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Replacing my BOV with a Recirc Valve?

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Old Jan 28th, 2020, 21:58   #1
rockheadrumble
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Default Replacing my BOV with a Recirc Valve?

Hi Guys

Ive just been reading a post from 2016 in regards to atmospheric blow off valves. The reason i was searching as i am in the process of fitting a aftermarket intercooler and and remaking the inlet pipes.

I previously had a atmospheric blow off valve fitted (close to the throttle) I fitted it many years ago when the 'whoosh' was more important to me than correct running! I thought i'd do more research before making the pipes and reinstalling it. Id read online and as mentioned in the post that cars with MAFs should alway recirc the air to the inlet pipe after the MAF as this air has been measured.

It was also mentioned that cars with atmospheric blow off valves could have poor running when throttling off, causing the car to nearly stall. This is a problem that i have and recently asked a question about. I thought it could of been related to the injectors I'm running not the BOV.

Im running a holset turbo with a load of other mods, hoping for 350bhp. In your opinions what is the best option for dumping pressurised air at throttle off?
If its best to fit an after market recirc valve where is the best place to fit it? Is close to the throttle best (i read this is the best place for a atmospheric BOV) or should I fit it before the intercooler to reduce the record pipe length? I have also read a couple of tuners saying they don't run either but I'm under the impression the pressure could then stall the turbo?


Thanks Guys

Joe
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Old Jan 28th, 2020, 23:14   #2
TonyS9
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The measured air story is total BS/myth. The ECU does not remember air from the past, all it knows is whats happening now. You need to dump the high pressure air so that the air flow measurement is valid and so idle can work. Also that turbo doesn't stall or get stressed. However the latter turbo problems are perhaps minimal, there is no definate evidence that running without dump is a real problem except for idle.

All dump valves are atmostpheric. The air has to go somewhere and the Engine cannot use it (otherwise you would not need a dump). The original one is insulated by lots of ducting and padding in the air filter box. A recirc dump is just as loud as atm dump if you use a cone filter rather than the mfrs air box.

Non-recirc valves are only different in that they have to close for idle, so they have an intermediate pressure that is close too but not atmostpheric, so they may take second or so to settle while the remaining pressure is lost and the MAF reading comes up to drive the injectors properly. It may get a little weak at this point.

Im running a closed breather and having oil leak problems, originally I blamed the dump (correctly) and now have recirc and atm dumps and 2 check valves, however I think I'm getting piston blow past now. I tried running on the track with and without out dump. It doesn't make much difference until you park up, when it needs a few seconds to depressurise before it will idle.

For tuning I wouldn't worry about it too much, everything seems to work close enough, but if you want perfect transition to idle then its probably easiest to fit near the throttle body and vent to the usual place between the MAF and turbo. If you are using a cone filter it probably won't be much quieter.

Last edited by TonyS9; Jan 28th, 2020 at 23:16.
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Old Jan 28th, 2020, 23:29   #3
Laird Scooby
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What Tony said ^^^^^

If it's mainly for gear changing, try changing gear without coming completely off the throttle. Takes a bit of learning but you can eventually get your foot to automatically mimic the right place so as you bring the clutch up and power back on, the revs are more or less matched. Kinder to your turbo, engine, clutch and gearbox and keeps the turbo spooled up too, improving your acceleration times.

If it's more for when you come off the loud pedal to slow down for a junction say, you could add a throttle damper, this would slow the last few mm of throttle butterfly movement as it approaches the idle position, allowing the turbo to slow down a bit and come off boost more gently.
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Old Jan 29th, 2020, 00:12   #4
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^ ...or just keep your foot hard in, pitch it sideways and slide that mofo like a boss!
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Old Jan 29th, 2020, 00:17   #5
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^^.. and while you're sideways show-boatin', the little old granny in her Daewoo Matiz is at the next set of lights wondering where you've got to!
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Old Jan 29th, 2020, 08:59   #6
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Morning Guys

Thanks for the quick replies.

From the replies I gather there is no real difference performance wise in relation to overall running.

Tony, you mentioned that the standard re-circ valve vents back to the airbox, i presume this is therefor before the MAF sensor? If this to be correct then as you mentioned the measured air definitely doesn't matter. This would help me to understand that a recirc valve is definitely unnecessary as the dumped air gets remeasured again at intake...is that correct?

When I'm changing gear i don't have a problem, its when coming to a stop that the revs drop really low.

Dave you mentioned a throttle dampener. I wonder if partly the reason for low revs could also be related to me having a bigger throttle body?

The low revs are bearable however, i just wanted to check there wasn't a more serious reason for it and from your replies i gather its nothing to worry about, just a bit of a nuisance!

Thanks

Joe
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Old Jan 29th, 2020, 10:09   #7
Laird Scooby
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Quote:
Originally Posted by rockheadrumble View Post
When I'm changing gear i don't have a problem, its when coming to a stop that the revs drop really low.

Dave you mentioned a throttle dampener. I wonder if partly the reason for low revs could also be related to me having a bigger throttle body?

The low revs are bearable however, i just wanted to check there wasn't a more serious reason for it and from your replies i gather its nothing to worry about, just a bit of a nuisance!

Thanks

Joe
The way the EFi is set up is generally to implement a fuel cut above 1200rpm on the over-run. In other words, close the throttle above 1200rpm and the fuel is cut by the ECU not sending the signal to the injectors to open.

With a smaller throttle body, the idle speed will need a slightly wider opening, even for the base idle speed of about 600rpm so more air will still flow through the throttle body compared to it's overall size.

It could simply be down to poor setting up on the base idle speed, blank off the IACV hoses and see what it idles at once hot. Should be about 600rpm, if not you may have to adjust the throttle butterfly to achieve this. You will then need to readjust the idle microswitch in the TPS by rotating the TPS so when the throttle starts to move on the cable, the switch opens and closes again when the throttle is released.

It could also be that because there is a comparatively smaller throttle opening for the idle speed, by the time the ECU registers the engine speed is under the 1200rpm threshold that it needs longer to restart the injectors - all of this will be in fractions of a second but all adds up to the overall effect.
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Old Jan 29th, 2020, 16:05   #8
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Quote:
Originally Posted by rockheadrumble View Post
Tony, you mentioned that the standard re-circ valve vents back to the airbox, i presume this is therefor before the MAF sensor? If this to be correct then as you mentioned the measured air definitely doesn't matter. This would help me to understand that a recirc valve is definitely unnecessary as the dumped air gets remeasured again at intake...is that correct?
Depends if you mean before = upstream. A recirc valve vents downstream of the MAF (the air is still dumped out through the air filter). Measurement doesn't matter during dump, it is invalid, the air is gone. Dumping makes the current measurement valid. If you don't dump there is no air flow through the MAF when the throttle closed, but the engine is still consuming air (more than usual due to the pressure), the ECU is not injecting enough fuel and it stalls due to a weak mixture.

As I mentioned the main difference with a recirc dump is that it remains open during idle. It therefore must vent downstream of the maf, upstream of the turbo.

Still this might not be the souce of your problem. It is common on standard engines for throttle bodies to gum up and not close properly or perhaps be too restrictive when closed. The ECU does not learn the throttle body characteristics, I think it is hard coded, but not exactly sure if its reset on power off. At startup the ECU is expecting a certain air flow from a closed throttle (equivalent to about 500rpm). If this is offset due to dirt it is a little slow to compensate with the IAC and can stall or drop. It might remember that until its switched off again, which might tend to indicate your problem is incomplete dumping.


Dave - On fuel cut, there is a big hysteresis, it doesn't engage immediately unless you close the throttle above 2000. Around 2000 it will engage after a few seconds of closed throttle. This is the time when it doesn't engage where I keep saying its more MPG efficient to coast in neutral by around +20mpg (say going downhill) as well as losing less momentum.
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Old Jan 29th, 2020, 16:28   #9
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Quote:
Originally Posted by TonyS9 View Post
Dave - On fuel cut, there is a big hysteresis, it doesn't engage immediately unless you close the throttle above 2000. Around 2000 it will engage after a few seconds of closed throttle. This is the time when it doesn't engage where I keep saying its more MPG efficient to coast in neutral by around +20mpg (say going downhill) as well as losing less momentum.
I'm aware of the hystersis Tony, i was deliberately trying to keep to the fact that it's above 1200rpm for the purposes of explaining what i think may be happening on this particular engine.
Your idea is just as valid, it may even be a combination of both but as it's modified, we might both be right to a certain extent.

On the original L-Jetronic cars, the fuel cut above 1200rpm was a definite thing, you could feel it. No hysteresis, fuel was either on or off and as the revs dropped below 1200rpm, you felt the fuel start flowing again.
With the LH-Jetronic, a softer cut was implemeted by the use of hysteresis, however the basic principles apply but it is more progressively cut which strangely improves emissions by decreasing oxides of nitrogen (NOx) that can allegedly be toxic, either to us or the atmosphere. These apparently occur due to incomplete combustion so by adding a small amount of fuel during overrun, more complete combustion happens, still with engine braking.
This might explain the increased economy for those few seconds of coasting in neutral.
Also as the engine speed drops, the fuel flow is increased more gradually so you don't feel the jolt as the fuel supply is restored - again we don't know if this was altered (or even removed) on the chipped version of the ECU that he has.
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Old Jan 29th, 2020, 23:29   #10
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Thanks guys for your replies, much appreciated and very educational.

Il continue to make the inlet pipes to accommodate the same dump valve that's already fitted.

I have a play around with the throttle and position sensor and try set it up better to eliminate the idle problem. The throttle will be clean as ive not long had the head off to change the head gasket, i expect that the idle gap will need to be increased.

I'll report back if i get it sorted

Thanks

Joe
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