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Message
I was reading the installation and operation
manual that came with my MicroTech electronic engine management system, and
under "setting up" there is a section entitled BstCUT, which says "This
feature prevents turbo engines from overboosting by cutting all fuel supply when
bost pressure exceeds the limit set in theBSTcut screen, i.e: if you set
this value to 10 psi, all fuel supply will be immediately shut off when boost is
11 psi or higher." Does this sound like a useful tool in installing and
setting up a turbo on my 13b? Please don't flame me, as I know less about
turbos than I know about understanding women. Does this mean I would not
need a wastegate? Thanks in advance for your replies. Paul Conner,
Mobile, AL
Hi Paul,
OK, here's my best turbo 101
theory:
In all stock turbo applications, there's a wastegate to
control the boost. This is just a "valve" of sorts to let exhaust gas go
around the turbine (less boost), rather than through it (more boost). It
can be built into the turbo housing as in most stock applications, and called an
internal wastegate, or it can be a separate unit called an external
wastegate. This valve is physically actuated by the wastegate actuator,
which is a big vacuum pot looking thing with a long arm that connects to the
wastegate lever. The actuator itself has a spring that's set to a
specific pressure, which is usually below the max pressure that you want.
In other words, if you plan to run 10 psi of boost, the spring might be 6
psi. The actuator is generally controlled by pressure on a hose from the
turbo compressor itself. This hose can also be Teed into an electronic
control of some type.
Basically, here's how it works. Say you have a
turbo, with an internal wastegate, and an actuator with a spring pressure of 6
psi. The wastegate is held closed by the spring, which causes all the
exhaust to flow through the turbine, and makes maximum boost. At idle
and low throttle settings, there isn't enough exhaust flow to produce any
significant boost, so the wastegate stays closed. When you open the
throttle, the extra exhaust flow spins up the turbine, and starts producing
boost on the compressor side. As the boost increases the pressure
increases in the actuator line as well. Once the pressure in the line gets
higher than the spring pressure in the actuator (6psi for our example) the
pressure starts acting against the spring in the actuator to open the
wastegate. As the wastegate opens, less exhaust flows through the
compressor, which reduces the boost. If the wastegate opens too much, the
boost is reduced below 6 psi, and the spring is able to close
the wastegate again, which starts the cycle over. Keep in mind
that this isn't a binary, open/closed operation in reality, the amount the
wastegate is open varies the boost, accordingly. The spring in the
actuator also doesn't just snap completely open when the boost hits 6.1
psi. It starts to open at 6, and opens more at 7, more at 8,
etc.
That was the most basic (automatic) way of
controlling the wastegate. Remember that you wanted 10 psi, so if the
wastegate is big enough, and the spring is set to 6 psi, how will you ever get
there? The answer is to bleed some of the boost out of the wastegate
actuator pressure line. You can do something as simple as putting a needle
valve in the boost actuator line, and open it some. By varying the opening
amount, you're changing the pressure that the actuator sees. Now you
can have 10 psi of boost, by allowing a 4 psi leak of the air in the
actuator line. (Turbo 201 begins here) In stock vehicles, you
tee this line into a solenoid, and let the computer control the opening and
closing of the solenoid. Since this is a binary open/closed situation, the
computer sends a square wave pulse to the solenoid, and varies the pulse width
to control the amount of time the solenoid is open.
Now, as for the question of using the boost fuel
cut. This is a safety device, and not a controller of any sort. You
do not ever want this to have to kick in. Imagine you've just lifted off,
running full throttle (the only time you're going to have too much boost), and
your engine quits. The computer saved your engine, while trying
to kill you. IF you can verify how this feature behaves,
and test it on the ground, it might be worth setting up, otherwise, I'd
leave it off, or set it to something like 30 psi so it never comes on.
I would want to know how quickly it reacts. In other words, say you
set it at 5 psi, then go to full throttle where it sees 6 psi almost
immediately. It will cut the fuel, but when does it turn it back on?
Boost will immediately drop due to loss of power, so does it come back on when
it gets back below 5 psi? If so, you could use it, but would have to teach
yourself to react to an engine failure by pulling the throttle back some.
One other word about wastegates. You can control
the wastegate manually, as I am trying to do. I just have a cable running
to the wastegate lever, rather than actually having an actuator. Depending
on how much boost you need, and how effective your wastegate is, you could
possibly just wire the wastegate wide open. For example, I know that I can
make more than 3 psi of boost (over ambient pressure) with the wastegate
wide open, because that's where mine is, and I have a relief valve that's
dumping the excess above that. The relief valve is also a safety item,
that I'll eventually get rid of, but I'm using it now to keep me from blowing
the engine while I prove that I can control boost via the throttle. So
far, boost is easily controllable with the throttle alone, though it does add to
the workload.
I hope that answered some turbo
questions.
Cheers,
Rusty
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