Boost contoller source

[91White-T]

The other day I switched my MBC from the intake man. pressure hose to the compressor hose, and noticed a few things... First off, boost builds more slowly, and the turbo isn't quite as loud, but it seems to pull a little harder. Overall it feels a little better the way it is now, which is strange since it really shouldn't make a difference....

[vrg3]

It should make a difference!

The intake manifold is further from the pressure source, and in particular it's after the throttle body. That not only means the feedback to the wastegate actuator is delayed, but also that the signal is different when not at full throttle.

If you have it connected to the manifold, then consider what happens when not at WOT. The throttle plate is a pressure drop, so the pressure regulated is lower than the pressure at the compressor outlet. So, for example, it might take 16 psi of compressor outlet pressure to achieve 10 psi of manifold pressure. Basically you work the turbo much harder.

This is where much of the part-throttle-full-boost problem comes from; you push the turbo way out of its efficiency range.

Your perception of boost building more slowly is due to the fact that before, the turbo was pushing boost before you let it into the manifold, so it was in a sense already spooled up when you called on it. The boost actually should build up at about the same speed if you're at WOT, though.

The reason it feels like it's pulling harder is that you're no longer pushing the turbo so far out of its efficiency island and at the same time causing less underhood heat soak. If you have an intercooler (particularly a top mount) this effect is much more pronounced.

In my opinion, the compressor outlet is the correct place to get the signal for a ball-and-spring manual boost controller. It's certainly possible to use any other positively-correlated pressure source (like the intercooler outlet, throttle inlet, intake manifold, or even exhaust manifold) but the closer you are to the source, the better off you are with respect to heat and efficiency.

[ericS2the6]

where exactly is this compressor hose or compressor outlet at? description or photo please.

[vrg3]

There's a nipple right on the turbo's compressor housing.

If you remove the intake tubing it's easier to get at and it will be plainly obvious.

In the stock configuration, there's a hose going from it to the boost control solenoid. The other two hoses from the solenoid go to the wastegate actuator and to the intake resonator.

[91White-T]

Hmmmm.. Makes perfect sense vrg3.. Wish I knew, I woulda done it earlier.

[2.2Turbo]

Which line do you do you use for a MBC? I am going to install a MBC and need to know which vacuum line to pick up. Do you use the factory wastegate solenoid when using a MBC? TIA

CT

[91White-T]

Use the 2 lines going to the solenoid for your MBC.

[vrg3]

You shouldn't use the stock solenoid when you use an MBC; the MBC should replace the stock control completely. You should, however, cap off the nipples on the solenoid and leave it connected to the wiring harness so that the ECU doesn't get mad. Or, alternatively, you could replace it with a power resistor (a stock automatic transmission dropping resistor works but gets warm).

The simplest way is to use the 2 lines 91White-T refers to (the third line from the solenoid goes to the intake resonator) and put the MBC next to the stock solenoid. But, it's better to run your own new vacuum lines of appropriate inner diameter to the compressor outlet and the wastegate actuator. Make them as short as possible for good response.

[LegacyT]

just a quick qestion-I always get these 2 confused time. The boost solenoid is the black cylinder and the brown thing is the map sensor on the passenger strut tower, right?

Thanks,
Mark

[vrg3]

The brown cylinder is the pressure exchange solenoid, not the pressure sensor...

You can look at this picture I made for boostjunkie to see the three relevant parts:

http://www.graphics.cornell.edu/~v/pics ... eplace.jpg

(Ignore the "pull off this hose" comment in the picture)

[czo79]

now I am confused. What exactly is the pressure exchange solenoid? The black cylinder thing on the end of it seems like it could be turned? Whats the function of it? I thought it was wierd cause my 91 turbo had one on the firewall, and my 97 legacy which has a phase II EJ25, it has one on the drivers side, sorta under the intake manifold I believe, above the block and heads.
THanks...
Micum

[vrg3]

The pressure sensor is used to measure both manifold pressure and atmospheric pressure.

The pressure exchange solenoid switches between the two. There's a vacuum line going from the manifold to one side of the solenoid, and the other side of the solenoid has a little black cylindrical air filter on it through which atmospheric air can pass. The top of the solenoid (its output goes through a short vacuum line to the sensor.

When the solenoid is off, the sensor is measuring manifold pressure. When it's on, the sensor is measuring atmospheric pressure.

Up through 1994, only turbo models had the pressure sensor and pressure exchange solenoid. In 1995 Subaru started putting them on all automatic-transmission models, including Imprezas (the naturally aspirated sensor is different though; 1.5-bar instead of 2-bar). Somewhere along the line (1997 or 1998 I think) they started putting them on all cars.

I thought the 1997 Legacy GT had the Phase I EJ25.

[czo79]

interesting....my car has a phase II EJ25 swapped in for the old phase I EJ25 that came in the 97GT. So I take it if the phase II EJ25 in my legacy gt has a pressure exchange solenoid, that it also has a pressure sensor....
So its the wastegate control solenoid that gets its place taken by a manual boost controller? Although you should leave it connected so as to not confuse the ECU?
What would happen to a turbo if you just disconnected the wastegate control solenoid? Would it just run with boost limited by the wastegate itself? Doesn't that limit it to 6psi or something?
thanks
Micum

[-K-]

Yes

[vrg3]

What would happen to a turbo if you just disconnected the wastegate control solenoid?

What do you mean by "disconnect?"

If you electrically disconnect the solenoid, I believe the wastegate would be directly connected to the compressor outlet, so you'd get whatever boost the wastegate spring was set to.

But, the ECU would get upset and worried about boost due to the missing solenoid and so you'd also get fuel cut above about 50% throttle.

If you mean disconnecting and capping off the vacuum lines to the solenoid, the wastegate actuator would get no pressure and so you'd get as much boost as the turbo is capable of putting out.

[legacy92ej22t]

You should, however, cap off the nipples on the solenoid and leave it connected to the wiring harness so that the ECU doesn't get mad..

Should we keep the line on the stock solenoid that goes to the intake resonator hooked up or can we cap both ends and remove it?

[vrg3]

Should we keep the line on the stock solenoid that goes to the intake resonator hooked up or can we cap both ends and remove it?

Your choice... I just left it there because the nipple on my resonator box is dangerously brittle and I didn't want to mess with it. It shouldn't make a noticeable difference. It would be cleaner, I suppose, to remove it and put a cap on the resonator's nipple as well.

[legacy92ej22t]

Right on, Thanks.