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How To Fix/avoid Reversion Stalling.

Philr32

By Philr32,
in Engines & Forced Induction

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Philr32 Proficient

Philr32

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Sorry this has probably been covered a million times but I searched for 45mins+ and couldn't find the specifics. I have an RB25DET from S2 33 with PFC and Z32 and GT3071, it has the typical reversion stalling problem. From what I've read it's to do with the AFM distance from the turbo inlet and something to do with the bends and smooth surface of the piping. What I want to confirm is, some bends are needed and it needs to be a decent distance from the turbo inlet, correct? I'm basically running some silicone hose joiners/bends and the AFM and a metal reducer. Should I be looking at trying to reduce the use of the silicone hose/joiner/bend things and going mainly metal? Would making up a bracket and attaching it to the body help or is that just stupid lol? Any tips, tricks etc?

Hope it was to long winded and confusing.

Cheers!

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Philr32 Proficient

Philr32

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  • Author
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Also BOV return angle plays a big part.

Currently have an aftermarket BOV in the standard place near the TB plumbed into the intake at a 90degree like a T fitting. How should it be plumbed in?

Philr32 Proficient

Philr32

Posted
  • Author
Posted

Thanks Ben, anyone else with more insight? Because it does it when there's not much boost and load too, for example if I'm cruising doing 100km/hr and put the clutch in for a few seconds or if I'm cruising and just clutch in and brake.

Trex Grand Master

Trex

Posted
Posted (edited)

Ben is on the money. Also correct me if I am wrong, but you will never fix the stalling if you are getting reversion from the turbo while using an afm. You will need a map sensor to fix it. (Or make it a blow through setup, though I have never used one or seen one that works well.)

Edited by Stagea97
GTSBoy Grand Master

GTSBoy

Posted
Posted

More or less, yes.

The factory plumbing is a good guide. There's a decent bend and a half in the rubber to the AFM, plus it is convoluted which helps to soak up some of the reverse flow. Smooth bends will always tend to allow a bit more reversion, but they are more desirable for a number of other reasons, so we have to live with them. The angle of the factory BOV return is also pretty steeply aimed at the turbo inlet. I think (per what Ben said) that that is probably the biggest effect.

Be aware though, that the whole factory system, AFM, turbo, BOV, pipework, was designed about an engine only pulling enough air for about 180 fwKW and about 7 psi of boost. Fit bigger turbos, run much higher boost, and there ends up being a lot more air needing to be dealt with when the BOV vents. At some point it becomes inevitable that you have more than can recirculate neatly and some will have to spill back through the AFM. It's hard to know where/when you'll get to that point on any given setup though, and hence whether effort spent on "fixing" reversion will yield results. That said, whenever your BOV return is clearly not right (as yours would appear to be), then you'd be silly not to try.

R31Nismoid Grand Master

R31Nismoid

Posted
Posted

Take a pic - show us how far away the AFM is from the turbo inlet.

A simple rule of thumb is have the AFM as far away from the turbo inlet as possible, then you have no issue. If you can move it even 5-10cm, that can be all the difference that is needed. Hence most people with singles and AFM is pretty much as close to the headlight/guard as it can get.

Ben is on the money. Also correct me if I am wrong, but you will never fix the stalling if you are getting reversion from the turbo while using an afm. You will need a map sensor to fix it. (Or make it a blow through setup, though I have never used one or seen one that works well.)

Totally wrong.

Trex Grand Master

Trex

Posted
Posted

Damn haha. What parts are wrong? I thought having air coming back through the blades towards the afm in bursts would cause the afm to read air in bursts causing fueling in bursts also. Also a map would be placed in the plenum and wouldn't be affected?

Philr32 Proficient

Philr32

Posted
  • Author
Posted

Damn haha. What parts are wrong? I thought having air coming back through the blades towards the afm in bursts would cause the afm to read air in bursts causing fueling in bursts also. Also a map would be placed in the plenum and wouldn't be affected?

I think he's referring to you saying it can't be totally fixed is wrong.

The car is in pieces so can't take a pic, also don't have any pics from before as the bay was ugly so not worth taking pics of ;)

R31Nismoid Grand Master

R31Nismoid

Posted
Posted

The stalling and AFM reversion can be two different things.

1. Stalling due to ATMO venting BOV while running AFM - this can be tuned around. ECU expects air to return, it isn't - so car stalls.

2. Reversion over the AFM - AFM can be too close to the turbo, reversion isn't just the recirc of the BOV and air directed in the wrong way but being to close to the compressor wheel where there is a lot of turbulence.

Simply moving the AFM further away will fix reversion over the AFM. Hell I've seen 500rwkw+ single setups with AFM working fine, but the AFM is as far away as possible from the intake.

Moving the AFM however will not fix the stalling issues that relate to BOVs, that comes to the tune.

Also remember factory BOVs leak at idle, hence they MUST be plumbed back. GTR BOVs do this as well, reason being is to reduce compressor surge in lower RPMs. People that have changed to aftermarket BOVs on GT-Rs and running larger twins - have at times have surging in low RPM/high load scenarios. This is partly the cause as there is no bleeding of the air (the second being the factory piping, but thats another discussion).

Trex Grand Master

Trex

Posted
Posted

I understand you can easily tune around having an atmospheric bov.

So you're saying that if you're having problems with having a too tight of a bov that's causing flutter, you can move the afm further away from the turbine and it will fix the stalling?

R31Nismoid Grand Master

R31Nismoid

Posted
Posted

I understand you can easily tune around having an atmospheric bov.

So you're saying that if you're having problems with having a too tight of a bov that's causing flutter, you can move the afm further away from the turbine and it will fix the stalling?

No that is not what I said at all.

hypergear Grand Master

hypergear

Posted
Posted

Because the AFM is a hot wire it can not identify which way air is traveling, in many cases the ECU commends for more fuel when there is a current change, that includes when air is bounced at it through the turbocharger. It causes an momentarily richness, hence the idle hunt or stall.

To fix it, Trace the tables in primary fuel map of when the engine is off throttle, deduct a small VE of fuel out of those blocks, and that usually does the trick.

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