Spring tension on a BOV

[JBT]

Hmmm. Very interesting theory.

[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.

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.

My SE doesnt appear to be experiencing idling that is causing it to run ruff. Sits steadily at 800rpm on idle, spluttering on accelaration or re-acceleration is not noticed, as far as the AF ratio is concerned, I will have it checked on the next dyno run.

From what i can see after good apply of the throttle in neutral to about 5500rpm then backing it off, the rev needle drops fast back down to 1500rpm as there is a slight hesitation around there and then drops down to about 600rpm and returns back to 800rpm. I dont know if it was doing this before applying the BOV as i really didnt pay to much attention to it. Perhaps some other SE owners could enlighten us. When reving to only 4000rpm in neutral, return back to 800rpm is constant with little or no hesitation or bouncing around set idle.

My only issue is after fitting the SP motorsport exhaust, im getting the occasional popping from the exhaust after comming off the throttle in the lower rev range then re applying the throttle. Not sure if its BOV or exhaust related, I have another spring with a higher tension for the BOV so I might give that a go and see if it makes a difference.

[JBT]

after good apply of the throttle in neutral to about 5500rpm then backing it off, the rev needle drops fast back down to 1500rpm as there is a slight hesitation around there and then drops down to about 600rpm and returns back to 800rpm.

My only issue is after fitting the SP motorsport exhaust, im getting the occasional popping from the exhaust after comming off the throttle in the lower rev range then re applying the throttle.

Both conditions sound like over-fuelling IMHO. The first problem (rev drop) being a rich cut and the second (exhaust popping) being afterfiring. Have you tried re-fitting the factory BOV?

[blackster]

after good apply of the throttle in neutral to about 5500rpm then backing it off, the rev needle drops fast back down to 1500rpm as there is a slight hesitation around there and then drops down to about 600rpm and returns back to 800rpm.

My only issue is after fitting the SP motorsport exhaust, im getting the occasional popping from the exhaust after comming off the throttle in the lower rev range then re applying the throttle.

Both conditions sound like over-fuelling IMHO. The first problem (rev drop) being a rich cut and the second (exhaust popping) being afterfiring. Have you tried re-fitting the factory BOV?

Not yet, Ive just fitted the other spring which has higher tension, Ill give it a run up the highway tomorrow and see if it makes a difference. Just driving locally havent heard any popping, throttle responce feels different with the higher tension.

Looking at the valve it is completely closed on idle, where as with the other spring it was slightly open by a few mm's and then snapped shut on acceleration.

[Garry]

I have no issues with the GFB unit, could it be the brand

He is using a dual port valve. So IIUC part of it vents to atmosphere? If the valve isn't completely closed at idle he will be having untold mixture problems as well as backfiring and popping during gear changes.

[blackster]

I have no issues with the GFB unit, could it be the brand

He is using a dual port valve. So IIUC part of it vents to atmosphere? If the valve isn't completely closed at idle he will be having untold mixture problems as well as backfiring and popping during gear changes.

Yep, part of it vents to atmosphere and other goes back into the intake. Looks like the higher spring tension has fixed the problem. Seems alot quieter now at low rpm there is minor blow off and higher rpm comes the pssssssssshhhhtttt.

[Garry]

How did the higher spring tension affect the throttle response?

[blackster]

How did the higher spring tension affect the throttle response?

I found with lower spring tension it would rev up much more slowly then it would with the higher spring tension and comming off gas then re-apply it torque/boost appears to be more evident with higher spring tension. A leaking BOV because of light spring tension or poor connection causes slower throttle responce and I presume the odd exhaust poping on deceleration / reaceleration, that is no longer in issue.

I take back what i initially stated in the thread, the first spring in the BOV is not the correct one and gave weird perceived results.

Took me a while to get spring/tension correct, but its finally there and Im happy with the net result.

[ASE05]

Blackster, I've just fitted the same BOV. Same as you, I'm finding I've gotta wind the adjuster a fair way in with the standard spring to avoid poor idle control after giving it a rev. On the road throttle response did seem to be better......I thought it may have been wishfull thinkin' on my behalf!

I'm gonna dice the venting to atmo cause the sneeze it gives doesn't appeal to me at all!

My question is, where did ya get the spring? I too think it could do with a slightly stiffer one.

[blackster]

Blackster, I've just fitted the same BOV. Same as you, I'm finding I've gotta wind the adjuster a fair way in with the standard spring to avoid poor idle control after giving it a rev. On the road throttle response did seem to be better......I thought it may have been wishfull thinkin' on my behalf!

I'm gonna dice the venting to atmo cause the sneeze it gives doesn't appeal to me at all!

My question is, where did ya get the spring? I too think it could do with a slightly stiffer one.

The original spring is is too light, it doesnt exert enough tension and you will get poor idle or the odd pop on deceleration even when fully wound. With the original spring even the lightest amount of boost gives very loud blow off.

If you change the spring to a higher tension about 3 twists from the redline, you get a faint bird like whistle at low boost (4000rpm gear change) you can barely hear it and a lighter sneeze on the more spirited drives (6000rpm gear change), youll also find the idle issue and deceleration popping gone but more importantly throttle responce is much better.

I got my spring from turbosmart direct, what you need is the spring that has pink paint sprayed along one side of the coil, sorry dont know the exact dimension, the spring that you have in there currently has green paint sprayed along one side of the coil as featured.



To get the spring out just keep unwinding past the red line and it should pop out. You should be able to order different springs from the place you purchase your BOV from or if your turbosmart reseller is kind enough will give you one for free.

Edit

Also add 10cm extension hose with the 5mm interlink to the stock vacuum line, as featured. Just running the factory hose onto the BOV nipple will leak a bit affecting idle.

[Moggy]

Hmmm. Very interesting theory.

I agree. An interesting theory, but I get the impression that it is nothing more than a theory. Simon from Nizpro writes as if he knows what he is talking about but offers no evidence to support his opinion.

And I don't really buy it. His opinion is that the airflow goes backwards through the turbo on gear changes without a BOV. And that such a quantity goes back through the AFM that it runs rich for that time. Does he think the turbo reverses or stops? In this case a turbo spinning from 100000+ rpm to 0 or reverse in a split second would, IMO, put a fairly impressive strain on the turbo over time. Or does he think that the airflow reverses through a spinning compressor? Because if this was the case I think his understanding of turbine aerodynamics is a bit lacking. The compressor is still spinning rapidly and still trying to supply compressed air to the engine. If the airflow tried to reverse through the compressor it would cause the compressor to stall which would mean there would be no air going through it and the air being sucked in from the AFM would not be able to pass through the compressor. You may get a series of pops as the air tries to pass the compressor and fails. You will not get air from the compressor to throttle body pipe travelling back out the AFM.

Also, he states the intake manifold pressure has to go into vacuum on a gearchange for it to operate which is untrue, it actually has to drop to a point where boost pressure overcomes the combination of BOV spring pressure and intake manifold pressure to open the valve. This will happen when the intake pressure drops only a few psi below boost pressure.

I would like to know where he got his information from, and to hear some information from studies that support his theory...

[The Pupat]

I agree. An interesting theory, but I get the impression that it is nothing more than a theory. Simon from Nizpro writes as if he knows what he is talking about but offers no evidence to support his opinion.

That's funny I thought there was a large amount of evidence offered up there IE the VL turbo having never had one yet is very reliable.

And I don't really buy it. His opinion is that the airflow goes backwards through the turbo on gear changes without a BOV. And that such a quantity goes back through the AFM that it runs rich for that time.

What the problem with that. You see it on 90% of AFM cars with atmo BOV's that they run like crap and overfuel to buggery on decel.

Does he think the turbo reverses or stops? In this case a turbo spinning from 100000+ rpm to 0 or reverse in a split second would, IMO, put a fairly impressive strain on the turbo over time. Or does he think that the airflow reverses through a spinning compressor? Because if this was the case I think his understanding of turbine aerodynamics is a bit lacking.

He doesn't state that it stops/reverses but he does say they it is slowing rapidly,which it is since it have nothing to drive it because they throttle just stopped the airflow into the motor. I very much doubt it would damage the turbo since even if the airflow reversed and span the turbo in the opposite direction there is not real force on the exhaust side to stop it except for the inertia of the system (which would be fairly minor compared to everything else that a turbo is subjected to). I'd imagine it's reversing through the "spinning compressor" since the speed would quickly dropping to the point that it would not be pushing positive pressure and hence not impedding airflow. Ohh and out of interest where does your understanding of compressor behaviour come from?

The compressor is still spinning rapidly and still trying to supply compressed air to the engine. If the airflow tried to reverse through the compressor it would cause the compressor to stall which would mean there would be no air going through it and the air being sucked in from the AFM would not be able to pass through the compressor. You may get a series of pops as the air tries to pass the compressor and fails. You will not get air from the compressor to throttle body pipe travelling back out the AFM.

Umm the throttle just cut off the power supply for the turbo how is it going to keep spinning rapidly? The turbine and the compressor doesn't weigh that much. Why would the air not flow out the compressor? Once the compressor slows down enough to stop supplying air at the pressure above that of the manifold it will have to allow air out the turbo and past the AFM. How could it not?

Also, he states the intake manifold pressure has to go into vacuum on a gearchange for it to operate which is untrue, it actually has to drop to a point where boost pressure overcomes the combination of BOV spring pressure and intake manifold pressure to open the valve. This will happen when the intake pressure drops only a few psi below boost pressure.

it won't neccessarily be only a few PSI depending upon what spring pressure is use and how the valve is built (IE the ratio across the valve areas on both sides), but I'm sure you knew that.

I would like to know where he got his information from, and to hear some information from studies that support his theory...

I would say over 16 years of playing with these things, plus he also comes into contact with a fair few cluey people on these matters.

[Sean]

I would like to know where he got his information from, and to hear some information from studies that support his theory...

I would say over 16 years of playing with these things, plus he also comes into contact with a fair few cluey people on these matters.

Agreed he and the whole Nizpro crew are amongst the most experienced turbo tuners in teh country and have had a real hand in a number of real race cars including a number of cars in the 24 hour production races and numerous other race events.

There are lots of sharks who pretend they have had real motorsport experience, but fail to back up thier claims...

[JBT]

The general reason given for having a BOV is to relieve the high induction system pressure during gear change (i.e. throttle plate closed) and thus keep the turbo speed from falling too much to give good throttle response (less lag) after the gear change.

I notice that most of Simon's examples where BOVs are not used are all high performance race/rally engines which would normally be subjected to flat-shifting i.e. throttle stays open and spark is cut for a very short time. I can understand why a BOV would be superfluous in those circumstances.

[The Pupat]

The general reason given for having a BOV is to relieve the high induction system pressure during gear change (i.e. throttle plate closed) and thus keep the turbo speed from falling too much to give good throttle response (less lag) after the gear change.

I notice that most of Simon's examples where BOVs are not used are all high performance race/rally engines which would normally be subjected to flat-shifting i.e. throttle stays open and spark is cut for a very short time. I can understand why a BOV would be superfluous in those circumstances.

In any car if you are going fast that's about how fast you should be shifting otherwise why are you bothering to "reduce lag" when you could just shift it faster.