yup you're right I guess it depends on the car...I loved the sound though

 

Hmmm.... a strange mixture of info here. I've been under the hood of some Audis and have owned 2 DSMs for the past 10 years so here's my input on the BOV. As Speedlimit points out, the BOV is located between the throttle body and the turbo, attached to the upper intercooler piping. As the turbo spools up, the pressure builds in the upper intercooler pipe. When you let off the gas, the throttle plate closes and that leaves only one place for the air to go: back through the turbo. This is bad for two reasons: one, the turbo took time to spin up and now the reverse airflow will cause it to 'stall' (spin down) and secondly a turbo spinning at 170k rpms doesn't like to be thrown in reverse. The BOV's job is to vent the pressure thus allowing the turbo to spin down freely and not 'stall' so abruptly.

A good BOV will help reduce turbo lag once the turbo spools up. It also has that 'whoosh' sound so many like. A poor BOV (like the 2G DSM's) will leak at an undesirable pressure thereby making you lose pressure that the turbo built up for you (PSI=hp remember). The plastic 2G DSM one would leak at about 13 psi though the turbo would spool up to about 15 psi ( 2 psi = roughly 14 hp lost!). The cool thing was that the 1G BOV was all metal and didn't leak at 13 psi or even 15. Plus it was OEM so delaers usually didn't care.

 

Well this will be my first mod if I get an evo, just for kicks.

 

Got one on the way.

 

::WARNING:: make sure you get a by-pass valve not a blow off valve. you people throw the term BOV around too much. don't get an atmospheric blow off valve because it may cause a rich fuel condition in the motor and will make it stall. also some cars see this as a major problem and will trip check engine lights and actually decrease the boost of the turbo until its goes back to the by-pass configuration. (VW's do this) blow off valves will actually increase turbo lag because there will no longer be recycled air to help keep the turbo spooled.

So, just get an aftermarket by-pass valve that recycles the air. you can have a blow off valve work but you need to do some tuning to make it perfect, and every car reacts differently to them.

A side note...take some time and learn all about the EVO and turbocharged cars if your actually going to be owning one. read books, search online, www.howstuffworks.com is great with info about motors.

 

zlancer is right the term BOV in 2G's should be redirect valve.
when pressure builds up in the intercooler pipe the BOV (mines a Greddy...purple screamer) redirects flow back to the inlet side of the turbo.
also the BOV can be set up for positive or negative control. the BOV has a small inlet on the under side of the valve (1/8") in the positive mode the line is routed to the turbo (compressor side)
to keep positive pressure on the BOV....
neg mode requires no line just cap the nipple off.
almost like a sense line from the turbo to the actual valve.

 

also, The EVO I believe has an electronic controlled bypass. The BOV and porting the o2 housing was done on Mitsu to balance intake pressure to the throttle body coming from the turbo......because the exhaust flow across the turbo would cause boost creep if the inlet is not controlled properly..........the twin spool turbo should fix this.

 

well now I'm pissed lol. Will it hiss like an aftermarket?

 

Thus, our first perfect example of why Im going to lay chill for awhile. Let peeps test out mods, etc. Then buy. Last thing I want to do is buy some needless junk for my car, only to replace it. Im sure with all the perfomance parts available for the EVOVII, it shouldnt take long. Ahh, cant wait for the choice on a 3 inch down pipe. Anyone know if Ralliart has exhaust available???

 

There is a lot of mixed up info in here.

Note that those in the Audi/VW world usually refer to CBVs or compressor bypass valves, which is the correct term, when venting into the intake. However, in the DSM world, we just call everything a BOV (blow off valve), though it is generally still venting into the intake.

Here is the normal path of air into the Evo:

Intake -> MAS -> Turbo Compressor -> Intercooler -> BOV -> Throttle Body -> Engine

The MAS measures the amount of air the car is taking in at any point in time. It tells the ECU how much fuel it should add to have a correct air/fuel ratio. If the MAS reports incorrectly, the ECU will use the wrong amount of fuel, and various bad things will happen.

When the throttle plate closes (you take your foot off the gas), it sends a pressure wave back up that chain, which can damage the compressor wheel of the turbo if it hits. To prevent this, the BOV vents the extra air back to just after the MAS. This eliminates the pressure wave, and keeps all the air the MAS has counted in the system. Once you press the gas pedal again, the throttle opens, and all the air goes into the engine.

Other than buying one for fancy colors and bling-bling value, the main reason to buy an aftermarket BOV is to have one that will allow you to hold more pressure before it vents off the extra air. This allows you to run higher boost levels, and get more horsepower. The 1G DSMs come stock with a metal BOV that will work fine under relatively high boost pressures (~18psi). The 2G DSM BOV is made of plastic and will start to leak at 11-12psi, which limits the amount of power you can make, and is also generally bad.

However, I don't know how much boost the Evo 8 BOV will hold. Until you see some knowledgable people (RRE, other DSM shops, guru members, etc..) testing it and telling you what the limits are, I wouldn't recommend upgrading. The people who do are going to be are those who want fancy colors and different sounds, and just like spending money on things that don't add any performance to the car.

One thing you should NEVER do on a car with an MAS (this means a stock Evo) is vent the BOV to the atmosphere. Remember how I said the MAS counts all the air that goes in the car, and tells the ECU how much fuel to add? Also, remember how the BOV vents excess air -after- the MAS, so each bit of air is only counted once? Well, in the pursuit of extra loud "PSSSSSSH" sounds, many people disconnect the vent hose from the BOV, and have it shoot the extra air out under the hood.

This makes the BOV a lot louder, but think about what else it's doing. The MAS has already counted that air, and the ECU is adding the appropriate amount of fuel for it, but the air isn't going into the engine anymore. Instead, it's just squirting out under the hood, making a lot of noise. All the extra fuel that would've been used for that air still gets added to the engine though, and creates what is called a RICH condition. There are many bad side effects to this, such wasting gas, hurting performance momentarily, and possibly causing your engine to sputter and drop revs.


In summary:

- Don't upgrade your BOV (CBV) on your Evo until the major shops test out the Evo 8 BOV and determine it needs to be upgraded.

- Don't vent to the atmosphere! Keep that air in the intake, where it belongs. Just get a bigger air filter, and it will make plenty of noise.

 

No doubt. Mitsubishi proved that it could make a great OEM Compressor Bypass Valve with the the 1G DSM. Then it showed that it could make a crappy one with the 2G's. Any BOV designed to hold 18-19psi of boost (like on a stock EVO) is probably pretty damn good and might not need to be replaced, EVER.

I like my GReddy type S, however it is all that I know. I would be intrested in seeing how a HKS or Blitz valve would sound, but it isn't worth the 2-3 bills I'd need to find out. (I think the BOV sound in the Grand Turismo games is a BLitz sound BTW.) Plus, ugrading the BOV/CBV was a necessity in my vehicle and one mod lead to another, so I got the the Type S to go along with their intercooler pipe kit. The one that I know I don't like is the Turbo XS RFL, and you know what "RFL" stands for. These things sound like a large cough even under partial boost. Two cars in my area in particular come to mind and even though they are probably reasonably fast vehicles (an MR2 and a WRX) I don't think they should sound like that unless they are 1000hp Supras or Skylines, it is really obnoxious. The megaphone design of this particular valve is what makes it so intense, but precludes any kind of CBV setup. You need a flat, cylindircal flange so you can clamp a hose on it to dump it back into the intake. If you push more air in, make more power and need more boost, you're going to get "that sound" even with a CBV, it isn't worth losing your cars practicality with a BOV that's loud just to make it more noticable.

 

Here is a page that has some BOV wavs. Check it out.

 

I've got it on my eclipse now. Most BOV's are adjustable. There's no real car-specific valve. You just need an intercooler pipe with a flange for it. I'm sure road race will have one very soon.

 

hahahahaha i have the turbo xs h-rfl. but i got that before i knew ne thing about blow off valves. this **** is sooo loud. but i converted it to a by-pass setup when i realized it was the cause of the engine stalling.

heeey another side note....some turbo cars use a MAP sensor instead of a MAS sensor. these cars can use atmospheric blow off valves since they read the amount of air coming in by the absolute pressure inside the manifold. but you still get the turbo lag.......

 

Haaa! Rev the engine to 6k without a load huh....That's real good for the engine bearings. Not to mention since I own a turbo car with a quite Bypass valve hearing one doesn't scare me away....As some poor schlep in an Acura found out after I showed him the tail lights on my car. Most of the kids couldn't drive their way out of a paper bag as I noticed with the big pauses between gear changes. Maybe if they watched Ronnie Sox drive his Hemi Barracuda then maybe they would understand how to shift their cars.

Anyway, if you ever want to hear a car without a BOV pull along side a 1986/1987 Buick Grand National or even an 1989 Turbo Tran Am. They didn't have a BOV/Bypass valve. The car simply made a weird sound like Curly from the 3 stooges Woo Woo Woooo...Hehe. The sound was made by the turbo trying to turn backwards after closing the throttle. I have seen the results of many years without one too. The nut holding the compressor wheel on shears clean off of the shaft. Not cool!

There you have it...