Forester XT Turbo question (USA)

[ZZ]

Is the turbo always in operation, as when just cruising at steady
city or highway speeds, or does it only come into operation when you
hit the gas pedal hard, as in rapid accelleration, passing etc ???
How about slowly accellerating from a traffic light versus getting
away from the light as quickly as you can?

Is there an air bypass that only throws the turbo air into the engine
when power is needed, and the rest of the time it just ducts it away?

I guess what I want to know is does the XT operate like a normally
aspirated engine when driving conservatively, and the turbo only come
into effect during hard accelleration ?

Other than increasing the amount of air in the combustion chamber I
really don't know how it affects normal day to day < and/or > more
aggresive driving. Anyone know of a primer on turbo operation?

Just curious, and thanks in advance.

 

[y_p_w]

Is the turbo always in operation, as when just cruising at steady
city or highway speeds, or does it only come into operation when you
hit the gas pedal hard, as in rapid accelleration, passing etc ???
How about slowly accellerating from a traffic light versus getting
away from the light as quickly as you can?

I believe it's always spinning to some degree from exhaust pressure.
But the turbo could be working at reasonable highway speeds too.

Is there an air bypass that only throws the turbo air into the engine
when power is needed, and the rest of the time it just ducts it away?

More or less.

I guess what I want to know is does the XT operate like a normally
aspirated engine when driving conservatively, and the turbo only come
into effect during hard accelleration ?

Not exactly. The compression ratio of the cylinders has to be
reduced to make up for the increased pressure when the turbo adds
boost. The turbo itself puts a bit of restriction on the exhaust
even there's no boost.

Other than increasing the amount of air in the combustion chamber I
really don't know how it affects normal day to day < and/or > more
aggresive driving. Anyone know of a primer on turbo operation?

Just curious, and thanks in advance.

This is a pretty easy to read primer:

<http://auto.howstuffworks.com/turbo.htm>

<http://www.honeywell.com/sites/ts/tt/turbofactsbenifits_howworks.htm>

 

[ZZ]

I believe it's always spinning to some degree from exhaust pressure.
But the turbo could be working at reasonable highway speeds too.


More or less.


Not exactly. The compression ratio of the cylinders has to be
reduced to make up for the increased pressure when the turbo adds
boost. The turbo itself puts a bit of restriction on the exhaust
even there's no boost.


This is a pretty easy to read primer:

<http://auto.howstuffworks.com/turbo.htm>

<http://www.honeywell.com/sites/ts/tt/turbofactsbenifits_howworks.htm>

Thank you very much for the info and links.

 

[Jakey]

The computer controls it based on the demands you put on the engine. There
is a wastegate that bleeds off pressure based on the computer control.
During normal driving the turbo is always spinning to some degree. A "turbo"
supercharger depends on exhaust gas pressure and velocity so more "turbo
power" is available at higher RPM. The computer/wastegate will allow a
maximum pressure at high rpms so as not to overpressurize the engine.

 

[Reece Talley]

An after market boost gauge would tell you the whole story. The turbo does
spin constantly. However, as RPM increases so does boost. At a
pre-determined point, you go from vacuum to positive boost. This is the
point at which the engine is working with mechanical aspiration as opposed
to normal aspiration. Unless you put your foot in it, the Subie will drive
just like a non-turbo car and the mileage will agree as well. Put your foot
in it and get the boost going and the power pours on...as does the gas with
a resulting decrease in mileage.

 

[y_p_w]

An after market boost gauge would tell you the whole story. The turbo does
spin constantly. However, as RPM increases so does boost.

But it's not purely a function of RPM, although higher revs usually
results in higher exhaust pressure.

 

[y_p_w]

The computer controls it based on the demands you put on the engine. There
is a wastegate that bleeds off pressure based on the computer control.
During normal driving the turbo is always spinning to some degree. A "turbo"
supercharger depends on exhaust gas pressure and velocity so more "turbo
power" is available at higher RPM. The computer/wastegate will allow a
maximum pressure at high rpms so as not to overpressurize the engine.

I believe the potential problem is more than just overpressurizing the
engine, but that the turbo can only spin so fast before its bearings
self destruct.

 

[Jakey]

Yes, some early high boost turbos had problems caused by the limitations of
the lubricant which would "cook" the turbo bearings. Remember when it was
recommended by car writers that before turning off a turbo engine, one
should run the engine at idle for 30 seconds, so as to permit a cool down
with fresh oil spash? Intercoolers and improved sensors and computer control
have made that recommendation unnecessary.

 

[Reece Talley]

From what I have observed, the amount of boost and the point at which boost
begins are both computer controlled. On my way back from Arizona last
Wednesday I noticed that at 80 mph I was pulling 15 lbs of vacuum. If I
pressed the accelerator more than a half inch, I got 10 lbs positive boost.
Three quarters of the way to the floor and I got 18 lbs boost. Flooring it
permitted up to 20 lbs boost. I could cruise at 85-90 and still be pulling
vacuum as long as the throttle position was less than 1/2" depressed.

Prior to the dyno-tune, the car would boost up to 22 lbs if floored. It did
this even though it would begin to ping at 19 lbs. Watching it on the dyno,
it was clear that the boost would continue and the computer would retard the
spark at the first sign of detonation. With the better gas and a fresh dyno
run, the boost was limited to 20 lbs max and the spark was allowed to
advance considerably farther. With a complete cat-back system, 3 " piping,
an injun intake and a front mounted intercooler and an after-market blow-off
valve, the car develops a regular 275HP with occasional jumps to 285 HP
after soaking the intercooler with water spray. This is at the wheels HP
measured on the dyno. With the bad gas, the best we got was 255 HP.

 

[y_p_w]

Yes, some early high boost turbos had problems caused by the limitations of
the lubricant which would "cook" the turbo bearings. Remember when it was
recommended by car writers that before turning off a turbo engine, one
should run the engine at idle for 30 seconds, so as to permit a cool down
with fresh oil spash? Intercoolers and improved sensors and computer control
have made that recommendation unnecessary.

Turbo "coking" on shutdown is a totally different matter than what I
was talking about. It used to be a function of reduced lubrication
(creating more heat) and lack of heat transfer from the bearings when
oil flow was shut off. The excessive heat produced would cause the
"boundary layer" of oil left on the bearings to cook - also called
"coking". Allowing the engine to idle before shutdown would allow
the turbo to slow down to the point where there wouldn't be as much
heat generated after the oil flow stopped.

The real reason why turbo cooldown isn't as critical these days is
because they're water cooled with a mechanism that allows the coolant
to circulate after the engine is shut off. As long as the heat is
drawn away by the coolant, the little bit of oil left in the bearings
after shutdown shouldn't coke in most conditions.

<http://www.spdusa.com/turbo_timer.htm>

I use Mobil 1 5W-30 in my WRX, just to be on the safe side. Even with
the Mobil 1 and water cooling, I still let it spin for a few seconds
before shutting it down.

I was talking about excessive pressure spinning the turbo beyond its
ability to survive even with oil flow. It's got its limits before
the bearings get fried. I understand that turbos can coke at
excessive revs even with oil flow, depending on various factors
(like oil type).