RotaryWeaponSE7EN, If you could please explain how a BOV would've helped in that circumstance I would love to know.

Turbonut, I though I already posted why they're an emmission device?
to SUPPRESS noise (compressor surge) and also because cars with MAFs don't tell you which way the air is flowing, they just measure the moment of it basically. So it would fool the ECU into thinking it should run the car rich, when in fact, it shouldn't.

and jon88se I dare say the reason he blew his 16g's was because he was running them at 20PSI, hardly a particularly smart idea (especially if they were not rebuilt second hand units......)

Gee lets see, if the bov was there when he let off the gas, the excess pressure and the backfire itself would have been released in the atmosphere. Rather than hitting the compressor fan and destroying the turbo.

Must've been a pretty hardcore backfire, so his throttle body also got blown away aswell then huh? Sure it wasn't that hitting the compressor wheel...?
and besides the BOV wouldn't have opened up if the motor was back firing through the inlet manifold, see they rely on vacume to open... backfires don't create a vacume.. :)

Yeah it was a backfire, took the throttle body and the front housing cracked. Happened at a SCCA event in topeka a couple yrs ago. Not my car but a neighbor of mine. Thats what she told me and she's been racing for decades. So I could be wrong, but I believe her.

http://www.enjoythedrive.com/content/?id=22417

Google is an amazing tool, isn't it? :D Of course don't believe everything you read, but I'm tending to agree with those who say that it's there to prevent compressor surge and keep the turbocharger spooled inbetween shifts. At the very least, through the debate, it would seem to be better to play it safe and release the pressure than to take chances with unsubstantiated claims that it won't damage the turbo if not installed.

Whether or not it needs to be vented to the atmosphere or back to the intake is personal preference. I kind of like the sound personally, but it's not necessary. If the same results (performance/reliability oriented) can be accomplished for a significantly lesser amount of money, then that's the route I would take.

Bent compressor fins
 

I used to have a Dodge 2.2L turbo. In the fastest drag run it ever made, I got a 35psi spike (I think, the gauge is 20 psi, but the needle went all the way past the word boost). It did stop the compressor and bend the metal fins. No BOV of course.

Tom

Get a BOV. They sound k3\/\/L!

richard.....LOL.

Thank you sir.
So the reason they put BOVs on cars is because they anticipate that people are going to run much higher than stock boost and they don't want them ruining their turbo's? I think not...
They are put on cars for emmissions reaons, sorry if you think I sound stupid, but its a fact.



And I can't believe that you 'Engineer' think that ~35 PSI of air pushing on a blade of metal would damage it? I tend to think that the bearings gave way and the wheel hit the housing.. thats generally what happens.
Now I have a compressor wheel sitting on my desk right here, and I can tell you that even with a POINT load of 20kgs doesn't do sweet F A to the wheel.

So if you'd like to explain to me how a distributed load of 35psi would damage it i'm all ears :)

if u think it will last then try it..
anything thats spinning around 100,000rpms nad then all of a sudden a force acts against it slowing it down dramitaclly does something. either to the bearings or the wheel itself

I love how people spread misinformation as a matter of fact...

BOVs do prevent compressure surge... do you have any idea how much force it takes to pressurize 15 PSI of air? how about 30 PSI of air? You said earlier that it "wasn't much" because it was air? Have you taken physics at all? LOL.

On a side note, atmospherically venting your BOV isn't a good idea either, it can cause a sudden lean condition (which is bad on your rotary, but you should know that by now). The proper way to vent it is back inline, after the MAF, before the turbo.

There's a little more to why race cars don't use BOVs then "because you don't need one" Not everything on a race car has to do with longevity. Here's a tip, wheels, tires, and brakes on a full out race car are replaced after every run, hell, connecting rods on any full race car are replaced every 2-3 events... but you're right, they'd never sacrifice longevity for performance. Idiot.

edit: Read the post you added. It's not completely about the pressure of the air, compressure surge has to do with sudden changes in speed of the compressor wheel. Inertia is the primary enemy.

I think we just do things different in North America..

Just because it cavitates (ie, the 'Chooff Chooff' sound) does NOT mean the turbo has stopped, quite the opposite in fact.

As Adsy01 said earlier, the turbo is slowing down at a massive rate anyway.

So of course if you shift slow, the turbo will slow down either way. Which way does it faster is a matter of debate, unfortunatly its not THAT easy to get an accurate rev/min reading from the compressor wheel.

But what is for sure is people who run race cars have found better shift performance NOT using a BOV. This tends to say that having the added pressure in the intake pipping out weight the benefit of trying to slow the rate of change of the compressor wheel by adding a BOV.

.....15Psi IS a force?..... 30Psi IS a force... I don't see your point, how much force does it take to create a force?

I'm studying mechanical engineering, so yes I have done physics..

Wrong it will go rich.. not lean.

Oh please, let me know of a race team that would not put a $50 part on their car that would save their $1000 turbo? I certainly know of a few that would, but they have killed turbos with and without BOV's. Now they don't run BOV's... something about better shift performance... funny that.

That post was in reply to the guy who said he bent his fin.
Nothing to do with inertia, sorry.

but if you wanna talk about inertia, go right ahead.
Have a look at the construction of a compressor wheel, where do YOU think the moment of inertia is? ... certainly not near the outside of it, thats for sure.

When I spoke of inertia, I wasn't meaning it's force in relation to bending a fin, so much as flexing the shaft and damaging bearings. When you start talking about lightweight ceramic wheels, you can risk breaking fins though. Much less malleable then pure metal. You're right about the rich vs. lean though, I got that one backwards, long day.

You're right about the shift performance though, no BOV will increase your performance during shifts, no argument there at all. HOWEVER, you forget an important fact, if a part in a race car will increase performance at the cost of longevity, they'll use it, that's why it's a RACE car. Which was the point I was attempting to make before. Just because race cars use it, doesn't make it a great idea for a street car, I return to my point about connecting rods. You're massively oversimplifying matters.

ME major eh? True massichist I see. Purdue EE alum myself.

I think its the whole northern hemisphere.

:D

It's not actually anything to do with the inertia of the wheel. (or, i should add, not for the most part, since it's moment of inertia is practically lumped right at the center of the wheel)
The sideloading on the bearing is created by the force/couple system made from this air pushing back against the blade and its distance from the centerline of the axis.
However, this force (call it 40Psi) has got nothing on the force exerted by the exhaust puleses hitting the rear wheel. which is fairly close to the same distance away from the centerline of the axis.

To a point yes I agree they tend to sacrifice performance for longetivity. However through conversations i've had with people running turbocharged race cars they, and also their turbo repair places I might add, agree that running a BOV will not help out the bearings at all.

Yeah I do also actually work at a machine shop on my holidays.. I started out doing EE, but looking at black boxes all day and measuring outputs vs inputs just wasn't my bag. :)

Australia... Home of the worlds only rotary powered vehicle to do 200mph+ in a quarter mile!! :D


Anyway....

It states in the Mazda factory service manual that the BOV is fitted to prevent noise. It's a fact. Anyone care to argue with Mazda about it?

I have noticed no performance increase during shifts with/without a BOV. And yes I've tested it at a racetrack. My stock turbo running upwards of 15psi is still alive and well after 12 months of no BOV.

Read this (it's long but worth it) - Click me

Wastegates have no place in this conversation.

Thank you S2-13BT!

I was trying to remember where that thread was.

Good read on the subject thats for sure.

Err....Okay you say bearings gave way. Could that still indicate that a BOV would have helped prevent damage to the compressor? Also have you considered the effect of fatigue loading vs. static loading? Compressor surge causes a large number of fatigue cycles on the bearings or compressor blades. I've seen compressors with blades prematurely broken from fatigue due to compressor surge.


Err.. Cavitation is a term used for viscous fluids ie liquids. Air doesn't cavitate... does it seriouly? Besides, is compressor surge really that loud? I've ran my car a few times with a non functional factory bypass valve(vacuum hose poped off) and as far as noise level goes.. there was no difference.

Actually some rally cars use small BOVs. Research and experience has shown that maintaining pressures of up to 4psi in the intake pipe between shifts is optimal in reducing power loss between shifts. I'm assuming that powerloss becomes minute above 4psi and is negligible so these rally cars that do run BOV's that releases pressures above 4psi to reduce surge.

Well... most racing teams that run turbos run ball bearing type turbos which are somewhat imune to compressor surge damage(please note.. somewhat and not absolutely).Also I don't remember who said this but it was one of the F1 car designers he said the best race car crosses the finish line in first place and falls to pieces


Well... rally cars do run them as stated above and ball bearing turbos are somewhat resistant to compressor surge.

Engineers aren't the most eloquent people in this world.. Could they have meant it prevent's noise compared to BOVs? (Factory Service Manual calls them bypass valves)

And err.. this is a little off topic but as far as the "Mazda engineers know best" arguement, why does everyone agree that mazda's engineers approached the fuel pump wiring design the wrong way? What about the subzero start assist system?

Cool, any datalogs? anyway rally teams have found that by maintaining at least 4psi of pressure in their intake tracts, powerloss when shifting is negligible.
Well... congrats on running it for 12 months... The turbo on my car has been on my car for 16 years and 170,000miles.......non-balbearing center section.

FUNNY!! Oh wait, not.

Exhuast pluses cause a rapid sucession of fatigue loadings on the exhaust wheel aswell?
I'm suprised you were able to determine that the blades broke because of compressor surge? What were the signs of this? because my old turbo ran without a BOV for quite a few years (before I got hold of it) and there do not appear to be any immediate signs of fatigue on the blades?

Quite true it is a term used for liquids, yes. But the same thing basically happens when you have any sort of pumping device like a compressor section of a turbo. You have areas of high and low pressure. While it may not technically be right to call it a cavitation it is basically the same thing in the sense that it interupts the flow badly. Just in this case (air) its not inducing a change of state.

I thought the system they had on rally cars (at least the one I saw a while ago) pumped the air from the BOV into the exhaust manifold to aid in spining the turbo up faster next time the throttle was smashed down again? (anti lag style..)
Got any links on the new style of just venting off unwanted air? I wouldn't mind having a read.

The example I was using for the race setup was actually _not_ a BB turbo.

And I always thought BB turbo would be more prone to having additional sideloads on the bearing due to them running larger clearances on the bearings?

Well This was a research done by a professor of mine. The blades were damaged due to fatigue since they had the characteristic where it transitioned from a hairline crack till and ended with brittle fracture.


Cool.. I think when talking about turbos or air pumps, the term used is surge. Im just guessing since they have a "surge" line on compressor maps.



The article I read was on a magazine quite some time ago so sorry.. I don't have a link. Damn I haven't been keeping up with that... I guess I need to start reading on rally systems again. I ddn't know BB turbo's had larger clearances on the bearings? But I guess from what I've read those BB turbos can take larger side loads(probable even if they have larger clearances).