Help me understand a turbo please...

[bigrobwoot]

So, I understand the basic exhaust layout, and how the exhaust gases go from the manifold, through the uppipe, into the turbo, and out through the downpipe, all the while driving the turbine to compress the air going into the manifold, but I do get lost in the details...

Can someone explain the relationship between a boost controller, blowoff/bypass valve, and a wastegate? Do turbo'd cars come with wastegates stock? And as far as I know, the bov/bpv is only for when you take your foot off of the gas pedal and the turbo keeps compressing air, but it has nowhere to go besides back into the turbo, which is really bad. Is this all it does, or is it involved with the other two things I asked about as well to control the amount of boost?

[sperry]

Okay, I'll take a whack at this:

As you know the turbo is made of two parts, the hot side and the cold side. The hot side is a gas turbine that spins up from the heat/pressure/flow of the exhaust gasses flowing through it. The turbine then spins up a compressor in the cold side that raises the pressure of the intake charge going into the motor.

Now, a turbo doesn't have to have a wastegate. If the turbo is big enough that it won't over-rev or generate too much boost when the engine is running at maximum power, then a wastegate is not needed. The problem with a turbo that large, especially on a small motor, is that it will have *tons* of lag. So, a much smaller turbo is used. The problem with a small turbo though is that if left unchecked it can spin so fast that it'll destroy it's bearings, or overboost the motor if there isn't some other means of limiting intake pressure. That's where the wastegate comes in.

The wastegate is a valve in the exhaust stream that variably bypasses exhaust around the turbo's turbine, limiting the top speed of the turbo. In it's simplest form, the wastegate is actuated directly by the boost level on the cold side. So there's a pressure actuator on the wastegate valve that at, let's say, 7psi the valve begins to open diverting away the pressure on the turbine and limiting the spool of the turbo. That's what's commonly known as "wastegate boost", since without further boost control the turbo won't make more than about 7-8psi of boost.

The problem with that of course is that straight wastegate boost control results in a fixed boost curve that tends to oscillate around the wastegate target pressure level. So to better control boost, a boost controller is used. The boost controller is a device that bleeds pressure off the line between the pressurized intake and the wastegate actuator. By bleeding off pressure, it can prevent the wastegate from opening, thus allowing not only higher boost levels, but also allowing control of the boost curve itself. A manual boost controller simply changes the target boost of the system and can react quicker than the wastegate itself, giving the user a means to set a higher target boost that the turbo can ramp up to very quickly. An electronic boost controller can actually map the bleed-off amount in reaction to things like engine rpm, gear, etc as well as react to the actual boost level seen in the intake to properly control the wastegate on the turbo.

With regards to the blow-off valve, as you mentioned it's there for protecting the turbo from surge. When the throttle body closes, the intertia of the turbo keeps it spinning and generating boost that no longer has anywhere to go. To prevent the turbo from spinning backwards, the blow-off valve vents the surplus pressure either to atmosphere or back to the pre-turbo side of the intake.

Anyway I just wrote all this in between conversations at work so I don't know how much sense it all makes. It would probably be much simpler to understand if I just had a picture. Maybe one later.

[Spencer]

You have the basic understanding of it

First of all The wastegate its self is located on the inside of the turbo. The part that is usually referenced is "usually" a little can looking thing with a rod coming out of the center. This is known as the wastegate actuator. You can get a wastegate actuator that is separated but it isn't really necessary for what most of us are doing. The wastegate actuator does just that. It activates the wastegate in the turbo. I believe the stock WG for the subbys is 7psi? So what this means is it will take 7psi to open the WG in the turbo thus relieving pressure to slow the turbine and reduce psi.

After the wastegate is activated the turbine begins to lose pressure and slow down. This is where the BCS (boost control solenoid) or MBC (Manual boost controller) comes in. For example my car is tuned at 19psi so my MBC will allow 19 psi to be built up before releasing the excess to hold that pressure.

So now a common problem I see is people wondering why they cant get more then 8psi or so of boost. chances are they are running at wastegate pressure and are either missing there "pill" (a little metal piece that sits in one of the boost lines that controls the psi.) or there is something wrong with the BCS / MBC.

Now back to the turbo. The BPV / BOV is a bleeding system, it blows off excess pressure to avoid surge to the turbo or too much pressure. During a shift when you take away all the RPM's and pressure that is pushing through the turbo the compressed air needs somewhere to go besides back into the turbo. If it was to go back into the turbo it could essentially force the turbine to stop abruptly or even begin to spin backwards which is obviously not good. So to relieve the pressure the compressed air is "blown off" In our case it is recirculated back into the system. On other cars you will see it "blow off" to atmosphere that is what causes that pshhhht noise. Do not put a BOV on a subby the car is not designed for it. All of the air that is calculated through the MAF needs to stay in the system for proper function. I believe it is possible to get tuned for an atmospheric blow off but I wouldn't recommend it. Also on that note there really isn't any need to change the stock BPV on these cars unless you are planning on running over 23-25+psi. The stock one works just fine.

[Spencer]

sperry beat me to the submit button, haha!

[van]

http://auto.howstuffworks.com/turbo.htm
IIRC There are some pictures/visuals in ^ there somewhere but I don't have time to look right now.

[sperry]

Originally Posted by van

http://auto.howstuffworks.com/turbo.htm
IIRC There are some pictures/visuals in ^ there somewhere but I don't have time to look right now.

I was going to link to that, but it's actually kind of a bad article and doesn't cover the WG feedback loop.

[bigrobwoot]

Thanks for taking the time guys. I think I get it, but I will probably have to read it a couple more times.

So technically is a boost controller even necessary if you just have a wastegate? Is the boost controller just to "override" the wastegate so you can get more boost?

[sperry]

Originally Posted by bigrobwoot

Thanks for taking the time guys. I think I get it, but I will probably have to read it a couple more times.

So technically is a boost controller even necessary if you just have a wastegate? Is the boost controller just to "override" the wastegate so you can get more boost?

Basically. But the boost controller not only allows more boost, it also allows control over the boost map so it's more than just a linear feedback loop between boost level and WG open percentage.

[zpeed]

Let me try, mainly from my old Talon. You might be confusing between wastegate and wastegate actuator.

Wastegate is the door in the turbo.
Wastegate actuator is the one that control the WG to open & close. (Spencer: explain in the 1st section)

Boost controller is necessary to get more adjustable boost. A WG actuator have a set PSI. You can try to bend the actuator rod or shim with some washer so it open the WG later.

Stock WG start to leaking a little when the turbo build up pressure close to stock setting (12PSI). On my old Talon I used manual boost control (MBC) set at 16 PSI. This is connect inline with the WG actuator. The actuator never see any pressure (0PSI) because of the MBC block all the pressure. So WG remain fully close. Until MBC open at 16 PSI it let all the pressure go to actuator. The actuator will fully open when see the 16 PSI which is more than stock (12PSI).

I hope that make sence!!

[bigrobwoot]

Yes it does, thanks. Thanks for everyone's help, I think I actually get it

[100_Percent_Juice]

A turbo works exactly like seminucleosis when you body ingests complex carbohydrates. Know what I'm saying?

[TrueNative]

Originally Posted by 100_Percent_Juice

A turbo works exactly like seminucleosis when you body ingests complex carbohydrates. Know what I'm saying?

And a BPV / BOV works like saying...... "pull my finger"

[bigrobwoot]

not at all juice, what's seminucleosis?

[cody]

It's like mononucleosis, only less intense, you feel me?

[bigrobwoot]

Umm, apparently I don't remember the symptoms of mono very well, and I'm totally raping your joke Joel...

[van]

Originally Posted by bigrobwoot

and I'm totally raping your Joel...

He's all yours

[bigrobwoot]

All I wanted was permission... Haha

[100_Percent_Juice]

Why don't you just buy a car without a turbo in it. oh.... yeah.

[bigrobwoot]

[van]

Originally Posted by 100_Percent_Juice

Why don't you just buy a car without a turbo in it. oh.... yeah.

?

[Kevin M]

Originally Posted by 100_Percent_Juice

Why don't you just buy a car without a turbo in it. oh.... yeah.

Maybe he went with a transmission instead.

[100_Percent_Juice]

That punch was pretty low Kevin.