Question about BoV on RB25DET NEO

[GTSBoy]

Driveability will be about the same with either externally venting BOV, or no BOV at all. Perhaps worse one way than the other, with me thinking that the definitely more flow going through the AFM through a venting BOV more likely to cause rich stalls than the perhaps more flow that the AFM might read on reversion.

There is no such thing as "turbo damage" from not having a BOV at stock, or even quite a lot higher than stock, power levels. You need a big turbo with a lot of mass spinning hard getting a horrible slowdown from a slammed shut throttle before there is anything like the shaft loads required to damage things. Not an issue on small turbos.

The ONLY 2 reasons that Nissan put a recirc valve onto the RB were:

1. It is a bypass valve. It is open when under vacuum. When not on boost, it bypasses intake air forward around the compressor, which unloads the compressor, allowing the turbine to sping more freely, making the whole lot a bit more efficient when just puddling around. Throttle response should also be faster via the shorter, smaller diameter BOV pipe (when in NA, ie before the BOV closes and boost is building) which is nicer for driveability.
2. Emissions. The reversion causes CO pulses. Eliminate the reversion (or at least, keep it away from the AFM) and you don't get that.

The stalling/driveability aspect could have been tuned around, as shown one example of by dose above, if Nissan hadn't put a recirc valve on. Many many turbo engines before the RB had no BOV. They did not stall. See the RB30 turbo as an example.

Nistune is definitely better than just stock ECU. It allows you to access and change things that are not excellent on the stock setup, and allows you to do mods like put decent injectors in, relocate the AFM, put a bigger turbo or even cams, etc, on, change to coil swith completely different swell needs, etc etc. All the things that you might need or want to do 25 years after the car was new.

Aftermarket replacement ECU is obviously better again, because it gives you even more freedom from the constraints of the stock ECU. I won't be needing to go any further than Nistune though, for the new turbo in the 250ish rwkW region I'm going to, with big injectors, and most other things being stockish.

[GTSBoy]

Oh, I also meant to say that it is possible to use a venting BOV and restrict its outlet down so that it does vent enough to prevent the sututu, but vents slowly enough that the AFM doesn't see all the air at once and makes it easier to avoid stalling.

[Kapr]

Yeah from what i read this couple of days about BoV i thought so too that atmo BoV would be "worse" in my case.
And yes i think/hope that on stock boost it should be ok. I dont want "no BoV" but that is just what i get. Will sort it this out later.
I drive it kinda gently even in N/A state so you bet your *ss i will be even more carefull with turbo...and of course still having the old box(but that is getting sorted) 
I thought that would be good to have at least Nistune. But i dont know what "tune" if any is there. Car(engine from that car) is running stock right now.
Yeah for me i think that Nistune should be enough :-) 

Oh i dont know that. And what outlet you mean?
The one that goes to the J-pipe or that "big" one that vents to the atmo?
If iam understanding this right you mean that circle one? So i have "booth worlds" like blocked but vented enough? 
Does this not make car/engine more confused? 😄 

 

Edited yesterday at 08:23 AM by Kapr

[GTSBoy]

4 hours ago, Kapr said:

Oh i dont know that. And what outlet you mean?

A BOV that both has an external vent and a return pipe connection is a hybrid BOV. I wasn't talking about one of those. If you bought one of those, I'd instead just buy a return only one.

I was talking about a BOV that only vents. I mean doing something, yourself, to restrict the outlet. Jamming something in there, welding something on with a ball valve on it so you can adjust it, etc etc.

[Kapr]

Yeah the hybrid you mean one of these? https://youtu.be/kL518ThmzSw?t=118

Yes that one that only vents is that i post above( GReddy FV2) right? That FV2 has some type of screw/valve where you can control how "hard" the spring would...but i think even on the hardest it would not be "good"...i would need to restrict this "more"...i consider this 🙂 
And car should be ok(or rather same) with this like with no BoV?

[GTSBoy]

Yes, that's a hybrid BOV.

The adjustable spring load ones are about making teh pressure build up higher before venting, so you get a more agressive whoosh. That is not what I meant or what you want.

By "restricting", I don't mean "keep it closed until the pressure is higher, then open it fully". I mean "open it as soon as it is required, but only have a small port area, so that the flow rate out of it is slowed down". The pressure will rise to be higher than a bigger ported BOV would provide, but it would do while it is venting. The idea is to slow down the rate at which the replacement eair is flowing past the AFM, to reduce the magnitude of the air flow signal peak, to reduce the amount of fuel that is added per revolution.

[GTSBoy]

Note that I'm not suggesting this is a great idea. Just saying that it is something that could be done to achieve a better outcome than the more shit ideas will provide.

[MBS206]

Something to note, on an aftermarket BOV, it's quite possible to end up with the worst of both worlds, where on light throttle, there isn't enough force to open the BOV so you get small reversion and driveability issues, and with the venting to atmosphere, it dumps a lot of fuel on shifts.

Realistically, dumping to atmosphere under power isn't a major driveability issues, compared to the issues of stalling from reversion. If you're getting up the throttle, then putting the clutch in, say on a shift up, but then don't release the clutch, yes, the engine can and will stall, but, just drive around it, and that's easy, let the clutch out in the next gear and the car rolling will keep it all running / going.who wants a hard hit of throttle and then to just let the motor idle anyway?

 

Main issue for driveability is blipping throttle and it wanting to stall, like when parking or trying to move in slow traffic.

Edited 19 hours ago by MBS206

[joshuaho96]

On 15/05/2025 at 4:07 PM, GTSBoy said:

Essay time.

First things first, an RB running stock turbo and boost levels shouldn't get so bad as to stall from reversion if the recirc valve has been deleted. Should get a little fluffy and annoying, but in my experience, not so bad as to stall. Of course, every car is a bit different, so it remains possible that stalling will happen.

So, running with no recirc valve is somewhat of an option, for otherwise stock stuff.

Atmo BOVs cause all sorts of shit, even on an otherwise stock setup. Only gets worse the higher the boost and the bigger the turbo. At that point you really need to go for a different ECU and no AFM.

Rebuilding the stock recirc valve configuration is not hard. You just need a stock or aftermarket BOV with the appropriate adapter for the 2 bolt flange on the back of the J pipe, and to get/make an appropriate ~1" pipe to get the air back to the turbo inlet, and to possibly modify the inlet (if it is not stock) to take the recirc pipe back in. Not hard. Just takes some cutting and welding.

Putting an R35 type AFM into the car anywhere is not as simple as just buying the AFM and throwing it in. You will also need to buy the appropriate boss that will then need to be welded onto the pipe where you're installing it. You can clearly see why by looking at the photo posted above. They are not a "simple" swap for a stocker. You can't put on in place of the stock AFM.

You can put one in place of the stock AFM, if you get the mounting boss and weld it to some pipe and otherwise make that pipe piece work like the stock AFM housing. Or you can buy such an adapter, either complete with the 4 bolt flange for the air box, or without, for varying degrees of work needed to then make it fit your stock airbox or some pod filter or whatever you have going on.

Oh, and the R35 AFM is not plug and play. The wiring is different, but changing that is trivial. The plug is also different so you either end up repinning the original wires onto the new plug, or you just use a short adapter.

If you weld a boss to the cold side pipe, the cold side pipe really wants to be 3", otherwise the scaling on the meter can get a bit weird, but whatever the pipe size, it's not as easy as just using the (fully documented in the Nistune doco) simple method for choosing R35 AFM in X" pipe size in the software, because the scaling will already be a bit different. Anyway, all of this has been comprehensively worked through on the Nistune forums, so there is full knowledge available.

I would use a Link/Haltech before I would bother putting an AFM into a cold side pipe. That's a lot of effort for a bodge.

Nistune is great, can work well even at fairly high power levels, but you are stuck with the limitations of it being the stock ECU, which includes needing to use an AFM, which is not always convenient for every set of modifications. You have to have a think about what you already have, what you want to have, and decide early if you'd be better off jumping ship to an aftermarket ECU. This so you don't waste time and money doing things 2 or 3 times.

Never heard of ECUmaster. Sounds like a backyard operation. If there are good tuners for it where you are, and it is a solid product, then it will be fine. We're only talking about an R engine here. Back in the day they all ran on crude nasty early 90s ECUs and they were fine. You don't need a rocket surgeon's ECU to run one.

I tell people all the time this stuff runs on a 2 MHz 8-bit MCU. The math is a bit arcane, they do a lot to get around their limited resolution and so on but it works great as-shipped. So if an 80 to 160 MHz modern 32-bit MCU can't make it work you've really screwed something up badly. Your average graphing calculator is faster these days, god forbid we start talking about what's hiding in vapes or some stupid internet-connected "smart appliance". 

On 15/05/2025 at 7:26 PM, MBS206 said:

ECUMaster is an Australian company from memory, been around a while, and seem to actually play really well on more unique engine setups. Seems to be more of them being used in the states, and they seem to support more cylinders than most others.

 

My opinion on stock ECU/Nistune with he AFM, is you can tune around the issue of stalling. It's still a bit of a band-aid, and not the most perfect smoothest setup, but it can make it drive able and not an absolute PITA. But you're still compromising.

A vent to atmosphere BOV has the same issue as no BOV with regards to the AFM. Both make the ECU think more air is entering the engine than there really is.

And I can't find any hard science/decent evidence that supports the claims all the "you MUST run a BOV of some sort of you'll blow the turbo up".

About the only one some people can give of why is "lag between gear shifts", and that is only an issue when your boost source for the waste gate is pre throttle body. I ran my boost source for the TD07S off the inlet manifold. Didn't have weird boost spiking issues, didn't have boost control issues, and on a gear shift the wastegate slammed shut, which meant all exhaust gases would be keeping it going round and round and not waiting for the wastegate to shut again when I stepped off the clutch and back on the loud pedal. It came back on boost in the next gear pretty much instantly.

 

ECU / tune / BOV choice I think really comes down to a lot more factors than just what you've stated. And personally, especially for daily driving, recirc setup for the BOV, or no BOV. I dislike the really loud BOV when everyone is taking off from the lights etc.

I suspect the lore about off-throttle surge damaging turbos must be very, very old if it was ever true. I still don't think off throttle surge is good for turbos because you're rapidly having air going in and out of the turbo on top of rapid loss in shaft speed which presumably does impart some kind of unusual load on the rotating assembly. As for the recirculating valve, I believe Nissan actually in a press slide deck of some kind explained for the RZ34 they found it improved response. Prior to that they kept the throttle cracked open instead which causes rev hang and because the throttle/engine is a substantial restriction a ton of the energy in the air stream is lost just to pumping past it. Recirculating helps reduce energy loss by bringing the intake side of the compressor up in pressure so it's not working as hard to pull air in. In DCT cars where they can slam gears I believe Porsche hangs the throttle wide open with no fuel injection to keep turbo RPMs as high as possible during the shift.

As for running post-throttle boost source, I would be concerned that at partial throttle on boost you would have driveability issues. It would be constantly trying to run crazy amounts of boost before the throttle to compensate for the throttling effect. Maybe with a much more sophisticated boost control setup you could activate a solenoid that goes from a conventional 3-port setup to something that allows the wastegates to see full vacuum in that specific case? 

[MBS206]

2 minutes ago, joshuaho96 said:

As for running post-throttle boost source, I would be concerned that at partial throttle on boost you would have driveability issues. It would be constantly trying to run crazy amounts of boost before the throttle to compensate for the throttling effect. Maybe with a much more sophisticated boost control setup you could activate a solenoid that goes from a conventional 3-port setup to something that allows the wastegates to see full vacuum in that specific case? 

Never an issue with boost control. No weird boost issues pre throttle at partial throttle like you're suggesting. It worked.

 

As for all the turbo damage claims, they all were started by aftermarket BOV companies.

At least turbo timer companies had SOME truth about them, except people failed to realise that except for in extreme circumstances, that small amount of driving you do to park/ through pit lane etc, is enough to not have a need for a turbo timer.

[The Bogan]

35 minutes ago, MBS206 said:

Never an issue with boost control. No weird boost issues pre throttle at partial throttle like you're suggesting. It worked.

 

As for all the turbo damage claims, they all were started by aftermarket BOV companies.

At least turbo timer companies had SOME truth about them, except people failed to realise that except for in extreme circumstances, that small amount of driving you do to park/ through pit lane etc, is enough to not have a need for a turbo timer.

I believe when they added coolant to cool the turbo it negated the need for turbo timers as water continues to flow even when the engine is shut off

I forget the actual terminology, I'll just say the coolant "percolates" through the turbo which stops the oil cooking itself to death 

Well....that's how I see it, as all the plant at work that has coolant lines to the turbo can be shut down straight away, whereas the older plant with only oil feed to the turbo you need to idle it for a few minutes

[GTSBoy]

1 hour ago, joshuaho96 said:

As for running post-throttle boost source, I would be concerned that at partial throttle on boost you would have driveability issues. It would be constantly trying to run crazy amounts of boost before the throttle to compensate for the throttling effect. Maybe with a much more sophisticated boost control setup you could activate a solenoid that goes from a conventional 3-port setup to something that allows the wastegates to see full vacuum in that specific case? 

In this case, the issue I see is that if the throttle is part open, to give the boost/power you're wanting, and the boost reference is from the plenum and thus there is less boost than the boost target, then the controller is going to have the wastegate more/totally closed and you're going to have higher EMP than you otherwise could have, if the boost reference was in the tract upstream of the throttle. More EMP is never a good thing, except in terms of response.

So, in this instance, you would have really good throttle response, because the turbo is already slogging away hard and there is pressure upstream of the throttle to be had. Just with the penalty of more ex manifold pressure, with the usual consequences of that.

[GTSBoy]

1 hour ago, The Bogan said:

I forget the actual terminology, I'll just say the coolant "percolates" through the turbo which stops the oil cooking itself to death 

Thermosyphon.

[hardsteppa]

2 hours ago, MBS206 said:

As for all the turbo damage claims, they all were started by aftermarket BOV companies.

Yes. And all the world's automotive manufacturers and their engineers fell for this myth and incorporate bypass valves on all their modern turbo cars, and leading turbo manufacturers such as Garrett and Borg Shane Warner also fell for this myth and incorporated bypass valves into many of their turbochargers.

[hardsteppa]

To the OP - I run that Greddy BOV (recirculated back into the intake pipe, not blowoff), as the stock R33 RB25 BOV did leak at around 19psi on a boost leak test. I haven't had any issues with the Greddy.

[GTSBoy]

32 minutes ago, hardsteppa said:

Yes. And all the world's automotive manufacturers and their engineers fell for this myth and incorporate bypass valves on all their modern turbo cars, and leading turbo manufacturers such as Garrett and Borg Shane Warner also fell for this myth and incorporated bypass valves into many of their turbochargers.

But they do so for the other reasons to have a compressor bypass. It's in the name.

[MBS206]

48 minutes ago, hardsteppa said:

Yes. And all the world's automotive manufacturers and their engineers fell for this myth and incorporate bypass valves on all their modern turbo cars, and leading turbo manufacturers such as Garrett and Borg Shane Warner also fell for this myth and incorporated bypass valves into many of their turbochargers.

I see you missed the rest of the conversation where they have benefits, but nothing to do with avoiding breaking turbos, which is what the aftermarket BOV made all the fan boys, tuners, and modders believe was the only purpose for them...

[Yeetus]

I've tried all different combinations of BOVs/ no BOV and stock bypass valves over the years, on gear changes the stock bypass valve seems to get the car back on boost quicker because in part the turbos wheel speed isn't being slowed down by reversion, although they have issues holding boost much over the stock setting.

Most aftermarket BOVs you can adjust the spring, tighter will make it open later and close sooner, but in my experience it'll cause a bit of flutter at low load/rpm anyway.

I've also got some input into this whole no bov causing turbo wear, never had an issue on any on my turbos HOWEVER, I got my R33 GTST with 200k kms on it, with from what I can see still has the original turbo, no lateral shaft play but has about 4-5mm of play in and out which to me seems like a worn thrust bearing from years (100-150k kms?) of turbo flutter running no bov, so maybe there is some truth to it in the long run. But that'll never stop me loving the Stutututu while I have the car.

 

OP just wants to know if he can run a atmo vented BOV with no major issues and the answer is YES, plenty of people do it, there's no harm in installing it and seeing how it runs before spending $$$ on an aftermarket ecu, last time I bought a Nistune it was $2400 for install and a tune , unsure of todays prices but you get me.

Crazy money to spend just to fix the minor inconvenience of stalling that can be overcome by letting the revs come down to near idle before putting the clutch in or a little bit of throttle to avoid it. You're better off leaving the ecu and tune for after a bigger turbo/injectors have been installed to take full advantage of the tune and get your moneys worth.

 

Let OP have his Whoosh sound without trying to break his bank haha

[Dose Pipe Sutututu]

BOVs do have a purpose, if you ever log pressure before and after the throttle body, you will see a spike pre throttle on lift off from a WOT condition.

Enough to bend throttle blades / damage e-throttle motors or simple assist in blowing off cooler pipes.
FWIW, the above on really applies to those running at least 2 bar of boost.

OP shouldn't have an issue, on the other hand, here are some videos of my shit box over a decade ago with some succulent dose with the airbox on and off. That shit box is unrecognisable these days 🫠