Anyone here run without a BOV?

[yoshimitsuspeed]

Good lord it's turned into lord of the flys in here.

I would have liked to have an educated discussion on the topic but I guess I was asking too much. Some of you have had good input but I have better things to do that read through all the 12 year old circle jerk and back patting.

A couple quick things.
My car is still running a BOV.
Talking about my car was in no way bragging about the size of my turbo or how much boost I was running or how much power I was making. It was merely an update as to what I have been doing in the last 10 years. Not that I recognize anyone on here who would care.
I took a different route. How many people here are running 7 PSI on 11:1 compression on a stock ECU designed for 8 PSI at 8:1 compression?


As was already posted several times on this thread, there are a number of people running 3:1+ PRs on MR2s, Alltracks and other 3SGTEs.
Sorry guys but you aren't as cool as you think you are if you think you have the monopoly on high boost motors. From what I have seen at least a good lot of them are running without BOVs.

As for those thinking I am trying to impose my beliefs on others not to run a BOV I think you should read this thread again and compare my posts to the disrespectful and condescending posts of others. I don't even have a belief one way or the other. I wish I could impose that belief on more of you.

Had we gotten to the point of having an intelligent conversation or had anyone actually asked for more proof or info I would have gladly posted more, searched more and likely learned more myself in the process.

Since that didn't happen I'll just leave a few things here that do provide more solid evidence against your general consensus than anyone here has been able to prove for it. Again, not to convince anyone but just to make people think.



AutoSpeed - Nizpro's Simon Gishus - Part 2

I've actually talked to an individual who works at Borg Warner's main facility about not running a BOV/by-pass valve and that the affects on the turbo itself are negligible. Here is his response:

"This is true, if you ever have a chance to see compressor surge tests done by the manufacture you will understand how much surge it takes to damage a turbo.

If you are on the top speed line (top speed line differs depending on wheel size) for example a medium sized turbo's max speed is somewhere in the neighborhood of 145 Krpm, now if you hit surge too hard at that point it can multiply trust loads 10 fold and cause a instant failure.

But the turbo will never see the condidtions that we test them under when applied to an engine."

The math has been done... The forces that cause compressor surge when the throttle plate closes pale in comparison with the other forces that are going on in there...

Numerous Video evidence has proven that pre-throttle plate pressures dont spike that much after a hard pull and and abrupt throttle close.

Turbo RPM speed sensor measurements have also proven little to no evidence that there are spooling improvements.

Ill be honest with you, I too used to think that a BOV was put in place to protect your engine from damage. But I think it was Texas awhile back who said that how much force do you honestly think 40psi of air in a 3 or 4 liter space has on an object that generates as much torque as your crankshaft spinning at 8000 RPM, Im talking of course about your turbo spinning at 100000RPMs...

Race car drivers NEVER use them... the only people that do are OEMs to reduce engine noise, and fanboy ricers...

The only time compressor surge is bad is if you are on boost. This means your turbo is improperly sized for you engine...

It depends on what you want. If you either like the BOV sound, or you like a quiet motor, then use a BOV.

If you are building a car that you actually want to go fast then go without...

The lie is that a BOV provides any degree of protection for your motor.
Its a tool used by BOV manufacturers to scare you into buying one.

Nothing in your link makes reference to damage caused by off-throttle compressor surge. I've yet to find ANY case of a turbo that was damaged by said phenomenon (that wasn't misdiagnosed).

Corky Bell, author of Maximum Boost has publicly stated the he's never seen a turbo damaged by off-throttle surge.

We could rehash this all over again, but it's already been said. Pablo posted the link.

What it boils down to is if it makes you feel better, run one. If you want better boost response between shifts, remove it. The turbo can handle it.

Me? I'm 2 years on a 20 year old turbo (from a vehicle that never had an oem BOV) at ~20psi with no problems. The car is faster without it, so that's the road I've chosen. Do the research and make your own decision.

Another happy BOV-less user here. LOVE the feel of the car without it and had zero problems with the turbo or ever seen any damage caused by off-throttle surge like said above.

On throttle surge is a whole different can of worms and can cause problems.

Also, small correction to an earlier post where i got credit for the math behind the turbo to flywheel comparison, that was actually scarecrow that did that math IIRC. I just checked it out and found it to be correct with some google searching. It even surprised me.

2years, 10000 hard/boosted miles (not a DD), 20-21psi, no bov, all is well.
Much better response between shifts.
Car came from japan with the ic pipe having no bov flange, so also ran who knows what boost and for how many miles without a bov on top of what I've done on it.

Granted it's an uprated apexi IHI turbo.

I will continue to run with no bov

Look up Crawford Performance. They build a GT35 turbo kit / FMIC kit for the Subaru STI, and do not provide any provision for a BOV. Everyone that has one loves it from what I could find.

Next is the fact that Crawford Performance offers their kits without BOV's... I checked for myself to be sure and on both the turbo and IC kits they aren't there. In fact, HERE is a quote from Crawford Performance in THIS thread on NASIOC:

Originally Posted by Crawford Performance
08-05-2009, 11:09 AM #21
Crawford Performance
Former Vendor

Member#: 42075
Join Date: Aug 2003
Chapter/Region: SCIC
Location: Oceanside

We have been working on turbo cars for twenty years that do not have any sort of bov.
We have seen a few charge pipes blow off but never had a turbo fail that incorporates a left handed thread on the turbine shaft. In the old days when the threads were right handed, some of the drag cars would spit off the compressor wheel at the end of the burnout

Team Crawford

Anyway I could go on and on and on and on.
But I have better things to do.

Take it for what it is. Information from a different perspective. That's it.

 

[Diamond Star Werks]

So how do you get better response from no bov that sends the charge backwards slowing shaft speeds or reversing it, than a recirculated one that keeps it flowing the same direction?

 

[Archer Fabrications]

I did ask what the be believed benefits were of a closed charge pipe sysyem. And if it was really worth the turbo system over an alleged few hundred rpm spool time.

No one said we had a monopoly on high boost motors, but the 4g63 is and has been the fastest 4cyl in the world for a while now.

 

[lasthope05]

I did ask what the be believed benefits were of a closed charge pipe sysyem. And if it was really worth the turbo system over an alleged few hundred rpm spool time.

No one said we had a monopoly on high boost motors, but the 4g63 is and has been the fastest 4cyl in the world for a while now.

+1

and Brent Raus 2g ran with a bov.

 

[LandSpeed-DSM]

How many people here are running 7 PSI on 11:1 compression on a stock ECU designed for 8 PSI at 8:1 compression?

speaking of back-patting..

But seriously, nearly anyone who has ever boosted their Honda can say the same. myself included.

If you do not find us receptive to your concerns, you can go stir the sh!t elsewhere.

Its pretty clear that you did not come here for a discussion. Your aspirations seemed to entail brief back and forth, before all of us were supposed to start fellating for dropping this knowledge bomb on the unwashed masses. Some of us are aware of the few OE turbo systems that came without. But what of the overwhelming majority that do?

We are to believe all of those engineers are simply rubes of the BOV cult? Or is it because they want the suckers to last 100k miles in all manner of retarded driving?

If an OE can save money and complexity they do. But the diverters and BOV remain a consistent fixture. You should examine why for yourself instead of pretending we are the idiots here for panning this silliness.

The benefits to not running one are specious suggestions at best. Negligible to the point you still run one. In truth I don't care what reasoning you hold for that. There are no downsides to run a diverter or BOV

The flow reversion is enough of a threat that in several of your cherry picked copy/paste quotes that blowing off IC pipes, shaft nuts backing off and thrust loads sufficient to cause failure over extended periods or in some cases immediately. We have first hand testimony from several end users regarding compressor stall leading to damage and failure. This includes someone who makes a living rebuilding turbos.

But we are the closed minded ones?

I'm quite familiar with the OE testing performed by the likes of Garrett, Holset and BW. With scant few exceptions in the myriad applications you will see their respective hardware.. the ones without a BOV are diesels.

Hers why some may feel that they are recovering faster without. The charge column not being evacuated means that volume does not need to be re-pressurized, but you are only looking at half the system. If the compressor is passing in and out of stall, shaft speed and consequently turbine speed have slowed too.

Now for those of us equipped to do so, you would see dramatic fluctuations in drive pressure and EGT as a result. This whole assembly now needs to be re-accelerated to reach your target PR, but has to deal with bringing drive pressure back in line while fighting to accelerate the compressor against the positive pressure upstream. A chaotic series of events, even by turbo system standards. This crazy set of alternating pressure differentials is going to dictate your cam choice as well, providing a major limitation on choices to sticks with low overlap and very short evo-evc intervals.

Instead of dealing with and tuning around that silliness.. just vent or recirc the charge and keep shaft speed up in the first place. Better yet, use a clutch switch activated ignition cut so you don't have to let off the throttle and fall out of boost in the first place

Stay slow, my friends.

 

[DSMBanditFZ6]

I did ask what the be believed benefits were of a closed charge pipe sysyem. And if it was really worth the turbo system over an alleged few hundred rpm spool time.

No one said we had a monopoly on high boost motors, but the 4g63 is and has been the fastest 4cyl in the world for a while now.

Correction it was the fastest. Pretty sure the GM 2.0 Eco tec is more recent.

But il add to the circle jerk and say, but none of those are mr2s and alltracs

 

[Archer Fabrications]

Faster than extreme psi's dragster?

6.57 @204?

Looks like the gm is 6.7

 

[LandoAWD]

Good lord it's turned into lord of the flys in here.

*Lord of the Flies"

I would have liked to have an educated discussion on the topic but I guess I was asking too much.

Posturing and irrelevant references ensured that would not be the case.

Some of you have had good input but I have better things to do that read through all the 12 year old circle jerk and back patting.

Your posts indicate that you do not, in fact, have anything better to do.

A couple quick things.
My car is still running a BOV.

Your 8cfm have little relevance to even the slow kids on here.

Talking about my car was in no way bragging about the size of my turbo or how much boost I was running or how much power I was making. It was merely an update as to what I have been doing in the last 10 years. Not that I recognize anyone on here who would care.

So; irrelevant to the "educated discussion", then?

I took a different route. How many people here are running 7 PSI on 11:1 compression on a stock ECU designed for 8 PSI at 8:1 compression?

On here? More irrelevant jibba-jabba by your own admission.

As was already posted several times on this thread, there are a number of people running 3:1+ PRs on MR2s, Alltracks and other 3SGTEs.

Irrelevant.

Sorry guys but you aren't as cool as you think you are if you think you have the monopoly on high boost motors. From what I have seen at least a good lot of them are running without BOVs.

Many can rebuild after a track day, too; irrelevant. Cars here live on jackstands for years at a time over a boost leak. Next?

As for those thinking I am trying to impose my beliefs on others not to run a BOV I think you should read this thread again and compare my posts to the disrespectful and condescending posts of others.

I read it. I mentioned posturing AND relevance, so...

I don't even have a belief one way or the other. I wish I could impose that belief on more of you.

Try doing so with a conversational attitude as opposed to a confrontational one. Good info was still posted, despite it having been posted before.

Had we gotten to the point of having an intelligent conversation or had anyone actually asked for more proof or info I would have gladly posted more, searched more and likely learned more myself in the process.

More proof of intent and posturing.

Since that didn't happen I'll just leave a few things here that do provide more solid evidence against your general consensus than anyone here has been able to prove for it. Again, not to convince anyone but just to make people think.

I'm trying to figure out what relevance several of the OPINIONS you posted (note the lack of DATA and inclusions of "fanboys and ricers" in your "proof") is to an "intelligent discussion". We have Google, and we can pull up posts/links/first-hand experiences on the opposite side of the "discussion".


I'm a GIANT asshole, and this whole thing stinks of the condescension and predetermined bias I'd use to rile up some folks.

 

[DSMBanditFZ6]

Faster than extreme psi's dragster?

6.57 @204?

Looks like the gm is 6.7

Oo your right.

 

[mustracetsi]

Everyones got thier two cents. Some run With some with out. Is all at your risk. When something happens. At the end of day is only your fualt. Every cars different may work for some but not for all. I had a friend whos teacher said bov was mainly for manual trans turbo cars. The fact that you got to clutch and shift gears was longer time that the tb is closed as apposed to and automatic. Then again autos got it too. I told him you better switch teachers because autos need them too. To me if a car has no bov. Then imo is really dont got power so the turbo setup like that not really doing nothing, when you got dsm that from factory everything in the car is setup for a bov and you can easily increase boost cause our motors can take it then is one less thing to spend money just for couple more psi. Once that non bov set sees real psi like max. You gona want to have had that blow off. Like fellow member said try holding hand over a hair dryer thats fine not really power thier but imagine trying cover over a jet turbine. Not only causing turbo flutter but you also putting strain on intercooler core, pipes claps and couplers. Your turbo may hold for bit but you causing your self more time working on the car then enjoying it.

 

[T is for TURBO]

Oh for ####s sake, I can't believe this thread is still alive. The following is straight from the horses mouth. Argue all you want with us, that's fine, but try to argue with Garrett Honeywell. They just MIGHT know something about turbochargers, I'd even venture they know a thing or two more than your MR2 buddies.

Q. What is the difference between a BOV and a Bypass Valve? How do they work, and are they necessary?

A. A Blow Off Valve (BOV) is a valve that is mounted on the intake pipe after the turbo but before the throttle body. A BOV's purpose is to prevent compressor surge. When the throttle valve is closed, the vacuum generated in the intake manifold acts on the actuator to open the valve, venting boost pressure in order to keep the compressor out of surge. Bypass valves are also referred to as compressor bypass valves, anti-surge valves, or recirculating valves. The bypass valve serves the same function as a BOV, but recirculates the vented air back to the compressor inlet, rather than to the atmosphere as with a BOV.

Source

Q. What is compressor surge?

A. The surge region, located on the left-hand side of the compressor map (known as the surge line), is an area of flow instability typically caused by compressor inducer stall. The turbo should be sized so that the engine does not operate in the surge range. When turbochargers operate in surge for long periods of time, bearing failures may occur. When referencing a compressor map, the surge line is the line bordering the islands on their far left side. Compressor surge is when the air pressure after the compressor is actually higher than what the compressor itself can physically maintain. This condition causes the airflow in the compressor wheel to back up, build pressure, and sometimes stall. In cases of extreme surge, the thrust bearings of the turbo can be destroyed, and will sometimes even lead to mechanical failure of the compressor wheel itself. Common conditions that result in compressor surge on turbocharger gasoline engines are:

-A compressor bypass valve is not integrated into the intake plumbing between the compressor outlet and throttle body
-The outlet plumbing for the bypass valve is too small or restrictive
-The turbo is too big for the application

Source

I took a different route. How many people here are running 7 PSI on 11:1 compression on a stock ECU designed for 8 PSI at 8:1 compression?

Also, your self back patting won't really impress anyone here, since there are plenty of people running 10:1 +25-30 psi. Or for example, a member on this forum, is running 9.0:1 compression with a compound turbo setup running 40 psi using dsmlink on the stock ecu. No AEM EMS here. No one gives a shit about your 11:1, 7psi hairdryer setup. 420A guys have been known to run more boost than that, and on bigger turbos.

 

[DSMBanditFZ6]

Oh for ####s sake, I can't believe this thread is still alive. The following is straight from the horses mouth. Argue all you want with us, that's fine, but try to argue with Garrett Honeywell. They just MIGHT know something about turbochargers, I'd even venture they know a thing or two more than your MR2 buddies.

Source



Source

You brought garret into the giant 12 yr old circle jerk too? Woah!!!!

Hey OP! Those are the wagon scientist you've been searching for!! I mean most of us use common sense, but your quotes are based on some guy DDing a car for 2yrs for 10000 miles. Some cars here have been driven for just a tad longer than that.

Efficiency and reliablilty.

 

[95talongirl]

My boyfriend ha been running an old Dodge Shadow 2.2L turbo car with a stock 80's garrett turbo for years with no real ill effect. However, this is a car that he doesn't care much about or if it blows up, as its a very rusty car in poor shape. It's sole purpose is sleeper look. It has gone 12's.

HOWEVER, as stated in this thread, BOV's do serve a rather important role in the turbo system. That surge can and will eventually wear on parts much quicker. Since cars that roll off the line carry a warranty and are designed with longevity in mind, they will runs parts that help aid this. People who are building cars for racing or power know that reliability goes out the window, so it doesn't matter at that point. You may get some time out of the car like my boyfriend has, or it could severely hurt the turbo pretty quickly as some have found out here. It s a risk, but if you don't care about longevity, then go for it!

 

[bradselph]

I will go and say a bov is completely necessary for a larger turbo even if it has surge ports, I have first hand experience from this. My last hx40 I had a small 1g bov and at 30+ psi when I let off you could hear the surge as the bov was to small to vent the amount of airflow I was pushing. Ask Justin what my thrust plate looked like within 1k miles.

To Justin, that's why my thrust plate was so worn out in such a short time, I realized this when I upgraded to my 50mm bov.

 

[PieEyedPiper]

In fact it seems more like the black sheep who do [use a bov] these days.

More please.

 

[T is for TURBO]

I will go and say a bov is completely necessary for a larger turbo even if it has surge ports, I have first hand experience from this. My last hx40 I had a small 1g bov and at 30+ psi when I let off you could hear the surge as the bov was to small to vent the amount of airflow I was pushing. Ask Justin what my thrust plate looked like within 1k miles.

To Justin, that's why my thrust plate was so worn out in such a short time, I realized this when I upgraded to my 50mm bov.

My Green at low boost (1 bar) with the surge housing would surge when my vacuum line popped off the bov. Sounded awful! Luckily I noticed it right away.

Also, how the hell did you get a 1g bov to hold 30psi on a hx40? I assume dodge garage mod?

 

[WES_393]

Also, how the hell did you get a 1g bov to hold 30psi on a hx40? I assume dodge garage mod?

You must be logged in to view this image or video.

 

[T is for TURBO]

Well that's one way!

 

[bradselph]

Wtf did that even open?

I just crushed mine a tiny bit and it held fine just had a hard time opening under 8-10 psi and obviously didn't flow enough for a hx40.

 

[snooopy365]

Wow this Bov is crushed real good.

I'm currently running the dgm and a slightly crushed BOV.
If you guys care about losing boost while shifting: just use nlts, even with no BOV closing the throttle body will kill boost.

If you are running no BOV, everytime the throttlebody closes there is a axial force on the turbine. Any unnessary force is bad.

 

[polarmoment]

This will be my only post in this thread.

Some background on me- I am currently going back to school for engineering. I try to read any engineering white paper I can get my hands on when it comes to centrifugal pump/impeller design. Some of you have read my analysis in the billet wheel thread (which have been shown to be accurate with datalogs from the various people using said wheels). I prefer facts and the scientific method to internet bullshit and "my friend did x," which is what this discussion usually devolves into.

With that said, several of Yoshi's quotes were originally written by me. I have not run a BOV since 2009. It wasn't a decision made lightly, it was a decision made by analyzing the physics behind centrifugal pump and turbocharger operation and doing the associated math and thought experiments. The conclusions I came to not only told me that it was safe to operate the turbo without a BOV (which is a conclusion loosely supported by no visible damage to the turbo in those 4 years, with pressure ratios up to 3.0), but that it would also improve how the car drove.

Here are some facts:

Turbo boost recovery
A turbocharger is a positive feedback device. The more boost you have, the more boost it will try to make and the faster it will try to make it. That is why you need a wastegate. This means that a decelerating turbo will re-spool significantly faster if there is boost in the charge pipes to drive the turbine when the throttle opens after a shift (or after getting out of the throttle for a slower car while trying to make a pass).

Since the wheel's stored energy at peak rpm is similar to that of an engine flywheel near redline, the effect of air reversing through the diffuser (pressure is converted back to velocity, since the diffuser acts as a nozzle when flow is reversed through it) that has lost a fair amount of energy through the conversion and re-conversion process is only a fraction of the energy of the rotating assembly. Consider a TD05h-20g at 30psi of boost:

This is a simplified approximation of the ideal energy (does not count the energy lost to friction, conversion, etc, so it's probably a fair bit lower than actual energy stored. I'm looking at worst case scenerio) stored in the charge pipes at 30psi of boost that is exerted on the impeller during a shift:

E= [P1V/0.4][1-(P1/P2)^(0.286)] - [P3V/0.4][1-(P3/P2)^(0.286)]*101.3 J/L-ATM

Where
P1= boost in ATMa
P2= atmospheric ATMa
P3 = boost left in pipes after the shift (typically in the 10psi range) in ATMa
V= the volume of the charge system (7ft of 2.5in pipe + ~3.3L for intercooler = 10L)


For a 7 foot charge system with intercooler at 30psi we end up with a very optimistic 1.4kJ of energy imparted on the turbo through reversion.

Total energy stored by a TD05h-20g (280g mass, 68mm diameter) at 150,000 rpm is:
E= 1/2Iῳ² which works out to a disc energy of about 20kJ. Again, simplified for purposes of discussion.

The energy released is less than 7% of the energy stored in the spinning impeller. Solve for energy lost and the 150,000rpm speed ends up around 145,000rpm. Even with perfect energy transfer in a worst case scenerio, you lose less than 4% of your impeller speed due to reversion. If I had to make an educated guess, I'd say it was probably closer to 1% The deceleration due to reversion is negligible in comparison to the deceleration of the impeller once the drive energy is shut off on the turbine side. The amount of force exerted on the wheel ends up in the 8oz region when you calculate it back through the area of the nozzle.

A BOV does not improve boost response for this reason. It may keep the turbo spinning a few thousand rpm faster, but the boost that's left in the charge pipes serves to spool it back up far quicker than the speed retained from the use of a BOV can re-pressurize the system from zero. The effect of this improved response increases as impeller mass increases. With a sealed induction system, it's possible to spin the tires on shifts despite lifting. No-lift shifting cannot duplicate this effect unless it's coupled with anti-lag (and we all know how good anti-lag is for turbos).

This has been common knowledge among race engineers for decades- BOVs cost lap times. Keeping the induction system sealed is worth full seconds per lap. Nizpro found that sealing the induction system was worth about 2 seconds per lap over a 1:40 course. Even cars that no longer lift to shift keep their induction systems sealed. The reason for this is twofold:

1. It removes a point of failure from the system. A leaking BOV will kill a turbo very quickly though overspeed. Minimizing failure points is key in endurance racing. This also speaks to how worried engineers were that off-throttle surge would damage the turbo- They weren't. Even though their turbos had to deal with thousands of full boost shifts and hours of full boost operation without air filters. Yes, they changed the turbo after the race. It's the equivalent of you changing your turbo at 200k miles.

2. It allows throttle response to remain linear. This is especially important in part-throttle steady state corners (the Carousel at Road America, for example). I remember driving the Formula Dodge cars around there years ago and even the slightest breath of throttle could induce rotation. Having a BOV dump boost upon breathing the throttle slightly would have been catastrophic. This is a big part in a driver's confidence in pushing the car.

These engineers chose sealed induction systems even for endurance races like LeMans and Daytona, where reliability was key. They were not worried about damaging the turbo without it, despite their turbos seeing more abuse than our turbos ever will in just that 24 hours. The IMSA Toyotas ran upwards of 40psi of boost, BTW. F1 cars also ran race distances at 40+psi. F1's turbos didn't usually fail outside qualifying, where they capped the wastegate and ran up to 75psi of boost.

Go on youtube and search "porsche 962 onboard" for a video of a 962 surging its way around the Sarthe. This is how everyone used to do it.

But my car came with one!
The misconception people have here is assuming they know what the engineers were putting a bpv on the car for in the first place. Cars used to not come with BOVs and their turbos lived long, happy lives- even at increased boost pressure. The reasons for adding a CBV (compressor bypass valve) were first noise, and then emissions. The MR2 came with a CBV while the all-trac did not because the MR2 was a production GT car near the top of Toyota's line and was expected not to make weird noises when one lifted off the gas. The ST165/185 all-trac was a race homologation (limited production run built to comply with FIA rules) and had no such concerns.

As cars moved toward hot-wire air metering, the new equipment was capable of reading the air oscillations in both directions which resulted in over-fueling when the throttle was lifted. This was bad for emissions. The CBV remedied this by keeping flow unidirectional across the sensing element.

But compressor surge will kill my turbo!

There is another common misconception that driven (on-throttle) surge is the same condition as undriven (off throttle) surge. It is not. Not even remotely. Here's why:

With driven surge, the turbine has full drive pressure differential across it (read: full turbine side thrust load). When the compressor surges, it loses its "grip" on the air for a second. This causes the average load on the compressor to drop, since it's only loaded part of the time. Simple physics tells us that the same amount of drive energy with less compressor load means the impeller will accelerate. When the compressor can "grab" the air again, it grabs it much harder, which results in an increasing hammering thrust load. The compressor is being driven farther into surge.

Hammering thrust load + full turbine drive thrust load + turbine overspeed = thrust failure. The overspeed of the impeller caused by this condition is also responsible for impeller explosions.

If you've ever read any engineering papers on centrifugal pump design, you will also know that the frequency of surge is inversely proportional to how damaging that surge is because less energy is dissipated per cycle. Driven surge is low-frequency, with very large pressure oscillations. I've experienced driven surge and the amplitude of driven surge oscillations were 10+ psi. This will destroy a thrust bearing in short order. Frequency/amplitude of surge is something no one ever talks about because most people have no idea they should.

Undriven surge is different. There is almost no thrust loading from the turbine side, and while the compressor is operating in the surge region, it is moving out of surge, toward equilibrium. Oscillations are higher frequency (10-15Hz) and much lower amplitude (<3psi). Here is an example charge pipe datalog with the throttle closed:

You must be logged in to view this image or video.

Actually, frequency of surge might be a good indicator for when you do need a BOV. If you are running a lot of boost and your off-throttle surge frequencies are under 2Hz with large amplitudes, maybe that's the point where you get a valve that will vent just enough to keep the surge frequency and amplitude in the acceptable range (>15Hz) while avoiding discharging the induction system completely.

Any oil of sufficient film thickness can withstand the milder oscillating thrust load from the compressor when there's little to no thrust load coming from the turbine side. Hell, my 20g held up to 2 weeks of part-throttle surge on Rotella T6 with no visible thrust damage. In fact, those bearings are still in my turbo to this day.

Those of us who run sealed induction systems are not afraid to post about failures, we just haven't had any! I've been looking for legitimate failures attributable to off-throttle surge for years and I've yet to find any. Everything I've found was due to some other issue (such as- like mentioned earlier in this thread- a disconnected BOV hose, causing a massive boost leak and overspeed).

The fact of the matter is that these "inevitable" failures just aren't happening. For the thousands of people who run sealed induction systems around the world (it's actually pretty common in Australia, England, and Japan), the actual failure rate (<1%) does not agree with the predicted failure rate at all (~100%).

Turbo failures due to other causes (FOD, improper oiling, etc) on cars with CBVs far outnumber failures with sealed induction systems. The only conclusion I can draw is that if off-throttle surge does damage the turbo, it doesn't do so at any statistically significant rate. While I am fully aware that absence of evidence is not evidence of absence, the disagreement in failure rates does let us draw some preliminary conclusions- the first being that it's very likely that it's not as harmful to run without a CBV than we had previously thought, or as harmful as those who try to sell us blowoff valves would like us to think.

I don't care what you do with your car. I'm not telling you to take yours off. I'm writing this to give some insight as to what's actually going on in the turbo under these conditions, and to show that some of the things we used to believe just plain aren't true (i.e. BOV keeps the turbo spooled) and the reasons why.

 

[Diamond Star Werks]

This will be my only post in this thread.

Some background on me- I am currently going back to school for engineering. I try to read any engineering white paper I can get my hands on when it comes to centrifugal pump/impeller design. Some of you have read my analysis in the billet wheel thread (which have been shown to be accurate with datalogs from the various people using said wheels). I prefer facts and the scientific method to internet bullshit and "my friend did x".

With that said, several of Yoshi's quotes were originally written by me. I have not run a BOV since 2009. It wasn't a decision made lightly, it was a decision made by analyzing the physics behind centrifugal pump and turbocharger operation and doing the associated math and thought experiments. The conclusions I came to not only told me that it was safe to operate the turbo without a BOV (which is a conclusion loosely supported by no visible damage to the turbo in those 4 years, with pressure ratios up to 3.0), but that it would also improve how the car drove.

Here are some facts:

Turbo boost recovery
A turbocharger is a positive feedback device. The more boost you have, the more boost it will try to make and the faster it will try to make it. That is why you need a wastegate. This means that a decelerating turbo will re-spool significantly faster if there is boost in the charge pipes to drive the turbine when the throttle opens after a shift (or after getting out of the throttle for a slower car while trying to make a pass).

Since the wheel's stored energy at peak rpm is similar to that of an engine flywheel near redline, the effect of air reversing through the diffuser (pressure is converted back to velocity, since the diffuser acts as a nozzle when flow is reversed through it) that has lost a fair amount of energy through the conversion and re-conversion process is only a fraction of the energy of the rotating assembly. Consider a TD05h-20g at 30psi of boost:

This is a simplified approximation of the ideal energy (does not count the energy lost to friction, conversion, etc, so it's probably a fair bit lower than actual energy stored. I'm looking at worst case scenerio) stored in the charge pipes at 30psi of boost that is exerted on the impeller during a shift:

E= [P1V/0.4][1-(P1/P2)^(0.286)] - [P3V/0.4][1-(P3/P2)^(0.286)]*101.3 J/L-ATM

Where
P1= boost in ATMa
P2= atmospheric ATMa
P3 = boost left in pipes after the shift (typically in the 10psi range) in ATMa
V= the volume of the charge system (7ft of 2.5in pipe + ~3.3L for intercooler = 10L)


For a 7 foot charge system with intercooler at 30psi we end up with a very optimistic 1.4kJ of energy imparted on the turbo through reversion.

Total energy stored by a TD05h-20g (280g mass, 68mm diameter) at 150,000 rpm is:
E= 1/2I&#8179;² which works out to a disc energy of about 20kJ. Again, simplified for purposes of discussion.

The energy released is less than 7% of the energy stored in the spinning impeller. Solve for energy lost and the 150,000rpm speed ends up around 145,000rpm. Even with perfect energy transfer in a worst case scenerio, you lose less than 4% of your impeller speed due to reversion. If I had to make an educated guess, I'd say it was probably closer to 1% The deceleration due to reversion is negligible in comparison to the deceleration of the impeller once the drive energy is shut off on the turbine side.

A BOV does not improve boost response for this reason. It may keep the turbo spinning a few thousand rpm faster, but the boost that's left in the charge pipes serves to spool it back up far quicker than the speed retained from the use of a BOV can re-pressurize the system from zero. The effect of this improved response increases as impeller mass increases. With a sealed induction system, it's possible to spin the tires on shifts despite lifting. No-lift shifting cannot duplicate this effect unless it's coupled with anti-lag (and we all know how good anti-lag is for turbos).

This has been common knowledge among race engineers for decades- BOVs cost lap times. Keeping the induction system sealed is worth full seconds per lap. Even cars that no longer lift to shift keep their induction systems sealed. The reason for this is twofold:

1. It removes a point of failure from the system. Minimizing failure points is key in endurance racing. This also speaks to how worried engineers were that off-throttle surge would damage the turbo- They weren't. Even though their turbos had to deal with thousands of full boost shifts and hours of full boost operation without air filters. Yes, they changed the turbo after the race. It's the equivalent of you changing your turbo at 200k miles.

2. It allows throttle response to remain linear. This is especially important in part-throttle steady state corners (the Carousel at Road America, for example). I remember driving the Formula Dodge cars around there years ago and even the slightest breath of throttle could induce rotation. Having a BOV dump boost upon breathing the throttle slightly would have been catastrophic. This is a big part in a driver's confidence in pushing the car.

These engineers chose sealed induction systems even for endurance races like LeMans and Daytona, where reliability was key. They were not worried about damaging the turbo without it, despite their turbos seeing more abuse than our turbos ever will in just that 24 hours. The IMSA Toyotas ran upwards of 40psi of boost, BTW. F1 cars also ran race distances at 40+psi. F1's turbos didn't usually fail outside qualifying, where they capped the wastegate and ran up to 75psi of boost.

Go on youtube and search "porsche 962 onboard" for a video of a 962 surging its way around the Sarthe. This is how everyone used to do it.

But my car came with one!
The misconception people have here is assuming they know what the engineers were putting a bpv on the car for in the first place. Cars used to not come with BOVs and their turbos lived long, happy lives- even at increased boost pressure. The reasons for adding a CBV (compressor bypass valve) were first noise, and then emissions. The MR2 came with a CBV while the all-trac did not because the MR2 was a production GT car near the top of Toyota's line and was expected not to make weird noises when one lifted off the gas. The ST165/185 all-trac was a race homologation (limited production run built to comply with FIA rules) and had no such concerns. Mazda states in the factory service manual for the RX7 that "a bypass valve has been fitted to reduce noise."

As cars moved toward hot-wire air metering, the new equipment was capable of reading the air oscillations in both directions which resulted in over-fueling when the throttle was lifted. This was bad for emissions. The CBV remedied this by keeping flow unidirectional across the sensing element.

But compressor surge will kill my turbo!

There is another common misconception that driven (on-throttle) surge is the same condition as undriven (off throttle) surge. It is not. Not even remotely. Here's why:

With driven surge, the turbine has full drive pressure differential across it (read: full turbine side thrust load). When the compressor surges, it loses its "grip" on the air for a second. This causes the average load on the compressor to drop, since it's only loaded part of the time. Simple physics tells us that the same amount of drive energy with less compressor load means the impeller will accelerate. When the compressor can "grab" the air again, it grabs it much harder, which results in an increasing hammering thrust load. The compressor is being driven farther into surge.

Hammering thrust load + full turbine drive thrust load + turbine overspeed = thrust failure. The overspeed of the impeller caused by this condition is also responsible for impeller explosions.

If you've ever read any engineering papers on centrifugal pump design, you will also know that the frequency of surge is inversely proportional to how damaging that surge is because less energy is dissipated per cycle. Driven surge is low-frequency, with very large pressure oscillations. I've experienced driven surge and the amplitude of driven surge oscillations were 10+ psi. This will destroy a thrust bearing in short order.

Undriven surge is different. There is almost no thrust loading from the turbine side, and while the compressor is operating in the surge region, it is moving out of surge, toward equilibrium. Oscillations are higher frequency (10-15Hz) and much lower amplitude (<3psi). Here is an example charge pipe datalog with the throttle closed:

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Any oil of sufficient film thickness can withstand the milder oscillating thrust load from the compressor when there's little to no thrust load coming from the turbine side. Hell, my 20g held up to 2 weeks of part-throttle surge on Rotella T6 with no visible thrust damage. In fact, those bearings are still in my turbo to this day.

Those of us who run sealed induction systems are not afraid to post about failures, we just haven't had any! I've been looking for legitimate failures attributable to off-throttle surge for years and I've yet to find any. Everything I've found was due to some other issue (such as- like mentioned earlier in this thread- a disconnected BOV hose, causing a massive boost leak and overspeed).

The fact of the matter is that these "inevitable" failures just aren't happening. For the thousands of people who run sealed induction systems around the world (it's actually pretty common in Australia, England, and Japan), the actual failure rate (<1%) does not agree with the predicted failure rate (~100%).

Turbo failures due to other causes (FOD, improper oiling, etc) on cars with CBVs far outnumber failures with sealed induction systems. The only conclusion I can draw is that if off-throttle surge does damage the turbo, it doesn't do so at any statistically significant rate. While I am fully aware that absence of evidence is not evidence of absence, the disagreement in failure rates does let us draw some preliminary conclusions- the first being that it's very likely that it's not as harmful to run without a CBV than we had previously thought, or as harmful as those who try to sell us blowoff valves would like us to think.

I don't care what you do with your car. I'm writing this to give some insight as to what's actually going on in the turbo under these conditions, and to show that some of the things we used to believe just plain aren't true (i.e. BOV keeps the turbo spooled) and the reasons why.

Very informative Thank you.

 

[Calan]

I have had farts flow more air than a T-25 @ 7psi.

That's some funny shlt right there.

 

[WES_393]

As cars moved toward hot-wire air metering, the new equipment was capable of reading the air oscillations in both directions which resulted in over-fueling when the throttle was lifted. This was bad for emissions. The CBV remedied this by keeping flow unidirectional across the sensing element.

Well I hate to quote myself, but...

I don't think it's totally a matter of lifespan, but more about performance. In my experience, off-throttle compressor surge (or "turbo flutter") can mess with perfromance in 2 different ways.

1. By messing with the MAF signal.

When the turbo flutters, the pressure waves flow back and forth past the airflow meter, so the same air is counted multiple times.

This isn't just "bad for emissions", it makes you run ungodly rich and possibly stall the engine when coasting. This is bad for the engine, hard on the trans, kills gas mileage, and is extremely unsafe with a mechanical power steering pump.

So turbo damage or not (which lets be honest, if you can kill an MHI turbo your doing something extremely wrong) it's still a bad idea for anyone not running Speed Density. And those running Speed Density likely have big $ in a big turbo and aren't willing to risk that for the mild (if any) performance gains from running without a BOV.

 

[Cdc1221]

Only problem I see with that theory, Wes, is that if the mas cannot measure the air going back out and write it off as unused, then what exactly happens if you are recirculating your bov back into the turbo's intake, and it discharges the unused air? Its being discharged at the inducer side of a compressor with no drive pressure, slowing it, and being released the opposite way through the maf. I could be wrong here, but it seems to be the same in my mind