...or you could buy a couple of bosch bypass valves and have about the same flow rate. Or order one from mitsubishi for an EVO, although I think thats made by bosch aswell? apparently very close to the same size inlet/outlet as the normal bosch bypass valves but a higher rated spring in it i think..

All the name brand manurfactures seem to harp on about how they improve your performance whilst shifting.

They never seem to harp on about how if you don't run one you will kill your turbo..

Strange...

 

If a BOV will infact improve performance while shifting, then isn't this whole debate moot? Regardless of whether or not it will or will not damage the turbo due to compressor surge, if it'll help keep pressure inbetween shifts so that turbo lag does not need to be dealt with in every gear, then the advantage becomes obvious and it will be installed. The added measure of safety for the turbo--whether it's actually an added measure or not--would be a bonus. If nothing else, it would add to the psychological effect of keeping the turbo intact; which might be enough to inspire enough confidence in the driver to win a race.

If you think it's faster, it'll be driven faster.

 

I do see WHITE FC's point...in addition there are many older turbo cars that left the factory without any kind of blow off valve. Nissan's first 2 series of Z turbos lacked them (81-83 280ZX turbo, 84-86 300ZX turbo) and I'm sure people have made it to 100K miles on the stock turbo. BUT, there is no advantage to running without a blow off valve and only advantages to running with one. There is an additional stress to the turbo being hit with reverse pressure once the throttle is closed without a BOV. A well designed BOV shuold improve boost/throttle response on WOT shifts as well. You can get a nice BOV on the cheap for sure...the 1G mitsubishi Eclipse/Talon/Galant VR4 piece is fantastic. Toss it in a vice and crush a used one to the proper spec and it will hold 25psi without any problems (crushing just stiffens the internals a bit to hold more boost). These BOV's are pretty much trouble free and very inexpensive. I wouldn't use one from a new EVO, they tend to leak a bit. However, the BOV from an EVO MR (in Japan now, coming to the US soon) is a nice upgrade for that car or any car that needs a cheap good BOV. I think they go for like $65. On EVO's I have driven, the MR valve is a night and day difference in throttle response and boost holding ability (and it's quiet). My Galant VR4 has a crushed 1G valve and runs 21 psi...it's very sedate sounding. My FC runs abour 13 psi and the GReddy Type S is deafening.

 

Think about it. Manifold pressure spikes while the throttles are closed with a blow-off valve, whereas pressure drops with a blow-off valve. Which do you think will offer better performance? More boost when the throttle plates are reopened after the shift, or less?
Here's more proof: When comparing STOCK turbocharged cars' factory 1/4 mile ET's, the automatic versions are usually faster. Here's why: the automatics don;t close the throttle during a shift, so there is no loss of boost.
A perfect example is the Starion: 14.9 manual trans, 14.8 automatic.

 

Out-of-norm bearing wear and cracking of the compressor wheel (which is typically a rarity.

 

I'll hazard this one too.
These big-name manufacturers are out there to get money. They would see decreased revenue from stating the product as a reliability product as opposed to a performance-adding part.
Example: How many people get a blow-off valve before a more effective radiator? If the radiator manufacturer claimed to add power, its sales would be higher.

 

I agree with you, as I said earlier many race teams have found it better NOT to use a BOV.

Edit noted reply above:So why do companies that sell aftermarket BOV harp on about how they improve power during a shift by keeping the turbo 'spining'? Why don't they talk about how it kills bearings if you don't have one?
Strange, no?

Edit: I Still find it strange though..

So you say compressor surge does kill bearing markedly faster.. I find this interesting. I don't see how 1) a large car company would knowingly sacrifice bearing life of the turbo by not running a BOV. 2) Exhaust pulse would put a greater sideload on the bearings than a compressor surge...

 

I thought I already explained this. I guess I didn't do a good job. The reason you get surge is because the exhaust gasses are trying to spin the turbine one way and the pressure in the manifold is trying to spin it the other way so the whole assembly tries to stall thus causing surge and that side load you are talking about.
I know you said that there "effectively" is no exhaust flow in between shifts but that I think is not true. Eventhough your tp is at zero your engine is still pulling some air through the tb and you still have exhaust flow. If you don't have exhaust flow, then you won't have a surging compressor. Again reason you have a surging compressor is because air in the intake manifold is pushing the compressor one way and exhaust is trying to push the turbine the other way. With the compressor and the turbine being connected with a shaft that's suspended on bearings at different locations while spinning, the problems becomes a dynamic one where the whole assembly actually vibrates.

 

actually most car companies have a way of releasing extra manifold pressure. most have a valve that is almost always open so you arnt always getting full boost. and some have bov, just not loud ones. these BOV go back into the intake piping.

 

It'd be interesting to see just how much exhaust flow there is when you lift off. I suspect it would be very little. You get the chopping air noise (surge) because the air is being forced back on the turbo which is spinning because of it's momentum, not any flow through the exhuast wheel. The surge lasts for about 1.5-2 secs after lift off without putting the foot back down (at least in my car, and that's only based on what I can hear).

There's no fancy calculations for any of that, just my assumptions!!

 

Yes, the exhaust flow is negligable, argue it until your blue in the face, but sorry your just wrong.

Do this, get someone to rev your car whilst you stand at the back with your hand a little way away from the exhaust outlet. While your accelerating the exhaust will be many orders of magnitude larger. Therefor negligable.

S2-13bt hit it on the head. Its momentum causes surge.
Incidently the momtum of the compressor wheel is much closer to the centerline of the axis than the exhaust wheel.

So that goes to say that the exhaust pulses hitting the exhaust wheel will give greater sideloading on the bearing per unit of force applied over the same area as compared to the compressor wheel.

SOoooo I don't buy the fact that the bearings will wear out faster.
And if surge did kill compressor wheels I don't see how so many manurfactures would have released cars without a BOV.....

 

look my face is blue Thanks for telling me I'm wrong when you don't even fully understand the topic yourself.


You know what's the funny thing? I was just about to suggest the same thing. Although under acceleration you will have more exhaust flow once you get off the throttle there is still significant flow. Remember that what goes in must come out and engine is still taking in air at 7000rpms when you are shifting. And what "little" resistance on the turbine that result will cause the whole assembly to vibrate.

Momentum itself isn't sufficient to cause the amount of surge people have measured in the piping.. it takes more than the momentum of the compressor wheel to do that.

 

That is the most non-technical, simpified, correct explanation....good job.

 

The point is though, that a proper BOV only has advantages. There is no downside to it...

 

Besides....Do the exhaust gasses ever try to spin the turbine the opposite direction from which it's already moving?

 

Wow yall still going at it.

 

I think im going to sumarize this whole post into a few lines. Hope i do a good job, let me know if i suck.

1. BOV are for letting air out of the intake track between the turbo and the throttle plate so that pressurized air doesnt smash back into the turbo compressor and screw the compressor blades and the turbo oil seals and bearings.
2. BOV are also for emissions because of the fact that when the fuel system is calibrated it has to recirculate that vented air, which is air that has been messured by the air flow metering device, computed for the propermixture by the ECU and its necesary to put it back in the motor otherwise you get that rich or poor idle condition.
3. BOV when vented to the atmosphere do have that cool "pssshht" sound that too many people who dont really know or care about cars enjoy so much.
4. BOV open when the throttle is closed and are different in functionality to Pop Off Valves, if your BOV is letting out air under idle the diaphram is leaking.
5. POP are like wastegates or actuators after a certain amount of boost they will open and let off that excess boost pressure, but will be limited to the size of their valve diameter. They are a safety precaution so you dont over boost.
6. If you dont use a BOV every time you let go of the throttle you kill the turbo little by little becuase the air has only that way to go, you can also burst a turbo hose or even an intercooler if your not careful.
7. If your turbo is not leaking oil or working fine and you dont have a BOV you will break something eventually.
8. Some manufacturers didnt put a BOV on their turbo cars due in part to cutting budget costs and if the system doesnt run more than 4-5 or even 6psi its really not that necesary.
9. You can keep boost levels high when you shift if you use a smaller than needed BOV. That way your turbo doesnt have to spool back up completely just maintain boost.
10. The other reason for recycling the vent gases into the intake pipe/track is to keep noise down to a minimum, even on non AFM equiped vehicles.

I hope ive covered the proper and improper ways of using BOVs. There where a lot of right opinions in this post and also a few worng ones. But hey, ist all about who goes faster and who breaks less parts doing it, right?

Peace out!

 

Forgive me for my arrogance but I feel I understand this a bit better than you if you feel an engine at 7000rpm will ingest the same ammount of air if the throttle is open or shut...

Think... Boost/Vacume....
Well.. I disagree.

 

Of course not? They do act to spool it up mighty quick in some instances though. But whats that got to do with the price of eggs in china?

We were talking about focers per unit area acting on a radius away from the shaft.

The bearings, with respect to sideloads, certinaly could not give two hoots which way it's trying to turn.

 

No problemo I know alot of arrogant people. Apparently you also think you understand this topic a little better than 99% of the turbonuts around a majority of which has more experience than you or me. That works for me. I never said that it would ingest the same amount, I just said that it was significant.


What are the price of eggs in China anyway? 1rmb?
You are over simplifying the problem. I already mentioned that it is a dynamic problem where vibration is one of the main problems.

 

Well all I can say is that a turbo repair place said to a race driver who told me that they said that a BOV would not solve any problems to do with turbos failing during a race..
They are obviously alot more experienced than I, goes without saying.
Theres also some much more experienced guys than I that replied to that thread that S2-13BT posted from ausrotary.com earlier who said a rather similar thing.
Probably just coincidence though..


Undersimplying it by saying that the exhaust flow is negligable? It is orders of magnitude smaller than when your at WOT. That for the sake of this argument should be negligable..

Do you hear your turbo keep pumping air out when you back off even with a BOV fitted? I certainly havn't. All i've ever head was a loud 'Pssshht' and then it stops, no matter what RPM, even be it 7,000rpm. The 'Psssssht' doesn't go any longer.


This still doesn't change the fact that they didn't start introducing BOV's until polution laws started getting tougher.
Even nissan didn't. Until the very late 80's early 90's.
Turbos were quite well understood by that stage (hell they pretty much were back in WW2)

That by itself is saying a fair bit in my opinion.

 

Well in this situation we are considering a condition where you are at 0% TP and not WOT. And comparing the pressure between the turbine and the exhaust ports vs the the pressure between the compressor and TB. Let's say you have 1psi exhaust back pressure before the turbine and you're running 10psi of boost. The pressure acting on the turbine is 10% that of the pressure acting on the compressor. I think that 10% is significant enough. Also like you mentioned the turbine is better designed to spin with that kind of loading.

Me neither but that's besides the point.


Nice history... I didn't know that. Maybe it's a coincidence? Maybe later on they stumbled on the other benefits?

 

Well those are certainly some interesting numbers.. But either way, going with your 1psi assumption that means basically (excluding vibrations which isn't an unreasonable one given the bearing design..) that there would be say instead of a 38psi pressure spike from a mean of 22psi there would be a 34Psi pressure spike.
Now given that somewhere in the RPM range of the engine whilst the turbo is able to produce this 22psi there will be (at least for nearly all street setup cars) an exhaust manifold pressure of at least double that 22psi.
So you could then have 44psi or more acting on a wheel with a mass moment of inertia closer to the outside of it and made of heavier material.
That to me says that the killing bearings faster from compressor surge is just a myth.


Not really beside the point at all.
If you agree that there is no difference in _duration_ of venting gases going out the BOV (once the turbo has reached its maximum set boost of course..) from say 3,000 to 7,000 rpm then how can the exhaust flow NOT be negligable?

Possibly a coincidence, but I strongly doubt it. Centrifugal superchargers were quite well understood back in the early WW2 days. I can't see how they left a major reliabilty hole left in the design of the system for that long (until 1980/90).

 

Why can you exclude vibrations? What bearing feature negates this? That 1psi is a very conservative estimate and more characteristic of after turbine pressures(back pressure). Since the whole assembly is spinning (at 5 to 6 digit rpms) you can't ignore that this is a dynamic problem. This is even magnified since pressure is acting against the rotating assembly. Even if you take that conservative 1psi on the turbine side vs the 44 psi on the compressor side and factor in weight difference and physical attributes, you should still get an unrealistically conservative 1% of loading on the turbine side. At 1% I still think it's safe to say it's(vibration and surging) a problem as the wheels are spinning at such great speeds. How accurately are the rotating assemblies balanced anyway? 0.01gram-inches?

I didn't say that exhaust gasses have enough punch to to boost to such great levels. I said it's besides the point because we are comparing the loads on the rotating assembly itself.



Gotcha. Although it was pretty well understood I do believe we have advanced by leaps and bounds since then. Ball bearing turbos, Anti-lag systems, Composite compressor and turbine wheels(not polymer composites but more like ceramic or carbon), twin scroll, etc ,etc.

 

The shaft is very well supported at both ends, yes there will be vibration, of course. But for the sake of getting a comparative view of sideloadings in the bearings from the exhaust and the compressor surge you don't need too worry about it. Of course it will factor in but i'm only talking in comparative terms here between the compressor and the turbine.


You said that there would be a significant force still acting on the turbine. This just can't be true... If it were true and you used a BOV there would be a continued flow of air since there would be no backpressure on the compressor yet there would be a force trying to turn the turbine. Hell it should even _accelerate_ the shaft to higher speeds if there was still a significant pressure acting on the turbine. However unfortunate, this just doesn't happen though.... The shaft in fact decellerates very quickly even whilst using a BOV.


We have, of course made, advances in the technology no question. But the dynamics of compressor surge would have been quite well nutted out by then
Changing wheels and bearing materials would not change this greatly.