Running Two BOV's / Dual Blow Off Valves [Merged]

[hurricanecris24]

I'm getting my intercooler piping custom made and I wanna add another flange to run two bovs, would I have any problems running it on a single turbo setup till I get my compound setup made? I'm gonna be using dsmlink with sd.

 

[Absit]

I'd love for someone to explain why it would cause problems.

 

[hurricanecris24]

I don't think it would just wondering if anyone has experience running two, only person I know who has is paul

 

[biglady112]

Single HX52. Two BOV's.

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[97gst_spyder]

Pretty!

 

[dsmrollerz90]

Okay, now what is the advantage of runnig two bovs?

 

[LandoAWD]

I ran 2 1g BOVs on my car for a while. It's different, but one Tial does the job as well or better.

 

[JusMX141]

Good reading on this topic here, and the handful of posts which follow.

 

[rEclipserGST]

Single HX52. Two BOV's.

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Is that things external wastegate dumping straight up into the engine bay?

 

[jjrock5]

Is that things external wastegate dumping straight up into the engine bay?

Probably above the hood when it's closed.

 

[DSMTyson]

Is that things external wastegate dumping straight up into the engine bay?

There is probably a hole in the hood that the pipe goes into

I love the way everything is setup in that engine bay. So much less cluttered.
____________________________________

Damn, u beat me to it!

 

[GRAVEDIGGER]

Is that things external wastegate dumping straight up into the engine bay?

Out the hood.

 

[GsXdSmKingGsX]

Kris, did you notice a big difference going from 2 1g's to that Tial?

 

[JusMX141]

Out the hood.

^ A buddy of mine has his run out the hood like that....his TiAL is mounted upside-down so the outlet faces upward.

 

[black_gst]

anti-lagging would look awesome!

 

[polarmoment]

Out of curiosity, Justin, do you have any data on turbos whose failures can be directly and solely attributed to off-throttle surge due to lack of any blowoff valve? I think you've probably seen more turbos here than anyone else, so if anyone would have real data I'm guessing it would be you.

Reason I ask is because there is a lot of internet lore out there about how utterly destructive off-throttle surge is, but no real evidence to back it up. A lot of very notable folks (Simon Gishus of Nizpro, Corky Bell, etc) have stated that it's not nearly the issue that people think it is and that blowoff valves serve a very different purpose than most enthusiasts would believe. I've found turbos that were misdiagnosed as killed by surge when they were actually killed by overspeed. I've seen people claim that it causes the compressor nut to come loose, but they were from people who typically didn't use any loctite. I've found cases where people have had turbo failures that occurred about the same time as a valve failure, but no other corroborating evidence beyond that, and logic dictates that correlation does not prove causation. There have also been many cars that came from the factory without them, most notably from Lotus, Ford, GM, Dodge and Toyota (the SW20 had one, the ST185 using the same engine/turbo did not. There does not seem to be any statistically significant difference in failure rates between the two models).

The physics side of it suggests that the guys saying they aren't necessary are probably right. Any extra thrust loading from the backwash of the charge pipes decompressing through the comp housing would be negated by the fact that about half the thrust loading is gone as soon as the throttle closes and cuts off drive pressure to the turbine.

The pressure wave in the charge pipes isn't what people think it is either. I've heard people say that pressure spikes to 50+psi, but that simply isn't true. This is a pressure log of the charge pipes without a blowoff valve. This is a log of pressure with a valve. There is no significant spike in boost pressure, and the only real difference is a slow ~3psi oscillation of pressure as the compressor stalls/unstalls while the charge decompresses and the rotating assembly loses speed. The "massive pressure spike" from the throttle closing is really only about 3-4psi above nominal boost.

There are also people who think the compressor wheel stops, or spins backwards during stall, but that would be in violation of the law of conservation of momentum (if the wheel stops/spins backward, and there is no exhaust drive due to closed throttle, what makes it spin forward again?) In reality, there is about as much energy stored in the spinning impeller assembly as an engine flywheel near redline. The assembly's change in speed is based almost completely on the loss of drive pressure (very high angular velocity with mass centered close to the axis of rotation- lots of stored energy), so I doubt the bypassing of the compressor has as much effect on wheel speed as it might immediately seem, given the energy involved.

While I wouldn't recommend running without one on a MAF metered car due to reversion, I run speed-density and I chucked mine in the bin years ago and haven't seen any evidence thus far that it's caused any damage at 20psi daily (turbo is on the bench now and looks perfect). The only effect at this point is better boost response between shifts- reasoning being the same as the turbo era F1 cars, IMSA GTP, Group C not running valves, and those turbos saw more abuse in the course of a single race at LeMans than most of our turbos will see throughout their lifetimes.

Thoughts?

 

[DSMTyson]

Look who wrote an essay ^^^ haha just kidding. Thats very interesting. The part where you explained how the compressor wheel couldn't possibly rotate backwards during compressor surge makes complete sense(the rest makes sense too, this was just what got my attention the most). I don't know why I never realized that.

So you are saying that you think turbo failure is mostly because of "over-spinning" the turbo? right?

anti-lagging would look awesome!

YEAHHH!!!

Is there any vids of him at the track?

 

[polarmoment]

Look who wrote an essay ^^^ haha just kidding.

Yeah. This is one of those subjects that people tend to get militant about, so I wanted to present my reasoning as thoroughly as possible. Most of the time when you mention that you're not running a BOV, people just start calling you stupid without understanding the reasons behind it.

So you are saying that you think turbo failure is mostly because of "over-spinning" the turbo? right?

Not necessarily, just that one of the best examples I thought I finally found of a turbo that failed specifically because the BOV hadn't worked ended up being misdiagnosed. What actually happened was that the spring in the valve failed, essentially locking the piston in a slightly open position. He'd get surge when he lifted off the throttle because the valve wouldn't vent much, but the real issue was the boost leak it created. The leak meant the turbo had to spin significantly faster to maintain the same boost pressure. Considering he had already been pushing the TD04 to over 20psi, the thrust bearing couldn't handle the extra load of spinning 20-30% faster just to maintain boost pressure.

Anyway, his turbo failed, so he sent it out to be rebuilt. He didn't fix the BOV though, so when the rebuild came back it failed about 5 weeks later for the same reason, but he attributed it to the surge induced by the valve not venting, rather than the overspeed caused by the boost leak.


I'm really just trying to get a better understanding of it all. I've been looking for actual, real-world examples of failure that can be directly and solely attributed to lift off compressor surge (NOT full-throttle surge) and haven't found any yet. I'm thinking if they exist, Justin would surely have seen one given the number of turbos he works with. As of right now, all the evidence I've been able to come up with suggests there's no real reason for them besides minimizing noise (Mazda even states this in their factory service manuals for the RX7 turbo) or upsetting mass airflow sensors with reversion. I do know that boost response on my TD06 is noticeably better without one. I have almost full boost as soon as I get back on the throttle after a shift.

It's also easier to modulate the throttle in the corners without it. The MR2's OEM valve is so quick to respond that even a slight drop in manifold pressure can open it, and suddenly you've gone from trying to modulate 7-8psi on corner exit to 0psi. It might not be a huge deal to do so in a DSM, but in a mid-engined car, significantly lifting the throttle/losing power mid-corner is asking for the car to swap ends.

 

[LandoAWD]

Kris, did you notice a big difference going from 2 1g's to that Tial?

Quicker BOV response, less hoses in the bay, and sound. The others acted like the SSQV does that I'm running as a temp BOV. One would whoosh, and the other would chirp off the rest of the charge.

Not a bad setup, but not as serviceable as a Tial setup.

 

[Boosted98gsx]

Okay, now what is the advantage of runnig two bovs?

Really? More mass flow out on boost discharge, durr.

 

[JusMX141]

Out of curiosity, Justin, do you have any data on turbos whose failures can be directly and solely attributed to off-throttle surge due to lack of any blowoff valve? I think you've probably seen more turbos here than anyone else, so if anyone would have real data I'm guessing it would be you.

None first-hand, but then again I don't believe I've ever rebuilt a journal bearing turbo which came from a car that had no blow off valve at all.

I do have experience with a 20G from my buddy Jeff's car a few years back....Jeff was changing motor mounts and forgot to hook the line up to his Dodge Garage-Modded 1G BOV then proceeded to drive the car around for a week or more like that at 22psi. The next trip to the track the turbo started howling. Upon disassembly the outer edges of the journal bearings were smooth, showing that the shaft was moving violently enough to cause the bearings to contact the journals as a result of the surge.

Early Chrysler turbo cars did not have bypass valves, but they ran 6-8psi from the factory.

I've found turbos that were misdiagnosed as killed by surge when they were actually killed by overspeed. I've seen people claim that it causes the compressor nut to come loose, but they were from people who typically didn't use any loctite.

Modern turbos are built with the nut's threads facing the opposite direction that the shaft turns....meaning a shaft that spins clockwise will have a left-hand threaded nut, and vice versa. Thus it is virtually impossible for one of these turbos to lose a compressor nut due to surge or excessive boost.

In addition- ALL of the 14B's I've seen which have lost their locknut were the early 01010 models that had a right-hand threaded turbine shaft.

There are also people who think the compressor wheel stops, or spins backwards during stall, but that would be in violation of the law of conservation of momentum (if the wheel stops/spins backward, and there is no exhaust drive due to closed throttle, what makes it spin forward again?) In reality, there is about as much energy stored in the spinning impeller assembly as an engine flywheel near redline. The assembly's change in speed is based almost completely on the loss of drive pressure (very high angular velocity with mass centered close to the axis of rotation- lots of stored energy), so I doubt the bypassing of the compressor has as much effect on wheel speed as it might immediately seem, given the energy involved.

It doesn't stop moving....it can't stop moving as long as there is exhaust energy from a running engine that is driving it forward.

What the surge does is cause the shaft to shake violently enough that the bearings will contact the housing and shaft through the oil film, causing rapid wear. Of course this does not occur on ball bearing turbos because the shaft is always 100% supported by the bearing and does not rely on a film of oil for protection.

 

[TSimage]

2 Tials w/ B&W S472

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I am contemplating running one of the new Tial Q bovs which do 60% more air than the regular Tials but I already have the 2 regulars so only time will tell.. +1 for ASAPerf.

 

[polarmoment]

None first-hand, but then again I don't believe I've ever rebuilt a journal bearing turbo which came from a car that had no blow off valve at all.

I do have experience with a 20G from my buddy Jeff's car a few years back....Jeff was changing motor mounts and forgot to hook the line up to his Dodge Garage-Modded 1G BOV then proceeded to drive the car around for a week or more like that at 22psi. The next trip to the track the turbo started howling. Upon disassembly the outer edges of the journal bearings were smooth, showing that the shaft was moving violently enough to cause the bearings to contact the journals as a result of the surge.

Is there any possibility that something else could have caused it? I only say that based on the relative lack of failures in comparison to the number of people who do run without them. Not only race teams, but it's actually pretty popular in Japan and Australia to ditch the valve, but every now and then there seems to be some fluke of a failure that may or may not have been caused by it.

Early Chrysler turbo cars did not have bypass valves, but they ran 6-8psi from the factory.

Nor did the Lotus Esprit, both 4 cylinder and V8 turbos, the Ford 2.3 powered cars (Tbird/Mustang/XR4Ti), sytys, GN, TGP, Celica all-trac, etc. There's actually a lot of precedent in not using them- much of it coming before the stricter emissions era. The ST185 was actually a homologation car for Toyota to run in the WRC, which makes the fact that they omitted it on the celica, yet used one on the MR2 with the same engine an interesting case.

What the surge does is cause the shaft to shake violently enough that the bearings will contact the housing and shaft through the oil film, causing rapid wear. Of course this does not occur on ball bearing turbos because the shaft is always 100% supported by the bearing and does not rely on a film of oil for protection.

That makes sense to a degree for sure. I'm just trying to reconcile the relatively small mass of air's destructive influence on a rotating mass with so much angular momentum and energy.

I think the best way to think about what's going on is to imagine the air like a slinky moving through the compressor. Each oscillation represents the air changing direction and moving backward for a split second before the compressor can catch it and pump it again. This happens as the slinky stretches back out to ambient pressure over the course of a couple seconds. The angular speed of the compressor wheel decelerates based on the speed at which it sheds energy from frictional losses. The slinky of air vibrating across it represents only a few grams of air mass for any given oscillation moving either way.

Also, I'm not here to tell people to remove their BOVs at all. It's sure as shit not going to hurt anything to run one and that's a decision that's up to the individual user. I'm more trying to figure out why the failure rate doesn't support the paranoia about it.

 

[DWR-DSM]

Anyone yet come up with the pros/cons of 2 bov's, such as the tials?

 

[JusMX141]

Is there any possibility that something else could have caused it? I only say that based on the relative lack of failures in comparison to the number of people who do run without them. Not only race teams, but it's actually pretty popular in Japan and Australia to ditch the valve, but every now and then there seems to be some fluke of a failure that may or may not have been caused by it.

Turbo was on the car for around 5k before that, no issues- rebuilt it with new journal bearings and seals and the turbo stayed on the car until the car was taken off the road a year or so ago for a complete makeover. Turbo was removed and rebuilt yet again to be sold- no unusual bearing wear.

I just find it odd that the same week the BOV wasn't opening was the same week the turbo started making noise and showed wear on the outsides of the journal bearings....which is enough proof that I'd never go without running one on my car(s).