Spring tension on a BOV

[blackster]

I have installed a turbosmart kompact dual port and im unsure how much spring tension should i apply.

Turbo smart have stated that the aim is to have the valve closed at idle and if your experiencing any low idleing after reving or backfiring to apply as much as much spring tension as possible. (ie turn the knob all the way)

I found with no spring tension, idle was sticking and after free reving in neutral idle would drop below 500rpm but returning to 800-900rpm more apparent when cold.

Spring tension at half way (3 twists) low idle doesnt seem to be much of an issue, no idle sticking and throttle responce appears really good.

Spring tension all the way (6 twists) no idle issues, but throttle responce isnt as good as half spring tension. Perhaps im paranoid about the throttle responce.

Not sure what spring tension to leave it at and im wondering with the spring tension at max is it venting at atmosphere.

[Garry]

Doesn't increasing the spring tension apply more pressure to the piston to keep the valve closed under high vacuum conditions like idling?

If you have a vent to atmosphere valve and it's not shut at idle then air is sucked in through the valve bypassing the AFM leading to a rough idle condition. Thats how I understand the way it works.

[blackster]

Doesn't increasing the spring tension apply more pressure to the piston to keep the valve closed under high vacuum conditions like idling?

Yes, i believe by applying more spring tension the valve should shut quicker comming back to idle or when changing gear.

If you have a vent to atmosphere valve and it's not shut at idle then air is sucked in through the valve bypassing the AFM leading to a rough idle condition. Thats how I understand the way it works.

Thats my understanding also, what I dont understand is how throttle responce can be better if the valve is closing at slower rate using half spring tension vrs the valve closing quicker using full spring tension and the throttle responce isnt as good.

Perhaps im just paranoid about throttle responce that being its alot warmer today and the turbo isnt performing at its peak

[JBT]

I thought the purpose of a BOV was to allow excess pressure in the turbo outlet - induction manifold line to escape when the throttle plate closed (i.e. gear change) when \"on boost\". I would've thought that it would always be closed under even light spring pressure at idle.

If there was too much spring spring tension it may not open at all or close too rapidly and cause the turbo to spool down during gear change.

[Benny]

I'm no expert when I comes to BOV's, but I've tried a few different ones on my SP, and I was also surprised that different BOV's give you different throttle responses.
I guess the best thing to do is to adjust it so the idle is OK and you're happy with the throttle response.
Just keep playing with it until you're happy. That's why it's adjustable.

[Garry]

I would've thought that it would always be closed under even light spring pressure at idle

According to my GFB instruction manual most recirculating type BOV's will be open slightly at idle due to the high vacuum opening the valve and the lighter spring they use. This isn't a problem as all the air has been metered through the AFM. If the vented valves are open at idle then air bypasses the AFM.

Maybe off boost throttle response changes with spring tension because air is able to bypass the turbo, intercooler and associated piping if the valve stays open longer with lighter spring tension?

[JBT]

Aaahah. I wasn't thinking \"recirculating\".

[blackster]

I would've thought that it would always be closed under even light spring pressure at idle

According to my GFB instruction manual most recirculating type BOV's will be open slightly at idle due to the high vacuum opening the valve and the lighter spring they use. This isn't a problem as all the air has been metered through the AFM. If the vented valves are open at idle then air bypasses the AFM.

Maybe off boost throttle response changes with spring tension because air is able to bypass the turbo, intercooler and associated piping if the valve stays open longer with lighter spring tension?

I just went and had a fiddle. Notice at idle the bov is slightly (1-3mm) opened, as more revs added it snapped tight shut, off the gas and it opens again. Valve appears to stay open longer with less spring tension.

[The Pupat]

I would've thought that it would always be closed under even light spring pressure at idle

According to my GFB instruction manual most recirculating type BOV's will be open slightly at idle due to the high vacuum opening the valve and the lighter spring they use. This isn't a problem as all the air has been metered through the AFM. If the vented valves are open at idle then air bypasses the AFM.

Maybe off boost throttle response changes with spring tension because air is able to bypass the turbo, intercooler and associated piping if the valve stays open longer with lighter spring tension?

I just went and had a fiddle. Notice at idle the bov is slightly (1-3mm) opened, as more revs added it snapped tight shut, off the gas and it opens again. Valve appears to stay open longer with less spring tension.

You should have just left it standard eh?

You'll need a higher spring tension since the spring tension just works on the a pressure difference between both sides of the valve. If the pipe pressure force is higher than the vacuum pressure force plus whatever the spring force is it will push on the valve and let air past and wreck your AFR's to it'll idle like garbage without enough fuel or in the case of overrun too much fuel.

:? I thought the purpose of a BOV was to allow excess pressure in the turbo outlet - induction manifold line to escape when the throttle plate closed (i.e. gear change) when "on boost". I would've thought that it would always be closed under even light spring pressure at idle.

If there was too much spring spring tension it may not open at all or close too rapidly and cause the turbo to spool down during gear change.

The point of a Bypass Valve, Blow Off Valve etc is to allow MAF sensed turbo cars to pass emissions testing and to reduce the noise from the turbo (IE turbo flutter). It doesn't stop wear on the turbo and it doesn't reduce turbo lag or if it does you aren't shifting fast enough.

[JBT]

The point of a Bypass Valve, Blow Off Valve etc is to allow MAF sensed turbo cars to pass emissions testing and to reduce the noise from the turbo (IE turbo flutter). It doesn't stop wear on the turbo and it doesn't reduce turbo lag or if it does you aren't shifting fast enough.

Well I'm confused now .

From this website:

\"What it does: The only job a blow-off valve is supposed to perform is to relieve excess turbo pressure that results from shutting the throttle rapidly under boost conditions.\"

From this website:

\"A blow-off valve is mounted in the intake plumbing between the
turbocharger compressor and the throttle plate. The blow-off valve is a
second safety measure against the turbocharger over-boosting and damaging
the engine.

The blow-off valve is more commonly used to keep the turbocharger spinning
when the throttle plate is suddenly closed. \"

From this website:

\"A blowoff valve (BOV, also known as a bypass valve, compressor relief valve, or sometimes hooter valve) or Charge Air Bypass Valve which is the correct term for this device according to SAE J1930 is a vacuum or in some cases a solenoid operated valve that is located in the intake tract on an internal combustion engine between a turbocharger and the throttle body. It's used is to prevent compressor surge by venting the charge air when the throttle is closed, such as during a gearshift. When the throttle is closed after the engine has been used with boost there will be an increase in pressure after the compressor while the flow though the compressor decrease to a very low level. This will cause a condition commonly called compressor surge, the flow in the compressor stalls and the compressor will no longer be able to uphold the pressure increase over it.\"

[The Pupat]

This'll be a long post. This is a quote of what Simon from Nizpro has quoted on his forums about the subject. To me from an engineering stand point what he says makes sense.

This is fairly simply if you have a good under standing of what goes on with the engine and how most engine management systems work. It still amazes me how many professionals don’t quite understand, and wont to add them to everything they get their hands on. The bigger and shiner they are the better.
Now there is no hard or fast rule that says you must or must not have one. It greatly depends on your application. There are also many reasons how and what purpose you use them for, so I’ll start with why they were invented in the first place and give some examples of why they were fitted to some vehicles and not others. These examples and the reasons also back up the truth of what they were designed to do.
Firstly they were never designed to keep turbo charger from blowing up on trailing throttle. Example 1 Lets look at the first of the modern mass-produced turbo charged cars that we will all be familiar with, you guess it the VL Commodore turbo. They were also known for having a very reliable turbocharger and yet not a BOV in sight, even with increased boost that 99% of them had the turbo charger gave great life often in the 200,000 k mark.

So why weren't’t they fitted with a BOV? This is simply, it was back in 1986-88 and the emission laws were relatively simple. Hey, I hear you say whats the EPA got to do with it? Well actually every thing. Lets have a look at what happens when the car is driving down the road changing gears accelerating and slowing. Lets also look at how the ECU is determining how much fuel is required with all this going on. The ECU is getting two main signals, one RPM from the crank angle sensor and the other, airflow ,from the air flow meter AFM. The afm measure the air entering the engine, it dose this by a heated wirer element like your average light bulb, inside a know diameter hole call it 55 mm. Now the ecu heats this wirer up to lets say 500 deg c with a voltage, normally 0 to 5 volts. The air entering the engine goes pasted the wirer and cools it down, so the ecu puts more voltage into it to restore the 500 deg c. It can now determine how much air is entering the engine by how many volts are required.

So now lets look at just one example of what happen when you decelerate. Your driving down the road accelerating with 5 psi of positive pressure in 3rd gear with an open throttle and are about to change into 4th , as you shut the throttle to push the clutch in and shift gears the incoming air now has no where to go it banks up against the throttle and the pressure risers, at the same time there is no air going into the engine any more so there is very little exiting the exhaust to drive the turbine wheel of the turbo charger. The turbocharger starts to slow, the air that is banked up at the throttle is pressurized and only has one way to go, back out the front of the turbo. Stop Again you say , this is what destroys the turbo. Wait I’ll get to that. Once the air goes past the turbo it then travels up the pipe to the air cleaner on its way passing the afm and at that time the afm measure the air AGAIN remember it’s already measured this air on the way in and the afm is not smart enough to know which way the air is traveling so it then tells the ecu to add fuel to the air that’s going in the wrong direction. A-HA VERY MIXTURE ON GEAR CHANGE. EPA says get stuffed go away.
So as emission regulation tightened up car manufactures came up with a devise called the recirculation valve AKA the blow off valve. As these valves are placed between throttle and the AFM with the turbo in the middle They recirculate the pressurized air that we just spoke about on the gear change so the afm dose not see it. No more black smoke.

[sabretooth]

There's a big big thread on the effectiveness of BOVs on ausrotary, and the general concensus there is that they're not overly required. I've got one on my car and I'm pretty much regretting it - the BOV was a bargain at the time though. But if it's not the sound of the BOV I have to listen to, it's compressor surge. The difference is that the compressor surge is quieter, as my BOV does not plumb back.

I personally can't say for sure how the BOV affected throttle response between gear shifts, as I had intercooler pipework done at the same time.

Ideally I want to swap my hybrid (atmo+plumb-back with the plumb-back not going anywhere) BOV for a quieter plumb-back one, but to do that I have to get my intake piping modified to accomodate plumb-back. Something I don't have access to right now.


edit: is it just me or has pupat fluffed his bbcode and made my post align to the right?
I don't believe BOVs save turbos from damage due to compressor surge, but I do expect (but have not tested) that they would prevent the turbo completely stalling when intake air tries to exit through the turbo - this return wave of intake air would be actiong against exhaust gases still blowing through the turbo wheel - therefore stalling it seeing as both lots of air want to drive the wheel in opposite directions.

[JBT]

The turbocharger starts to slow, the air that is banked up at the throttle is pressurized and only has one way to go, back out the front of the turbo.

That's what all the manufacturers/suppliers/boffins say too. At best it causes the turbo to surge or slow down and, in the worst case, to stop/reverse. Now if the latter was to happen, I guess there would be horrendous temperature rises in the turbo and induction system at that stage followed by some large mechanical stresses and significant lag when the turbo reverses or starts spooling up again. None of that would be good for turbo life either.

Once the air goes past the turbo it then travels up the pipe to the air cleaner on its way passing the afm and at that time the afm measure the air AGAIN remember it’s already measured this air on the way in and the afm is not smart enough to know which way the air is traveling so it then tells the ecu to add fuel to the air that’s going in the wrong direction

Good theory. I've also read the theory that the AFM airflow can almost halt causing a snap reduction in fuel flow (just as the throttle is about to open) causing the engine to stumble or quit.

[The Pupat]

Oops I hit the character limit.

(continued from previous post)
Now as the EPA laws got tighter engines became smaller [GTR, EVO etc] but more powerful, intercooler became std and turbocharger compressor wheels got bigger this problem became worse as there was more air being banked up, which meant more air traveled the wrong way and was measured. So recirculation valve were fitted to all of them. And that the truth the whole truth and nothing but the truth, and it sorta makes sense don’t ya think.

Then came the aftermarket they made really big shinny ones, didn't’t quite understand what they were for, thought it was to save the turbo from spinning backwards, they normally got you to plumb them up so all the air just blew out to atmosphere again this was measured air so the ecu added fuel although the engine never saw any of it, and it then ran really rich on gear changes but they did make a dunny flush noise that seems to have attracted a cult following. Funny thing is that Indy cars and world rally cars don’t have them. May the EPA isn't on to them yet. They also don’t blow up turbo’s very often. There goes that theory.

So you asked about the BA XR6T. Guess what it hasn't’t got an AFM. It dose its calculation via the manifold pressure sensor [map] and its mounted after the throttle so it never sees the pressure rise on gear changes. So why do they have a BOV STD? Makes you wonder, may be the after market guys are not the only ones that haven’t quite got a handle on it. The other reason would be they properly didn't’t like the dunny flush sound. That my guess.

So now we come to compressor surge.

This really isn't the noise you here on decell with no BOV in fact that noise is[THE GOBBLE GOBBLE] the compressed air going passed the compressor wheel of the Turbocharger in the wrong,direction. Dose it hurt the turbo. Not in my experience.Compressor surge is where the turbo is making to much boost pressure for the amount of volume its supplying. In other words the turbo is to big for the engine at the surge point.What can happen at this point is that the turbo tries to to compress the air to a higher level than the compressor can make so it slips or surges. Think of it like when your Tyre's break traction on a wet road the engine speed increases this is effectively what happens to the turbo and at this point the turbo over speeds and can often fail. BOV have absolutely no effect at this point as they are close when all this is happening. So what about throttle res ponce on gear changes. Again the only thing that drives the turbo is the exhaust gas and once the throttle is close for a manual gear change the turbo is slowing very quickly. If the gear change is fast the BOV hasn't got time to open as the inlet manifold needs to go into a state of vacuum to operate the BOV and this is unlikely on a snappy change. Plus on a snappy change as the throttle is closed the boost as we have previously discussed actually increases momentarily and this is then immediately ready to enter back into the engine the moment the throttle is re opened.

So if the gear change is slow and the manifold goes into vacuum and the BOV opens the boost pressure is then vented to atmosphere or if your lucky back to just in front of the turbo, so if your cars fitted with an AFM it doesn't run ridiculously rich for a split second, you have now dumped all the boost pressure in the intercooler and plumbing, you stand on the throttle at this point your car is back in a N/A state so you can now wait for the boost to build up and away you go again. Doesn't sound all that fast when it explained like that. Which is why WRC rally cars don't have them and went to boost enhancement instead.

[The Pupat]

Next part

(continued from Previous Post)
OK now that you understand what compressor surge is I can now go back to my first comments about the gobble gobble noise not being surge. Well actually it is, But the reason it doesn't damage the turbo is that in that circumstance the throttle is closed so there is no exhaust gas to drive the turbine so it cant over speed. I'll take you back to the car doing a skid. Think of the car up on a trolley jack in gear doing 100 kph steady state throttle. Now lower the jack, yes the car will skid the Tyre's as it hits the floor but with very little accelerate it wont cause the engine to over rev.I hope I've explained it in a way that makes sense

So why do you have one?

As in the first post states emissions is the main reason and a must if the ECU is controlled by AFM.Secondly noise as far as a car manufacturer is concerned to get rid of the noise not make it. A well designed factory plumb back valve is almost silent.Thirdly there is some thought that a small increase in fuel economy is gained, as the BOV is opened while in cruise driving as the manifold vacuum is holding it opened the turbo doesn't have to push itself against the small amount of positive pressure it could be making, thus allow the turbine wheel to spin more freely and this then lowers back pressure.