Let's talk boost control and Blow Off Valves(BOV)

[slow4dr]

I get this question quite often. "Are your wastegates adjustable?"

The answer is yes, not with the supplied parts though. First you must understand what a wastegate does. It bypasses a certain amount of exhaust around the turbine wheel to control the speed in which the turbo spins. On a typical external wastegate you will have two vacuum/boost ports one above and one below the diaphragm on the head of the wastegate. When you can control the vacuum/boost signal to the diaphragm you can control the boost above what the base spring pressure is.



In this picture you can see on the wastegate on the right there is a port at the top of the wastegate. This port can be used to hold the diaphragm shut longer when pressure is applied which in turn will raise the boost level by not allowing the valve to open. This is for the extreme side of boost running 20 + psi.

However, not pictured is the port on the lower side of the diaphragm. This is the one we want. If you run a vacuum/boost line to the intake manifold the wastegate will open and control boost at the exact spring pressure in the gate(6 or 12 psi included in kit). When the manifold line sees 6 psi it has enough force to push open the 6 psi spring in the gate. The Banks kit includes these lines.

Boost controllers(electronic or manual) are a function of this line from the manifold to the gate. If you bleed off some of the boost in this line you can keep the gate closed longer and raise the boost level. These controllers can be found in many different configurations. I have been using Hallman manual controllers for many years. http://hallmanboostcontroller.com/ These are great bang for buck controllers. For the guys/gals that want the best you want an electronic controller. These are a little more expensive but are still very easy to hook up. I really like the greddy Profec series. http://greddy.com/ These can be mounted inside the vehicle and can be configured to control boost in a hi-lo button switch or even as advanced as controlled by speed or RPM.

The Blow Off Valve(BOV) seems to be very misunderstood. People often think the BOV and the wastegate hold a similar function. The BOV releases pressure from the intake when the throttle is abruptly closed. The reason for this is the turbo is spinning at a high rate of speed when boost is being made. When you suddenly close the throttle plate the pressure has nowhere to go but backwards and into the turbo forcing it to stall. The BOV works off a vacuum line under the throttle plate. As soon as it sees vacuum the signal goes to the BOV opening it and releasing the pressure. This is the PSSSSH sound you hear when turbo cars shift. The air released can be routed back into the intake tract before entering the turbo or vented to atmosphere. On a MAF sensor engine management you want to route the BOV outlet back into the intake tract before the turbo as the air has already been accounted for. Some people refer to this as a bypass valve. In a MAP(speed density) you can just vent it to atmosphere.


Sorry for the rant!

[907rs]

Thanks for the info, Jason!

[Y-TRY]

Thanks for the help, Jason. I have some turbo-related questions in the technical forum that you sure could help me out with. But here's one of them that no one has addressed:

Why have I not seen any turbo applications with the centrifugal-style BYPASS valve in lieu of the BOV and/or Wastgates? Is it because there is something different with how the supercharger produces boost compared to the turbo, that somehow negates the use of a bypass? Or is it simply because I've never seen a turbo/blow-through at all, thus never seen the symptom or it's solution? (Maybe this isn't a problem with EFI'd turbo cars so it's never been needed) It seems like the Bypass valve would work great in a turbo application and could possibly fix the problems I'm having. There must be a reason why I've never seen it used. Have any of you guys had experience with this type of valve in a turbo application like mine? What are the pitfalls? What kind of info can you give me?

[slow4dr]

Originally Posted by Y-TRY

Thanks for the help, Jason. I have some turbo-related questions in the technical forum that you sure could help me out with. But here's one of them that no one has addressed:

Why have I not seen any turbo applications with the centrifugal-style BYPASS valve in lieu of the BOV and/or Wastgates? Is it because there is something different with how the supercharger produces boost compared to the turbo, that somehow negates the use of a bypass? Or is it simply because I've never seen a turbo/blow-through at all, thus never seen the symptom or it's solution? (Maybe this isn't a problem with EFI'd turbo cars so it's never been needed) It seems like the Bypass valve would work great in a turbo application and could possibly fix the problems I'm having. There must be a reason why I've never seen it used. Have any of you guys had experience with this type of valve in a turbo application like mine? What are the pitfalls? What kind of info can you give me?

I have never heard someone use the term centrifugal BOV or wastegate. Tial & Vortech make BOV's that release pressure 360 degrees to the atmosphere. Is this what you are after? A bypass(recirculation) is a BOV that is routed back into the intake pipe before the compressor. This is necessary if the vehicle uses a air metering device before the compressor. The metering device has accounted for that air and placed the proper pulse width at the injector. When you release the air out to atmosphere you will have a rich condition and the vehicle will surge. A Manifold Absolute Pressure(MAP) or Speed Density(SD) system can release the pressure out to atmosphere. This is because the MAP sensor is placed in the intake manifold and the air isn't accounted for until it reaches this point.

Superchargers also use something called a bypass valve for releasing pressure while cruising. This is something all together different as well. Superchargers produce unwanted boost during certain cruising situations and the bypass valve will release this boost for driveability and mileage.


Jason

[Y-TRY]

You're a plethora of info, Jason. Thanks.

It's the "centrifugal-style BYPASS valve" I speak of. I understand that these leave the intake plumbing open under most every condition but WOT. But I think you answered the question. I'm guessing the boost map is different between the super- and turbo-charged compressors, and that type of valve is used to address a specific characteristic found in the super-charged applications only. Right?

Also- I was thinking, in my ignorance, that wastegates are either fully open or fully closed. Is this the case, or are they "smarter" than that? Do they open partially under certain loads, or boost signals, or do they suddenly pop open and closed at the given time?

[slow4dr]

Originally Posted by Y-TRY

You're a plethora of info, Jason. Thanks.

It's the "centrifugal-style BYPASS valve" I speak of. I understand that these leave the intake plumbing open under most every condition but WOT. But I think you answered the question. I'm guessing the boost map is different between the super- and turbo-charged compressors, and that type of valve is used to address a specific characteristic found in the super-charged applications only. Right?

Yes, this would be a supercharger only type bypass valve. The supercharger will make boost based off of RPM alone. A turbo is only going to make boost when it is needed.

Originally Posted by Y-TRY

Also- I was thinking, in my ignorance, that wastegates are either fully open or fully closed. Is this the case, or are they "smarter" than that? Do they open partially under certain loads, or boost signals, or do they suddenly pop open and closed at the given time?

They are somewhat smarter than that. They will open partially when running higher boost levels. Back pressure & spool charcteristics have a big effect on how quickly they open.

[hotwheels]

i just built a turbo system for a mitsubishi........i installed a bpv (bypass valve) instead of a blow off valve.......the guy didn't want to hear the whoosh sound.........was pretty simple actualy, i just welded the bpv to the line to the throttle body and then ran a 1" black hose to the intake side, to the back side of the mass air meter...........i was kinda skeptical at first, but after the work was done, it worked rather well............anyways, just thought i would share that........you can use a supercharger bypass valve if you desiree..........

the above post is awesome though, great information.............hotwheels of turborides