FLUTTER sound as BOV

[Steve]

AndrewD, perhaps you should do some research, instead of listening to BOV salesmen and the rest of the speculation posted on interenet forums.

Pray tell why the compressor surge is harmful? it isnt, the turbo is not a positive displacement pump and the fact is that turbos actually speed up when they surge, as there is no more load on the compressor wheel.

I have tried with and without, have just upgraded my turbo after nearly two years and 10,000kms, with a large amount of track work - NO bov and the turbo is fine

perhaps the engineers who have chosen to run NO bov in the past know more than you (hard as it may be for you to accept) Perhaps there is also a reason Mazda state in their workshop manual for turbo engines that the bov is there for emissions purposes - absolutely no metion of protecting turbochargers from imaginary gremlins

lastly, if you dont want to listen to others on here, perhaps you will listen to the words of Simon Gishus, who actually has done research, not just read articles on the internet like you

From Auto speed article

In Part 2 of our interview, we speak to a leading industry expert Simon Gishus about Nissan engines, Holden Gen 3 V8s, and directions for modifications...

What are some areas where people often make errors modifying their turbo car?

"The classic blow-off valve.

"The blow-off valve is designed as an emissions control device for OE manufacturers. It came about when smaller engines made more and more power using larger turbochargers and bigger intercoolers. As you close the throttle, the build up of pressure and the larger volume inside the intake has to go somewhere; it can't go into the engine because the throttle is shut. Instead, it has to do a U-turn and it comes screaming out the airflow meter. That creates the 'gobble-goggle' sound.

"The gobble-gobble sound is something the public has grown to love.

"The airflow meter is not all that smart and does not realise the air is going in the wrong direction; it therefore measures the air twice (once going into the engine and again going out in the wrong direction). The computer now tips in twice as much fuel as what's required, making it run rich - making it not pass emissions.

"Therefore, manufacturers fit a blow-off valve - or a recirculation valve as they are actually called. A recirculation valve opens when it senses manifold vacuum, returning the air trapped at the throttle body to between the airflow meter and the turbocharger. As such, the airflow meter does not take a double reading - the car now passes emissions.

"Unfortunately, we've had people ringing up and wanting the "audible gear change alarm".

""What audible gear change alarm?" we ask. "You know, when the Sierras were running around and just when they went to change gear it used to go whoda-whoda-whoda" they tell us.

""No pal, that is the dump valve..."

"Some people do think that at the absolute upper extremes of boost levels - about 30-plus pounds - the blow-off valve does, somewhat, save the compressor wheel and shaft from trying to rotate backwards. It doesn't actually rotate backwards at all - all you're hearing is cavitation. What happens is, you've shut the throttle, the turbocharger is doing 100,000 rpm and now has a boost spike of 50 psi. Because it's working in a higher region than what it's designed for, it slips; it basically does a skid like a car tyre does when you dump the clutch. That's the noise you hear - the whoof-whoof-whoof is the air doing a skid."

the article does go on, but the above sums up nicely the fact that you AndrewD, should practice what you preach, and not just come on here and sprout your psuedo technical (and flawed) babble, and refer to people as 'idiots'

/rant

[Steve]

http://autospeed.drive.com.au/cms/A_1457/article.html

[Link]

I have few points to rebutt,

-The physics do suggest it can happen. In applications where the throttle is POST compressor, when it is closed you increase the pressure between the compressor and the throttle. As pressure increases, pressure ratio sky rockets. Most compressors for road cars (not race cars especially F1) operate with a maximum pressure ratio of no greater than 2. Above this surge occurs (but not only under this condition). Surge also occurs when the throttle is closed, the compressor cannot 'push' the air any further and it reverts and flows over the tips of the blades. As JC Marhsall stated earlier this creates vibrations in the blades and coupled with high hydrodynamic pressure loadings on the blades (at a distance from the blade / wheel interface) this causes them to bend or shear. It happens, go to garrett they have plenty of compressor wheels on display that have failed due to surge.

-Racecar engines that have not failed. I call dibs on this one. Lets define a racecar shall we. I think you will find that most racecars especially in the elite class, are often rebuilt after every race and the majority of their parts replaced. Not to mention that typical race cars with turbo applications run high boost (higher pressure ratios ie around 4 or 5) this means that compressor wheels are larger, and have a greater saftey against failure. However,

I would like to point out that most of the earlier F1 cars had PRE compressor throttle. This means that while the the throttle is closed, still increasing pressure ratio, the engine is unhibited to breath the pressurised air. And this surge doesnt occur. Rather now you get cavitation. The need to run a recirculation valve to 'raise the inlet pressure' is not so pronounced and typically improved seals are all thats required.

-Before you start complaining that people are getting all upset about falous claims, take the time to read many of Society of Automotive technical papers on this 'phenomenon' as you put it. Why do you think most compressor applications outside of automotive use are constant throttle / load?

I'll leave you with something to think about, being as it is i have access to a race engine using a garrett GT15v (yes its small, but its also the very same kind used on the new porche engines) we have a shaft speed sensor and have clocked it at in excess of 200,000 rpm. Now consider that you have surge at a pressure ratio of 3. Almost instantaneously the shaft speed changes to 0. Now no matter how light that wheel is, there is a significant amount of inertia force on the blades which is tramsitted to the key and shaft.

So before you go all internet mechanic on everyone, do some research. Peace out.

How do you explain the equivalent of 650 1/4 mile runs on MANY Ford Sierra's running at Bathurst over 6 hours of an endurance race at 36+psi boost ?

Or the worst punishment of over 2500 1/4 mile runs on the typical Le Mans endurace racer during a 24hr race again at very high boost ?

There are many examples of cars that are far more highly stressed running the same old "shitty" turbos we use now days and not having ANY issues or instant failures.

Please turbos cant and dont cavitate - this is rubbish - Cavitation is a non issue in an air system.

In a fluid system cavitation happens when small air bubbles (in water) can be formed around the compressor due to a large pressure head or an overspeeding compressor for a certain flow capacity - both have absolutley nothing to do with an air based pumping system

The resulting air bubbles explode against the surface and can eat away a whole compressor!

The loads assosiated with WG only flutter are very very small (not enough to cause any adverse effects), that is why this was never an issue is years gone past, why is it only an issue now ?

Using the reasoning that race cars throw away the turbo between races is all well and good, just remember how much "work" they are doing compared to your average street/drag car and you will see it far out strips any other motorsport situation. That is why they call em endurance cars

Misleading marketing and value added sales are the biggest reasons, though this again is only my opinion and nothing more. I will choose to follow past history from the smartest brains in the world who proved their systems (in the toughest situations) rather than some hype rubbish printed in HOT4's for example

End of the day do whats comfortable for YOU, justify it how you like so long as you are happy and think your making the best choice. If your going to be paranoid about not running a BOV then its not worth it for you.

[AndrewD]

Im still laughing.

"Turbochargers cant cavitate"

Any rotating machinery can cavitate, especially air systems. The type of cavitation you described was what is experienced in centrifugal water pumps. Doesnt mean it requires water.

Btw those articles are by industry professionals. The same very people that design those turbocharger systems. Yes i understand there is an emissions, fueling related function of BOVs but its not their only function.

Ive got logged data that proves the fluctuation in turbo speed, so maybe Simon is talking about a different situation.

[Steve]

of course, you know better than simon.. silly me

and as for your comments about cavitation, perhaps you know better than encyclopedias, dictionaries etc - unless you are talking about 'a different situation' - just like simon

http://en.wikipedia.org/wiki/Cavitation

there are dozens more definitions available on the net too http://www.google.com.au/search?hl=en&...ation&meta=

ALL talk about fluids - perhaps there is a reason for that?

perhaps we all need to go back to uni - except for you of course

[Rolls]

As JC Marhsall stated earlier this creates vibrations in the blades and coupled with high hydrodynamic pressure loadings on the blades (at a distance from the blade / wheel interface) this causes them to bend or shear. It happens, go to garrett they have plenty of compressor wheels on display that have failed due to surge.

The kind of surge you are talking about is different to when you throttle off, as when you throttle off the exhaust stops spinning the exhaust wheel as well so the force is far less.

[AndrewD]

ALL talk about fluids - perhaps there is a reason for that?

air is classed as a fluid

a fluid can take either gaseous or liquid form

[Rolls]

AndrewD what have you got to say about the factory produced cars that run no bov, my old 31 skyline came with no bov from the factory, its still had the standard turbo on it running 11psi at 106kms (apparantly) its was a little noisy but was still working fine and it fluttered all day long.

Edited October 24, 2006 by Rolls

[Steve]

air is classed as a fluid

a fluid can take either gaseous or liquid form

maybe when talking about fluid dynamics, but with cavitation, liquid is more specifically used (which is also referred to as fluid), but you obviously like splitting hairs, so what say you then to the following quotes

- The formation and instantaneous collapse of innumerable tiny voids or cavities within a liquid

- (noun) The production of voids in a liquid system due to extreme reduction of internal pressure

- The boiling of a liquid caused by a decrease in pressure rather than an increase in temperature

to quote but a few from a quick google definition search

Or are you now going to expand and say that as fluid includes liquid, there for incudes gasseous variations (bubbles forming within air)

And also, please point me to one external definition of cavitation that specifically states it can occur within a gas - you made the call, shame you cant back it up.

[Cubes]

Steve,

Running no bov. Any drivability issues?

A mates VLT without a bov felt perfectly fine. That had up near 200,000km's on the clock, its old bush turbo was still fine.

[Steve]

drives better imo - I originally removed it cos Bai suggested it was best for throttle response, he doesnt use one on his drift car for the same reason. The 2ic of signal also agreed that no bov was best.

Never looked back, never use one again - throttle response is much crisper, and pretty much instantaneous, especially when on/off throttle trying to control a slide

Idle, coming off revs (fuel cut recovey), etc, all perfect

[Link]

Can we say Myth Busted yet guys?

[Cubes]

Hey ppls,

Out of my own curiosity I blocked my BOV off today.

The sound.. well it does that vlt noise, its fairly quiet though as I run a stock airbox with a 100mm cai running in to it. So all the noise is out the front of the car.

Boost is also up ~1psi.

Apart from that, boost response is most definitely up. It boosts a hell of a lot earlier and on full throttle gear changes and part throttle back off's (i.e drifting) its definitely much much better.

Now for the down side..

Mine for what ever reason suffered a few drivability issues.

Plodding around changing up at around 2000rpm with ~7psi being made initial acceleration in the next gear halt/motor die for a fraction of a second then accelerate as per normal, sometimes it would fart and blow flames on the gear change.

Driving up my drive way with a little free rev also saw the car stall.

But I'm sure the stalling issue could be ironed out with an adjustment of the fuel recovery point.

Apart from the low throttle low rpm issue; if I were to use the car for track or primarily drift, drag etc I would most definitely do the mod.

[Steve]

Something else to try, adjust your idle bleed solenoid (AAC Vv), open it up a bit and see if that fixes it.

[Cubes]

I'm certain bumping idle and the fuel cut recovery rpm would sort the occasional stall issue.

I'm going to drive around on it for a few days and see how it goes.

Your are 100% on the mark regarding response. Definitely much nicer through the hills.

[Steve]

ah adeliade hills, god I miss them

[Cubes]

Hows the police over in WA steve? A little easier than over here?

[Steve]

bloody awesome, I passed my car over the pits with heaps of mods (coilovers, 2way, slammed etc, etc), got permits for it all

Only time I have been pulled over was getting sideways on an on-ramp, just gave me a fine, no defect, no hooning bullshit, nothing - loving it

[RaZ_TuRbO]

unless this post is closed this thread will go on forever.

I think people should express there experiences WITH and WITHOUT bovs and then let the owner of the car make there decision.

Coming from a background of vl turbos and currently driving an r32 i really think bovs are unnecessary and if not having one does damage at ALL it must be very very minimal.

As for the turbo damage issue why not get the best of both worlds..just buy a cheaper small bov and tighten the spring really hard so on low boost it flutters and high boost it vents the air?(i think somebodys already said this)

my thoughts

cheers

Raz

[Cubes]

Well not really.. The thread did die until I decided to try blocking off my bov.. I posted my experience.

After driving the car last night and today for in total a good few hours I so much like the less lag between gears and how much quicker it spools.

With a larger turbo that lags a little more I think no bov would be a good thing. In my case I'm going to have to go back to having a bov as its just too harsh when driving in traffic.

I'm 100% convinced running a bov actually increases gear change lag. It still freaks me out how blocking off the bov has removed that fraction of a second delay before it boosts again.

But unfortunately I can't handle the roughness at low 2000rpm gear changes when its making reasonable boost.

You rb20 and rb25 blokes probably won't experience what I'm seeing as if loaded up I get the boost in by under 2000rpm, not loaded up it comes on hard at 2000rpm, ~2200rpm with a bov.