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Being closed at idle helps spool. If it's not closed the turbo will take longer to spool since it also has to work to close the valve.

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I fight going back and forth in my head on this. If we're talking a simple application with an OEM BPV installed running with manifold reference, as soon as throttle increases, the manifold vacuum decreases and the valve closes near immediately. If the valve is always held closed, the turbo can build some pressure pre-TB, but not much. If the turbo is allowed to freewheel, it can reduce exhaust restriction (minimally??) and be at a much higher rpm. When the valve closes, the turbo is spinning faster than if it had the valve closed the entire time and then could create boost faster. That's how I'm trying to reason it anyway.
 

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The OEM valve should be closed at idle. There will be compressor outlet pressure on both sides, and the very light spring will hold it closed.

Its commonly the goal to keep the valve closed and thus the compressor capable of pressurizing the inlet to help response, with the exception of times where the valve is open to increase net turbocharger acceleration. From Ford's patent on this I linked in the other thread with a similar concern as this, they utilize this at high altitude on some vehicles, however the ST I'm pretty sure is not one of them. For a huge majority of the operating time of a turbocharger, the optimal bypass position is closed.

As such, you want the valve closed at idle. Anything else has the compressor rotating at a higher speed than intended by the ECU as its expecting the valve closed. In a lot of cases, because of the throttle plate delta pressure desired, it may even be commanding a throttle inlet pressure higher than ambient, and thus want the bypass valve certainly held closed.

While a speed density or blowthrough setup will meter air fine with the bypass open, the intent is to keep it closed. It'll operate with it open, but it will pretty much never be the "optimal" configuration.
"with the exception of times where the valve is open to increase net turbocharger acceleration"

would you expand upon this?
 

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Most who have a great urbo want it to spool as soon as it can depending on the turbo size. So the larger the turbo the more you want the valve to stay shut at idle. Yes our tiny turbo spools really quick anyway. So for the oem turbo you might not see any difference in the butt dyno department. With a larger turbo this maybe more noticeable. I haven't test thus with a larger turbo since I'm just running the stock one for now. In fact I've been debating about swapping back to a bpv instead of my TiAL to help spool even more. Since I auto x the faster my turbo spools the better.

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Thanks Bugasu. On other applications where you are running manifold reference directly to the valve, would those also be closed at idle in an OEM application? I see the benefit of keeping the valve closed, but also see the benefit of letting the compression "freewheel" to allow the car to be in boost sooner when the throttle is depressed and the valve closes.
Yes, typically they are. That's the point of the process. Its two pronged, yes it helps prevent issues on MAF vehicles, but the excess pressure is the same.
There is as you say, slightly increased resistance to turbine rotation and prevention of free-wheeling, but the OEMs typically keep it closed, and rarely if ever use free-wheeling.

On the other Ecoboost performance vehicles (Fiesta, Mustang, FoRS), the valve is actually electronic. No control via pressure reference, and its held shut at idle.
You're right, there's really not too much negative holding it open and free-wheeling, and with the stock spring, it does indeed close pretty quick, but you do get spool benefits in most (not all!) scenarios with it closed. This is a common OEM discussion actually, and there's a section in one of the recently released ECU development graduate studies book on exactly what you're bringing up: Response vs efficiency and proper control schemes to minimize the backpressure (thereby improving idle efficiency) while maintaining off-idle spool.
 

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Yes, typically they are. That's the point of the process. Its two pronged, yes it helps prevent issues on MAF vehicles, but the excess pressure is the same.
There is as you say, slightly increased resistance to turbine rotation and prevention of free-wheeling, but the OEMs typically keep it closed, and rarely if ever use free-wheeling.

On the other Ecoboost performance vehicles (Fiesta, Mustang, FoRS), the valve is actually electronic. No control via pressure reference, and its held shut at idle.
You're right, there's really not too much negative holding it open and free-wheeling, and with the stock spring, it does indeed close pretty quick, but you do get spool benefits in most (not all!) scenarios with it closed. This is a common OEM discussion actually, and there's a section in one of the recently released ECU development graduate studies book on exactly what you're bringing up: Response vs efficiency and proper control schemes to minimize the backpressure (thereby improving idle efficiency) while maintaining off-idle spool.
Good info and thank you, looks like I was mistaken on this.

@UnfocusedST, my apologies. You are correct.

Most who have a great urbo want it to spool as soon as it can depending on the turbo size. So the larger the turbo the more you want the valve to stay shut at idle. Yes our tiny turbo spools really quick anyway. So for the oem turbo you might not see any difference in the butt dyno department. With a larger turbo this maybe more noticeable. I haven't test thus with a larger turbo since I'm just running the stock one for now. In fact I've been debating about swapping back to a bpv instead of my TiAL to help spool even more. Since I auto x the faster my turbo spools the better.

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Makes sense, and knowing that the valve closed does help spool, definitely makes sense. I guess the trade-off is going to be how well can you keep the valve closed before you run into surge issues. With the electronic control, you can do whatever you wish as long as you can tune the tables. With manifold reference, you'd have to find the sweet spot with spring tension to keep the valve closed as long as you can while still allowing it to vent.
 

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Good info and thank you, looks like I was mistaken on this.

@UnfocusedST, my apologies. You are correct.



Makes sense, and knowing that the valve closed does help spool, definitely makes sense. I guess the trade-off is going to be how well can you keep the valve closed before you run into surge issues. With the electronic control, you can do whatever you wish as long as you can tune the tables. With manifold reference, you'd have to find the sweet spot with spring tension to keep the valve closed as long as you can while still allowing it to vent.
No problems. It's good to have a debate everyone in awhile.

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"with the exception of times where the valve is open to increase net turbocharger acceleration"

would you expand upon this?
Certainly.
This boils back to the "basics" of turbochargers. A very rudimentary view is: The exhaust gas goes in, hits the turbine, and the turbine extracts the energy in the exhaust gas, and this extraction is said to "produce" a torque about the turbocharger shaft.
Meanwhile, the compressor is doing the opposite. The compressor pushes relatively slow intake air up to a high speed, and then using a diffuser causes all the gas to run together "compressing" it.

So, the compressor is consuming power to speed up the air and get it through the diffuser into the charge pipes. The turbine is extracting power from the exhaust gas.
Now for a given pressure and airflow, the compressor requires a certain amount of power in order to do its job to maintain this pressure and flow, and this power is produced by the turbine, and transmitted across the shaft to the compressor. But now what happens if the turbine produces more power than the compressor needs? Well, you now have a net torque on the rotating assembly. Torque is moment of inertia x acceleration, and because our moment of inertia is constant (we hope, unless the turbocharger is blowing apart ;) ), a net torque means we have net acceleration, i.e. we're now speeding up the turbocharger.

This is what "spooling" a turbo is about. You're spinning it up to higher speed, and at these higher speeds the compressor generally operates better (you have to look at a compressor map to see what exactly the compressor does for a given speed). So, better spooling turbos basically mean they have more acceleration for a given power output, usually because of a lower moment of inertia (smaller turbo) or because their compressor doesn't require as much power as its spinning up (a better compressor). They could also have a better turbine design to extract more power overall as well.

Now, the next question is how the bypass valve enters the situation. When you have the bypass valve open, you're essentially letting air go. Typically, you're now operating at a low pressure ratio, with the same mass flow, which means dropping the power requirement. So now, your turbine was still producing the same power, but your compressor requires less, and so you have more net torque, and thus more acceleration.

So you now are spinning up your turbo! Great right? Well, except for what happens when you close the bypass valve. Once you close it, you'll now ride the speed line up the chart (you're going to start building pressure), and as such your compressor power requirement starts to ramp up. However, your turbine was still making the same power it was before generally, which may not be enough to meet the compressor power requirement, so you'll start to have a net power loss, and thus start to decelerate undoing what you just did.

If however, you play your cards right, you can find scenarios, depending on your compressor and turbine arrangement, where your final operating condition has a balance of turbine power produced and compressor power consumed, and thus using that moment of lowered compressor power requirement allows you to accelerate to that condition faster. I.e. you can spool to a condition faster than if the valve was closed and the compressor consumed more power along the way.

This however, isn't the norm for most operational conditions for a turbocharger on an engine. Ford's own patents on it indicate they may use it for high altitude lag compensation, but I don't believe that logic exists in any form on the Ecoboost vehicles I've seen to date.


@LMStone510 gave an analogy that I thought was quite good, so I'll steal it here.
Assuming no wheelspin, if you want to launch your car as quickly as possible do you:
A) Sit at idle, let the clutch out while flooring it, and then accelerate to redline?
B) Sit at idle, keep the clutch in (akin to keeping the bypass valve open), rev the engine to an area you know it can make more power at, let the clutch out while flooring it, and then accelerate to redline?

Generally, you do B. Why? Because the engine can produce more power at the higher RPM and maintain that RPM shortening the need to accelerate to it from an idle.
This is somewhat analogous to those situations. The difference is the situations where the bypass valve being open is beneficial is not as often as "everytime I want to spool as fast as I can" and much like launching requires you to know which RPM will work best to produce the best power for your engine load, this requires a compressor map to know which turbocharger RPMs you can do the same thing with :)
 

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I have a stage 2 stratified tune, and currently have a Mountune BPV but I have a turbosmart dual port on the way. Will it cause issues since I am switching that would require me to have Stratified change anything in my tune? I'm very new to turbos so this makes me a bit nervous, though I'm super excited about my BOV.
Any help regarding this @[email protected]? I also just installed one step colder plugs. Will it be necessary to adjust my OTS tune?


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Discussion Starter #52
having the bov shut at idle also prevents crap from going into the intake tract. on my evo its closed, and my evo is speed density. not a big fan of maf's and i dont know many who are running a gtx3582 with a maf. im sure there are but i am not.

on a side note im really not digging this solenoid controlled bov. its open a lot. sometimes at low rpm while cruising it opens and stays open for a few seconds. the whole cycling (pshhh........pshhh........pshhhhh) while cruising at autobahn speeds is annoying too. i kinda just want to grap the exhale from cpe and a tial and call it a day. my bank account says i cant though. but seriously, if it were safe to just get a vaccuum source and hit the bov from there and bypass tihs damn solenoid i totally would. its really irritating. thank God i dont have a ssq on it or i would be going bat$!#@@ crazy
 

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having the bov shut at idle also prevents crap from going into the intake tract. on my evo its closed, and my evo is speed density. not a big fan of maf's and i dont know many who are running a gtx3582 with a maf. im sure there are but i am not.

on a side note im really not digging this solenoid controlled bov. its open a lot. sometimes at low rpm while cruising it opens and stays open for a few seconds. the whole cycling (pshhh........pshhh........pshhhhh) while cruising at autobahn speeds is annoying too. i kinda just want to grap the exhale from cpe and a tial and call it a day. my bank account says i cant though. but seriously, if it were safe to just get a vaccuum source and hit the bov from there and bypass tihs damn solenoid i totally would. its really irritating. thank God i dont have a ssq on it or i would be going bat$!#@@ crazy
Actually if your bov is open at idle all you have to do is take a piece of paper to it. This will show you air is not being sucked in, but blown out.

Also when cruising at 80mph here on the interstate my defi boost gauge stays pretty much at 0 psi. So if your going faster on the autobahn your in boost alot more than you think.

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Discussion Starter #54
i go faster than 80 in a lot of cases :)

but honestly im not that much in boost. im just annoyed by the cycling
 

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So are there any other solutions besides changing the tune (which I'm currently working with Raffi from fswerks on) and running a line from the bpv to the symposer delete which sounds like a bad idea.
 
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