? would be interesting.

Since the "reverted" air goes out via the AFM (or just slows the air coming in through the AFM), emissions are not affected

this does effect emissions - crank case gasses are mixed with the charge air from a turbo. Emissions requirements state that all crank case gasses are burned in the engine. Air reverting out of the compressor would contain these gasses and therefore a car without a bov would not comply. With a recirculating bov, the compressed air is just fed back into the turbo inlet, and therefore back into the engine.

Any air going out the afm would increase the amount of fuel being dumped in by the ecu, it cant tell what direction it travels, only that it has passed.

[Steve]

Oh, and tonba, the wastegate is on the exhaust, or turbine side, it controls boost - has nothing to do with charge air directly.

[Steve]

airflow in the wrong direction and a stalled compressor logically must display poorer throttle response than airflow in the right direction

yes, but that is a bit simplistic IMO

How much does the turbo slow down? how long does it take to slow down or stop completely ( I doubt it does stop completely until the engine stops running)

The turbo doesnt just stall as soon as you lift off the throttle, it takes a several seconds to wind down completely.

Exhaust gasses will spin up a turbo very quickly, and provide more energy to spin the turbo than the boost pressure (if this wasnt the case, a turbo wouldnt work - or need a wastegate) - more pressure on the inlet side, means more exhaust gasses more quickly.

Throttle LAG (time taken to build boost) is going to increase the larger the quantity of air needed to be pumped to pressurise the intercooler and pipework to full boost pressure - sometimes, with a bov, it seems like seconds (thats what it seemed like at the time - waiting, waiting, damn too long, better clutch it go get the damn thing back on tap).

The quicker you get exhaust gasses moving at speed, the quicker you will have boost, the less air that needs to be compressed in the intercooler and inlet pipes, the quicker you will have boost.

As someone else mentioned, if you stop the air flow around a fan, it increases in speed, perhaps due to the lack of resistance (bit like a vacuum cleaner getting blocked - no air being moved, speeds up), so reverting air through a turbine wont have the same effect as through a positive displacement pump.

[RedEmblem]

Well, I'd like to add three things after reading this thread.

1. I've got a stock BOV on my 33, sounds okay.

2. I've heard from alot of people that BOVs do not increase performance, or, if they do, they increase it by only a very tiny margin.

3. Steve, I like that avatar.

[Roy]

LOL...once again one of my disclaimers, dont state any of the following as fact, just thinking out loud:)

How much does the turbo slow down? how long does it take to slow down or stop completely ( I doubt it does stop completely until the engine stops running)

The turbo doesnt just stall as soon as you lift off the throttle, it takes a several seconds to wind down completely.  

Depending on the turbo and application turbos can spin upwards of 80,000rpm (many go higher when running over 1 bar). Now say you have a 63mm compressor, looking at a few compressor maps for say 3-4 psi you need between 30,000 to 40,000rpm. So in order to get an extra 12-14psi you need another 40,000 to 80,000rpm of the compressor. From idle driving off in 1st and 2nd gears think about how long it is taking your engine to get the first 2-3psi (ie 30,000 to 40,000rpm of compressor blade)

So not having any idea of what rpm a compressor is spinning at idle, 2,000rpm etc...too many variables such as displacement and turbo size...but you get the idea that if i stall the compressor to the point where the (LOL) the chooo-chooo-chooo noise is stopping you are effectively running out of the cfm that will actually provide response when you crack the throttle open.

So you are slowing the compressor to approx 20,000rpm, and to get full boost/cfm back when you crack the throttle you need to accelerate the compressor some 40,000 to 60,000rpm again, which puts heat into oil, adds back pressure to the exhaust gases etc etc all on top of dulling throttle response

The idea is for the turbo to never slow down at all, reality is without anti lag etc it is going to but its something you want to minimise

Exhaust gasses will spin up a turbo very quickly, and provide more energy to spin the turbo than the boost pressure (if this wasnt the case, a turbo wouldnt work - or need a wastegate) - more pressure on the inlet side, means more exhaust gasses more quickly.  

.

The turbo isnt providing energy, its converting energy. What i think you are saying is that the energy provided to spin the turbine with exhaust gases is higher then the energy required to overcome the inertia of the compressor and friction of bearings etc.

And at closed throttle there isnt a lot of air passing thru an engine so little exhaust gas to spin anything for that fraction of a second...so again we are not wanting to have to spin up a turbo, we want it to stay at as high an rpm as possible

Rather then more pressure on the inlet side, its is a matter of storing the kinetic energy in the turbo and using what exhaust gases are available to maintain/increase turbo rpm.

Throttle LAG (time taken to build boost) is going to increase the larger the quantity of air needed to be pumped to pressurise the intercooler and pipework to full boost pressure - sometimes, with a bov, it seems like seconds (thats what it seemed like at the time - waiting, waiting, damn too long, better clutch it go get the damn thing back on tap).

Thats interesting that you notice that. I have found the opposite, perhaps because the TD06 is on the larger scale of things for an RB20 it is a little more sensitive to shaft rpm/cfm/boost etc. If you have shaft rpm then you will have boost on demand when you crack the throttle. With the BOV open you have an open system so never going to have pressure.

Consider this and see if it makes sense. The BOV opens when you back off, and closes when you are back on the gas. So an inlet without a BOV will naturally spike in pressure as it is effectively a fixed volume with the turbo trying to pass air into it. Now because of the tolerances of the turbo being what they are the air/pressure will actually bleed out past the blades and filter. In doing so it slows the compressor, the extent of which is debatable.

Now imagine you have a BOV, then when you back off the inlet is an open system so is never going to be easily pressurised. With air being lazy its taking the easiest route out of there and passing out the BOV. This means the compressor isnt trying to squeeze air into a fixed volume and the airflow direction is more uniform and the compressor shaft speed is less likely to drop. Its still spinning and supplyign scm, its just that its being routed somewhere other then the throttle body.

Now imagine if i shoved my ham fist in there whilst the throttle body was still closed and jammed the opne BOV closed. With the tb closed its like pretending the BOV is the open throttle body. What you would witness is the exact same thing if there was not a BOV installed and the throttle body closes, a spike in pressure and nowhere for the air to go, But this is not the case. The BOV closes when the throttle body opens...so the air is still there.

So what happens when you crack open the throttle...you have a closed system that already has a good supply of flowing air from the spinning turbo all ready to pass thru the engine.

Now the thing i cant see working is the no BOV pressurised system etc. Consider the volume of air in the pipes/cooler etc and then open the system because the throttle body has been opened.

1 bar is jack all pressure in the real world. And to lose so little pressure would require only a small increase in volume, ie the plenum and hungry inlet valves. So what stored energy/pressure you had in the inlet pipes/IC is now gone and you are left with a shortage of cfm because the turbo has slowed down and airflow that is idle/sitting around as it had nowhere to go. Its possible the airflow is headign back towards the air filter.

As someone else mentioned, if you stop the air flow around a fan, it increases in speed, perhaps due to the lack of resistance (bit like a vacuum cleaner getting blocked - no air being moved, speeds up), so reverting air through a turbine wont have the same effect as through a positive displacement pump.

I dont really understand that will have to have more of a think, but it doesnt sound right...for one they have a 240V control system, which when it isnt passing/sucking the right amount of air they could have a form of speed control that winds up the frequency...not really comparing the same system. We are talking about something that is translating the exhaust energy from an engine.

A better example using your scenario would be to have two vaccum cleaners, and isntead of a 240V motor providing the energy, you are using the gases/flow from one to power the other...now imagine what would happen if you turned off the vacuum powering the second vacuum and expected the other to keep sucking air when you have given it a volume of duct to pick up (ie increased the systems pressure) rather then let it just suck/pass air.

LOL...no one will read this...i am mad i tell you, mad. But will be great if ppl take the tiem to read it and offer their thoughts:)

[Steve]

Theory is such a wonderful thing bit like statistics....

I dont agree with what you are saying, for two reasons.

1/ I dont see how the volume of intercooler/pipework can be pressurised as quickly by a turbo spinning at near full speed, versus a slightly slower turbo with a nealy fully pressurised intake system - a BOV will dump ALL the inlet pressure almost instantly, vs reversion which takes time.

2/ Unlike you, I have experienced this myself, as have others I know.

With the whole fan thing, speeding up, look what happens when a turbo crosses the surge limit, the wheel increases in speed, but no extra air is being pumped.

Its amazing that a debate like BOV still attracts such conjecture and differing opinions

Any of you guys who are into drag racing your street cars ready to take up the challenge of bov vs no bov - so its not a racing car or drag only car. It would be interesting, as I know of one guy who said he consistantly lost between 0.1 and 0.2 seconds without a bov. Woud be nice to compare.

Here is a challenge for you Roy, next time you have your ride in an appropriate place, try getting it sideways and mid slide just back off ever so slightly util the bov lifts - then see how long it takes to come back up again.

[Sydneykid]

Let's kill the fan thing.................

Are we talking about

1. blocking the airflow into the fan (ie; from the rear)?

2. blocking the output of the fan (ie; from the front)?

If you are blocking the rear, then the blades are spining in a partial vacuum, lower resistance, hence the fan goes faster. If you are blocking the front then the fan slows down, because the air is bouncing back (reverse direction) and this creates drag on the spinning blades.

When you have reverse airflow (no BOV) it is the same as #2, not #1 (that would be like blocking the compressor inlet)

Here is a challenge for you Roy, next time you have your ride in an appropriate place, try getting it sideways and mid slide just back off ever so slightly util the bov lifts - then see how long it takes to come back up again.

We do that twenty or so times every lap in a race. On the data log I can see how long it takes the driver to apply acceleration (via the TPS). I can see how long it takes for the boost to build (via the MAP sensor). How long it takes for the car to accelerate and how fast it accelerates (via the G force sensor). On our cars we tune the BOV to ensure the fastest boost build and acceleration.

It is worht mentioning that we have a PPG dog box which changes gear in milliseconds, so there is very little time for the BOV to open or the airflow to change direction. The real advantage comes when you have to get of the throttle without changing gear. Then the rate of acceleration is very important and that where the BOV is worthwhile.

Lastly, on the engine dyno we can actually see the compressor blades turn backwards when we abruptly shut the throttle with no BOV. It is very easy to see using a timing light as a strobe. In fact you can see the large puff of reversion back out the compressor inlet.

[hypntk]

Thanx, i think i figured it out. BOV are important in the job they do, whether you have a stock one or spend money on an aftermarkert one is personal preference. Ok heres another question for you all while im on the topic. My stock boost is set at 7psi ( for now anyway ), the aftermarket BOV i wanna put on is from a m8s R34 which was set at about 15psi, so the question is: Can i just wack on the aftermarket one and it will be ok or do i have to adjust the BOV so it works with 7psi. This may be a silly question but does the amount of boost make any difference to the air pressure left in the piping when the throttle is let off??