Blow Off Valve?

[turbohcar]

I got to talking with a member here that frequents the forum and we couldn't come up with an answer for my question so, here goes.

Why don't the GN cars have a blow off valve? Every factory turbo car I've ever seen has had one, and from what I know, for good reason. The idea is that when the turbo is making positive manifold pressure (i.e. boost) and you let off the throttle, air is going to back up against the throttle plate. Now, that air has to go somewhere, so it progressively backs up through the intercooler and plumbing and back to the turbo. When this happens it causes compressor surge which is essentially when the two wheels stop spinning instantaneously because it cannot create any more pressure on the intake side and causes the turbo to "stall". This is the reason blow off valves were created. When a turbo "stalls" it can significantly increase the risk of bearing failure and decrease turbo life. The blow off valve gives the pressurized intake charge somewhere to go instead of backing up. You simply hook it up to vaccum port on the intake, attatch it to an intake pipe (between the turbo and throttle body), and when the intake sees vaccum (i.e. when you let off the throttle) it opens the valve.

Now, after that novel, why is it the GN cars can get away with this and I can't? Thanks

 

[GNguy]

Do a search on this topic, It has been discussed about 3 times in the past month.

 

[turbohcar]

Whoops, sorry. On the local Mitsubishi board I'm usually the one making the "use the search button" posts. Kinda makes you feel stupid when you're on the other end of it.

In any case, I found a little bit of useful information. Most of it was just preference and others found that it might have more to do with the GN cars being automatic versus the manual transmission. This is what I still don't understand after all that reading. What does it matter what transmission it has in it? It's virtually impossible to "let off easy" every time because eventually you're going to run into a situation where you need to put the brake pedal to the floor (say some idiot cuts you off and then stomps on his brakes). I realize that some of you may not like the ricer effect that putting a blow off valve on your car has but, in my experience, it's a lot cheaper than replacing your turbo because of compressor surge. I can't explain why, even though physics tells us its not right, you can run on a stock turbo for 100+k miles (Tom Tom Turbo's car for example) when everything I've been taught has told me that you shouldn't even be able to get away with more than 300miles without some sort of a dump valve.

Automatic or not, compressor surge still happens. I just can't make sense of why. I'm not trying to determine in my mind whether or not you should get one because in my mind there is no choice. It's more about why the turbo 6 cars can get away with it and I can't. As I see it right now, there is no logical explanation. It may be one of those things that can never be explained, but who knows.

BTW, I did see that some of you had some questions about recirculating your blow off valve. If you are running a BOV after the MAF meter, you need to recirculate that line back into the system before the turbo, after the MAF meter. It goes like this: MAF meter, intake tube with recirculation line, turbo, intercooler piping, BOV, throttle body. If you are running a BOV before the MAF meter then you can vent to the atmosphere without problems. The idea is that when running MAF before the BOV, once the air passes through the MAF, it is already measured (telling the computer how much fuel to use in relation to air) and if you try to vent it to the atmosphere it won't be able to adjust itself for the air that is vented. So, that's why if your atmospheric dump BOV is after the MAF and you let off, the car dies or stumbles on itself. I have seen some cars get away with it but they are far and few between and there is no logical explanation for them.

 

[49-blues]

Where should you recirculate it to?

 

[turbohcar]

It depends on where your BOV is. If it's after the MAF sensor (following the intake piping from the filter to the throttle body) and the MAF sensor is before the turbo, you need to recirculate the air back to the intake pipe between the MAF and turbo. So it should look like this:

Filter--MAF--intake pipe with recirculation inlet--turbo--intercooler piping and intercooler--BOV--throttle body

If you are running a MAF on the pressure side (after the turbo) and you're running your BOV before the MAF then it's safe to do an atmospheric dump (which is also louder).

The basic principal is making sure that the air that passes through the MAF doesn't get lost.

 

[49-blues]

At what kind of pressure should the BOV be opening?-thanks

 

[turbohcar]

Yet again, it depends. The BOV opens under vaccum so, you'll need to run a vaccum line from the corresponding nipple on the BOV to a vaccum port on the intake manifold. It's hard to set a BOV without a "test vehicle". What you want to do is set it first on about it's lightest setting and work your way up until it leaks a little bit at your max target boost number and then tighten it a little more than that. You want it to open just after you let off and not leak while making boost. At the same time you don't want it not to work at all (or not enough). The best way to set it is just to set it light and work up from there. You should be able to tell by your boost gauge (or hear it) if it's leaking at pressure.

 

[GNguy]

I run mine on th up-pipe with no recirculation with absolutley no problems.

 

[turbohcar]

^^^Exactly. Some DSM's are the same way. It varies from car to car and mostly it's the 95-99 Eclipses and Talons that are able to vent to the atmosphere with theirs on the up-pipe. It's unexplainable because in all reality the MAF is measuring more air than what's getting there.

 

[PaCemkr86]

I run mine on th up-pipe with no recirculation with absolutley no problems.

mE TOO. I ve used the tials 50mm and the greddy type R, works good no problems

 

[soundguy]

well it seems all you're doing for one is making a big circle. If you put the BOV before the TB and have it vent between the MAF and turbo your doing nothing more than the piping form TB to turbo is doing. Second, if the BOV doesn't activate till you see vacuum at the intake then there's no pressure to vent in the first place. Any pressure before the TB is gonna be seen in the intake. If you take your foot off and the plate is actually fully closed, the Idle Air control is gonna open up and will still allow any pressure before the TB to pass into the intake, it'll be slower passing thru there since the hole is only about 1/2 inch big, but then without the throttle down exhaust flow is gonna drop fast too and the turbo will stop pressurising the intake piping. If you watch your boost gauge and take foot off throttle you will see boost drop almost instantly. Between that and what little pressure that is still before the TB will be passed by the Idle Air control valve all happens in a split second.

 

[turbohcar]

well it seems all you're doing for one is making a big circle. If you put the BOV before the TB and have it vent between the MAF and turbo your doing nothing more than the piping form TB to turbo is doing

I think you're thinking of it wrong here. The main concern is the boost pressure backing up against the throttle body plate and backing itself up until it gets the the turbo and creating compressor surge. By venting some of this pressure off, it prevents compressor surge and puts the air back in front of the turbo where a vaccum is already present which vaccum is less than air pressure at sea level anyway. It may seem like a big circle but, that's only created while the engine is slowing down and it more or less helps take pressure off the intercooler plumbing until the IAC takes over again.

Second, if the BOV doesn't activate till you see vacuum at the intake then there's no pressure to vent in the first place. Any pressure before the TB is gonna be seen in the intake.

The BOV is only actuated by vaccum in the intake manifold and releases pressure between the throttle plate and turbo.

If you take your foot off and the plate is actually fully closed, the Idle Air control is gonna open up and will still allow any pressure before the TB to pass into the intake, it'll be slower passing thru there since the hole is only about 1/2 inch big, but then without the throttle down exhaust flow is gonna drop fast too and the turbo will stop pressurising the intake piping

I'm not sure about the GN cars but I know on my car there's no way that the IAC hole is big enough to release enough pressure into the intake manifold quickly enough. Besides, even if it were, your car wouldn't slow down nearly as fast as it does and your boost gauge wouln't fall instantly into vaccum when you let off.

If you watch your boost gauge and take foot off throttle you will see boost drop almost instantly

Correct. But remember, your boost gauge is showing what the intake manifold sees, not the intake piping before the throttle body.

Between that and what little pressure that is still before the TB will be passed by the Idle Air control valve all happens in a split second.

You're right about it happening in a split second but there's no way the IAC can take care of 20+ pounds of boost by itself. Air may flow through there but the turbocharger is still going to be creating that 20+ pounds of boost or more (in the intercooler piping) after you let off. The engine is still running, though it may be slowing down, but instead of having somewhere to go it gets backed against the throttle plate and is trapped. So, it builds like all gases do, equally dense throughout it's container (in this case the intercooler piping) and when that air charge in the intercooler piping reaches a pressure equal to what the turbocharger is creating, the turbocharger instantly stops rotating (because it cannot create any more pressure), air backs up through the turbo and out the intake piping and once pressure drops off a bit the turbo can continue spinning. This is compressor surge.

 

[Alaskabuick]

Bov

Turbohcar,

check your PM's

 

[turbohcar]

I can't explain why, even though physics tells us its not right, you can run on a stock turbo for 100+k miles (Tom Tom Turbo's car for example) when everything I've been taught has told me that you shouldn't even be able to get away with more than 300miles without some sort of a dump valve.

Now that we've established what a BOV does and what it can do, back to the real question. Why is it that the GN drivetrain can live without one and no other turbo car that I know of can? If anyone can shed some light on this, it would be much appreciated. Keep in mind, I'm not saying everyone needs one and you're stupid if you don't run one, I'm just confused as to why and how it is you could run without one.

 

[mybuick]

i have a 2001 subaru wrx bov and redirects it back through the intake before the maf, as subaru does from the factory. it also stopps the falling on its face and stumbling when you have to slam on the brakes.

 

[8AV8]

What does your talon run in the quarter?

 

[turbohcar]

Well..... are you counting bench racing or not? :biggrin:

Truth be told, the Talon hasn't been to the track yet. I've had it for three years and haven't even put plates on it yet. The entire car, save for body and wiring harnesses, has been replaced. Little maintenance parts that never got replaced before is what really made this project go on for so long. In fact, right now it is up on jackstands waiting for a new clutch pedal (the stock one has plastic bushings that wear out easilly, I'm getting a set CNC'd out of aluminum) and then FINALLY, it'll be ready for plates and insurance. I'm still breaking in the clutch so it may be awhile before I get to take it to the track. Atleast another 1,450 miles anyway.

 

[BLACK6PACK]

I think the reason most people don't run a BOV on a GN is from experience. My stock GN turbo lasted 175,000 miles before it took a dump. Most have similar experiences. The 54 turbo I have now is probably 5 years old. We don't let off 4 times during the 1/4 mile, just once at the end after spanking the car next to you. Why spend $500 on a BOV if the turbo is going to last many thousands of miles. Also, a BOV adds one more thing that can go wrong or that can give you troubles. I don't know how many threads I've read where the BOV is leaking boost or isn't working properly.

 

[turbo2nr]

Do a search on "compressor surge". It is NOT the noise when you let off the gas. I bought a BOV to combat comperssor surge when I went with a big turbo. Some combos get it, some don't. It's about drivability. Stick shift cars drive smoother with a bov. I have messed aroung with four of my Subarus a lot, and Buicks also. Never had compressor surge untill I got a T70 turbo. Car bucked and surged at shifts and at part throttle. Now it drives smooth again.

Buicks ignore the MAF when you let off the gas for the most part, no need to recirculate.

 

[BoostKillsStres]

Its definetly about the compressor surge, I also didn't have any until I went with a big big shaft turbo, now the car is not much fun to drive. When your at part boost and lift slightly, the surge hits and your maf numbers can go from 50 to 150 and back to 50 and cycle like that for some time, well the injector duty cycle also goes from 5% to 15% and cycles because of the maf, you can imagine what this does to the car and fueling, bucking and such from the rapid changes in air and fueling. I'm trying a couple different BOV's to see if I can cure the problem.