Aftermarket BOV, performance or wank factor

[Steve]

Maybe we could e-mail a link to this tread to 'Myth Busters'

[archie01]

Steve

Great find re the link to the autospeed article.

It's funny b/c i had this conversation with Simon back in 1998 when i use to take my VLt in and i agreed with his reasoning back then. Good to see 4 years later (when the article was written) has hasn't changed his opinion.

[psybic]

Maybe we could e-mail a link to this tread to 'Myth Busters'

Hell yeah!!! Thats the best idea to come outta this thread. I'd luv to watch that! THey'd come up with some cool tests indeed.

[Cefiro_PWA]

Hell yeah!!! Thats the best idea to come outta this thread. I'd luv to watch that! THey'd come up with some cool tests indeed.

hahah who wants me to do it?

[jezzerrr]

I'm no pro regarding turbos, and I'm not going to pretend to be (but i know a bit about pumps, turbines and PRV's, which i'll go into shortly). Nissan enginneers put a BOV there for a reason, and with a plastic inlet spool, I'm not going to argue the point too much. My BOV stays. I do worry about the leaking-overspeed thing tho, which it does atm, so i'm buying an aftermarket jobbie. The beauty of actually having a BOV is you can mimic having no BOV if you want, just wind the bastard right up. for me tho, I have to agree with sydneykids' line, and can see how you need to "tune" your bov for best performance.

To clarify, the "turbine" section of a turbo is actually on the exhaust side, it is an impulse turbine, to be precise. The induction side is indeed a pump, and is definately not a turbine,. If it was a turbine, then you'd be able to call any old centrifugal pump a turbine. Although people do still like to call it one, it is not, by definition, a turbine, just attached to one.

So in the case of a turbo, as with any pump, it will pump up to it's maximum pressure/flow, and then it will dead head/cavitate. So this is fine if you cannot reach the top of this pumps' efficiency curve, ie you cannot reach it's max pressure/flow. however, running no BOV you will very quickly reach this point, at which stage, if the pump still had power, it would increase RPM. HOWEVER, as this is a unique pump, in that at the same time you are dead heading it, you are also removing it's power source, it will not increase in rpm, it will simply slow down , and possibly stall and reverse, as it increases pressure in the run to the throttle butterfly, using nothing but the momentum of it's revolution.

No Bov

It is understandable, how no BOV can increase power. If you manage to change gears quickly enough that your turbine has not stalled, you will have increased pressure after the turbo by a substantial amount, at which point you re-open the throttle, and you get a handy rush of 15+psi into the plenum, and hopefully the turbo hasn't slowed down too much to say, 40000 rpm, using the example Sydney Kid provided before, and is able to speed back up quickly and provide boost. But given how quickly it spools up, I'd imagine it would slow down fairly quickly too, so you'd have to be pretty swift in the throttle all the times to not stall the turbo, which is why BOVs probably aren't used so much in racing. Which would tend to point to the fact that if you're car is not a racer, you might be doing a bit of harm to your turbo by NOT driving your car at full noise all the time and dong snap changes.

Race cars, PRV's

In regards to race cars, most of them run pneumatic boxes nowadays, so the need for a BOV is even less, basically, the only time it would possibly be needed is when they are slowing down for a corner. Indy cars still run pop off valves, which I believe are set slightly higher than their on throttle boost pressure, and so act like a "style" of BOV, in that they release excess pressure (why would they want to do that, i wonder?). Another name for a pop off valve is a pressure relief valve, which actuate at one pressure, and reseat at a lower pressure, by way of two diameters that the passing air acts upon. once the PRV is open, the air acts on a greater surface area, allowing it to reaseat at a lower pressure than it actuated at.

Leaky Bov

It's piss easy to understand how this can stuff things up. one, the overspeed issue mentioned before, and two, also mentioned, you lose all your boost pressure as soon as the bov opens. As my understanding of bovs goes, I dont think that either of these scenarios was what BOV's were designed to do, it's just what we're used to them doing.

Best of Both worlds

If you tune your bov to act like a PRV, voila! you have the perfect system. And the beauty of a BOV over a PRV, is that is is alot more tuneable than a PRV, so you can effectively fine tune the pressure at which it actuates, and the pressure at which it reseats, so as to maintain pressure, and at the same time, keep the turbine spooling. All in all MUCH more controllable than running no bov at all, and you dont run the risk of over pressuring your turbo. And the other advantage it has over a pop off valve is that it is vacuum actuated, so at times when you aren't running at full noise, it will open enough so that your turbo doesn't stall or reverse.

So all in all, If your BOV is leaky or worn out, that is bad. If you race, or drive hard ALL the time (ie not really anything you can do legally on the street), and never just stab it and drop off again (to get that "pigeon" sound, non BOV people! and you say the BOV sound is a load of wank!), you can probably run without a BOV. If you drive your car daily, and dont want to stress too much about doing bad things to your turbo, then a correctly set up BOV is the way to go.

[jezzerrr]

I'd better mention that all that is only IMHO! purely based on what I know about the individual components making up the system, and not necessarily the parts themselves. I dont race, I dont tune. But I do know that if my turbo goes bang cause of a mod I made based on what Mr/Mrs X said, they aint gonna be there to pick up the pieces!

I also notice on that le-mans car, the incredibly short run from turbo to butterfly... ie not much stored energy to try and push itself back past the impeller when(if) the turbo comes to a stop... Not nearly as much as something running an intercooler, complete with runs to and from...

The autospeed article is a very good explanation and could be a very good argument against BOV's, and it is if you're racing, and you can keep the "skidding" noise going for the duration of the gear change, which would be pretty easy. You could even say that the skidding noise is a good thing! But I was thinking, what does it mean when the "skidding" noise stops? and there is 50+PSI infront of it, atmo or vacuum pressure behind it, and no exhaust gases powering it? hmmm. HMMmmmmmmm.....

[gun32gtr]

is there anyway of externally venting the gtr bovs?might be a dunb question but thought i would ask?

[czy18e]

just thought i would ad to the discussion.....

I currently have a supersonic bov on 3" i/c pipes and when the car was running 12 psi it worked fine now the car is running 19 psi i get a loud flutter before the psht noise when i change gears and the car pauses for a second so i am fitting a second bov to solve the problem.

Now that should prove that bov are definently needed.

[Steve]

Lol

[Cubes]

First I'll also add a LOL!

czy18e, what sort of siiick power are you making to require a second bov?

Must be sick mate.. fully sick.. I bet Thorpy even thinks its sick.

[RB30-POWER]

lol, my thoughts as well....

[czy18e]

No worries fellas!

maybe the bov i have is stuffed but thats what it does.

294 rwkw.

[jezzerrr]

thorpy says it's fully sick

I smell bait. lol

[predator]

:whackit:

[czy18e]

Steve can you tell me how to fix my problem then as you have helped before with useful info i can always just sell the second bov if there is another way.The sound doesnt bother me i just want to get rid of the deadspot on gearchanges.

And you other tools should stop feeding so much sausage to thorpie,and not attack people for no reason behind the safety of your computer screen.

I am not a mechanic so any help would be appreciated.

Thanks.

[Cubes]

heheh Thorpy would think 294rwkw is fully sick..

http://www.fueltech.com.au/auto_turboaccessories.html

It states Highflow version available upon request.

i get a loud flutter before the psht noise when i change gears

Where is the bov located? Near the throttle body as the stock setup?

Maybe its adjusted incorrectly.

[Steve]

czy18e, your post was pretty funny so many factors that need controlling (and examining)to ensure they dont change, before something can be considered proof

Did you change anything when you would up the boost?

Have you tried adjusting the BOV, ie, wind it fully shut, as tight as it will go?

Did the same guy tune it for the higher boost as did it at lower boost?

There are alot of different things, but by the sounds, you have throttle lag (ie, it takes a while from when the throttle is opened to when you are making power) - it could be the fact that you are using a bov that is causing the problems (as I found when I started making over 300rwkw) or it could be a poor tune, if you have changed anything else (eg cams) you could be falling off boost (power band is narrower).

First thing I would be looking at is blocking off, or winding shut the bov as much as possible AND checking its not lifting under boost (as cubes is pointing to)

The reason I say this, is when under full boost, with no bov, and lifting off the throttle, you will get a high pitched squeal, not a flutter sound, the flutter comes later (about 1/2 to 3/4 of a second later) once the majority of the compressed air has reverted past the turbo. On quick changes you cant even here the flutter, just a very quick squeel.

Faults like the one you have can be alot easier to trace when on a dyno though, but definately start by looking at adjustment of the BOV

good luck, let us know how you go.

[czy18e]

czy18e, your post was pretty funny so many factors that need controlling (and examining)to ensure they dont change, before something can be considered proof

Did you change anything when you would up the boost? 

Have you tried adjusting the BOV, ie, wind it fully shut, as tight as it will go? 

Did the same guy tune it for the higher boost as did it at lower boost?

There are alot of different things, but by the sounds, you have throttle lag (ie, it takes a while from when the throttle is opened to when you are making power) - it could be the fact that you are using a bov that is causing the problems (as I found when I started making over 300rwkw) or it could be a poor tune, if you have changed anything else (eg cams) you could be falling off boost (power band is narrower).

First thing I would be looking at is blocking off, or winding shut the bov as much as possible AND checking its not lifting under boost (as cubes is pointing to)

The reason I say this, is when under full boost, with no bov, and lifting off the throttle, you will get a high pitched squeal, not a flutter sound, the flutter comes later (about 1/2 to 3/4 of a second later) once the majority of the compressed air has reverted past the turbo.  On quick changes you cant even here the flutter, just a very quick squeel.

Faults like the one you have can be alot easier to trace when on a dyno though, but definately start by looking at adjustment of the BOV

good luck, let us know how you go.

<{POST_SNAPBACK}>

Thanks steve i will try blocking the bov tomorrow and let you know.

The only other things i changed when turning up the boost was fuel pump and injectors.

The same guy tuned it but we didnt do a road tune and so i didnt find out about the throttle lag until i got out of the city(as it only does it on gear changes on high boost)

the cams are stock.

I am going on the dyno again on the 17th and want to have this sorted by then if i can.

And so when i have the bov jammed shut ie.no bov, it wont hurt my turbo ??

Thanks ill post tomorrow with results but if it doesnt fix it ill have to wait for the dyno.

cheers.

[someonestolecc]

I recently fitted an aftermarket BOV which is yet to be plumbed back. Prior to this my OEM s15 re-circ valve used to flutter when I was on full noise.

My aftermarket bov - anything under 0.5 bar I hear flutter. Anything 0.6-0.7 bar it will blow off.

My question is - what is "correctly" setup. Should I set it up so the BOV vents lower than 0.5 bar?

[someonestolecc]

Steve can you tell me how to fix my problem then as you have helped before with useful info i can always just sell the second bov if there is another way.The sound doesnt bother me i just want to get rid of the deadspot on gearchanges.

And you other tools should stop feeding so much sausage to thorpie,and not attack people for no reason behind the safety of your computer screen.

I am not a mechanic so any help would be appreciated.

Thanks.

<{POST_SNAPBACK}>

If you're having dead spots on gear changes do you have a plumback BOV or atmo? Mine was doing that when I had mine atmo because it would be getting too rich. Plumb it back and try then.

Otherwise it might be set too tight or you have no BOV. When my BOV was too tight (i.e. pretty much like not having any BOV) my car would stutter as the turbo shat itself, almost like a surging.