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1999 ver7 coupe
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506 Posts
Pros: Intercooler stays mostly pressurized between fast shifts giving you better response while accelerating quickly. Loud chirpy noises.

Cons: Makes the car less drivable and a bit jerky rolling on and off the throttle. Increased stress on the turbo. Loud chirpy noises.


I have a garrett turbo and have been told by many car builders and racers who are far more knowledgeable than I am that it will not damage my turbo. One of those same people said they wrecked a VF39 running no BOV. *shrug*

I posted a bit more info in this thread: http://www.rs25.com/forums/showthread.php?t=78480
 

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2015 WRX
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2,575 Posts
You can operate a compressor in surge for a limited time (a couple seconds at a time), and achieve a relatively lengthy lifespan. However, repeated and extended operation in surge will result in a high cycle fatigue failure of the impeller blades. At the risk of sounding like a text book, surge is the phenominon of backflow through the compressor. This rapid reversal of flow makes the fluid in the compressor highly unstable resulting in vibration and high stresses in the blades. The way a compressor operates, the higher the backpressure imposed on the impeller, the lower the massflow and higher the pressure rise. When you release the throttle and throttle body closes this creates an increase in backpressure on the impeller, but a decrease in massflow since the flow follows the path of least resistance back through the compressor. By definition, this pushes the compressor towards surge. Every time the compressor surges you rack up a certain number of fatigue life cycles in the material, and eventually you will reach the fatigue limit (high cycle fatigue) and the material will fail. If you have a wheel spinning at 60k+ rpm, you can accrue millions of life cycles in a matter of minutes. The newer ball bearings are certainly more stout than journal bearings, but that doesn't not make the turbocharger indestructible. You may be able to operate sans bypass valve for a while, but it will prematurely fail the impeller or destroy the bearings.

Surge is the worse possible operating condition for a rotating wheel. Try being in the same building as an industrial gas turbine engine when it surges, it feels like an earthquake.

Now decide for yourself if you should run a blow off valve or not.
 

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RSTi, STi V6, BRZ Turbo
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3,947 Posts
You can operate a compressor in surge for a limited time (a couple seconds at a time), and achieve a relatively lengthy lifespan. However, repeated and extended operation in surge will result in a high cycle fatigue failure of the impeller blades. At the risk of sounding like a text book, surge is the phenominon of backflow through the compressor. This rapid reversal of flow makes the fluid in the compressor highly unstable resulting in vibration and high stresses in the blades. The way a compressor operates, the higher the backpressure imposed on the impeller, the lower the massflow and higher the pressure rise. When you release the throttle and throttle body closes this creates an increase in backpressure on the impeller, but a decrease in massflow since the flow follows the path of least resistance back through the compressor. By definition, this pushes the compressor towards surge. Every time the compressor surges you rack up a certain number of fatigue life cycles in the material, and eventually you will reach the fatigue limit (high cycle fatigue) and the material will fail. If you have a wheel spinning at 60k+ rpm, you can accrue millions of life cycles in a matter of minutes. The newer ball bearings are certainly more stout than journal bearings, but that doesn't not make the turbocharger indestructible. You may be able to operate sans bypass valve for a while, but it will prematurely fail the impeller or destroy the bearings.

Surge is the worse possible operating condition for a rotating wheel. Try being in the same building as an industrial gas turbine engine when it surges, it feels like an earthquake.

Now decide for yourself if you should run a blow off valve or not.
Word.
 

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1999 ver7 coupe
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506 Posts
It's pretty easy to go back and forth :)

Everything Suby25 posted is of course true, but in my experience I have not heard of a single bov related failure on a decent turbo, and that includes talking to people on race teams that run their subarus this way.

No doubt you are applying a lot of additional stress on the blades and bearings. However, there are many engine components subjected to considerably increased repetitive stress when modifying a car, many of which are still extremely unlikely to fail...ie, I don't worry about the rods bending in my ej207 even though I've bumped it up by 150hp.

I completely agree that you are adding a lot of unnecessary wear on the turbo, but by how much? What's the standard lifespan of a nice garrett-style turbo? If it's ~30 years, and no bov abuse reduces it to ~10 years, I'd still happily run no bov.

To be honest though, turbo damage concerns aside, the *driveability* downsides of running without a bypass are enough to make it a stupid mod for many people. Try it out, see what you think...you might find that it's annoying enough for you not to do it.
 

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2015 WRX
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2,575 Posts
I completely agree that you are adding a lot of unnecessary wear on the turbo, but by how much? What's the standard lifespan of a nice garrett-style turbo? If it's ~30 years, and no bov abuse reduces it to ~10 years, I'd still happily run no bov.
Typically when a rotating bladed element is designed, a certain fatigure life is factored in, say 100 million life cycles (not strictly a mean time between failure part). That may seem like a lot, but as I mentioned before, if you have a wheel spinning at 60-100k rpm and you're operating in surge or an unstable flow range, you can rack up millions of life cycles in a matter of minutes. Granted a turbocharger is a variable speed machine, so you won't spend alot of your time at the high speeds, but it still is detrimental for the health of the turbocharger.

However, with that said, IF the peak stress in the wheel is below the yield strength of the material, the material will have "infinite life" (not taking into consideration any other deterioration due to operation - blade tip erosion, etc...).

Some more info to chew on....
 

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01 RSTI / 03LeggyH6
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9,435 Posts
My Nissan has no BOV currenty (YIkeS!!!) and I am running a JDM sr20DET with Garret GT28R. I definitely notice the surge when letting off throttle before shifts. Also the Chirp through the wastegate is also quite evident and although sounds nice, itis a sign of pressure entering the turbo that should otherwise have been diverted/bypassed. I wouldnt reccommend running large amounts of boost w/o some kind of BOV or BPV. So I have to get a universal type BOV and install it. Plus my motor has individual throttle bodies which means a vac line must be fed into each one so that there is consistent boost going into each cylinder.
 

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1998 Impreza WRX STi
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303 Posts
Alright this is an old thread but im reviving it because im curious.

My stock BOV stopped working recently, and i replaced it and in the process i plugged the line to the BOV just to mess around with it a bit. So here i have to ask, im running an APS SR30 Ball Bearing Turbo, and im wondering how much damage am i really doing and am i really risking blowing my turbo or what? because over the years ive gotten nothing but conflicting information from everyone ive ever talked to so...
 

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2005 Baja Turbo
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2,385 Posts
I don't recall seeing any kind of recirc or BOV on my 1985 GL Turbo, but it was only running 7 PSI.

I would strongly suggest running the proper kind of pressure relief valve for your application, a BPV for TMIC equipped stock turbo cars, and a BOV for anything with a FMIC that has a blow through MAF setup. If you're running 7 PSI of boost, then you probably could get away with running no pressure vent... Running 25 PSI, put a freaking BOV on it.

Also would like to mention... If you're running a turbo without any kind of vent system, and you have a backfire into your intake, you could damage things. Get a pop-off valve, it could save your turbo.

Most diesels don't have them, Crawford doesn't use them. I'd say it's all in the tune.

cheers

poorman
Diesels don't even have any kind of limiter on the air intake. There is no butterfly on the air intake, the only throttle control is on the fuel supply. That's a horrible comparison. Why do you think you have to buy an aftermarket kit to get compression braking out of a diesel?
 

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06 SGM Impreza 2.5i
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336 Posts
8 psi on my car has been doing fine with no BPV. Then again, it is an Eaton Supercharger and I don't think BPVs existed when they were first designed lol
 
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