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Last Updated
26 May, 2002
MR2 Turbo Basics and Boost Control
Reproduced from
MR2 Message Board post by permission of its author, Bill Wotschak
THE TURBOCHARGER
The turbocharger is a centrifugal compressor driven by the
otherwise-wasted energy in the exhaust stream. It is a 2 chambered
housing with a shaft through the center extending into both
chambers. A turbine wheel is mounted on one end of the shaft and is
in the exhaust stream, and an impeller wheel is mounted on the other
end. That is the compressor end, and it is connected to tubing that
goes to the intercooler, and then to the throttle body. A
turbocharger is really a very simple device, and as long is it
regularly fed clean , high quality oil from the engine lube system,
and allowed to cool down before engine shutdown, will last nearly as
long the engine itself.
THE WASTEGATE
There are two alternate paths for the exhaust flow at the turbo. One
is across the turbine, and the other is out the wastegate, allowing
it to bypass the turbo. Since more energy at the turbo means more
air to the engine, which means more energy to the turbo, which in
turn means more air to the engine, which means...well, I think you
get the point. The wastegate is necessary to limit the airflow
output from the turbo.
The wastegate isn't simply open or closed; it modulates to maintain
very precise control over the turbo's speed and output.
SOME BASIC PHYSICS
Compressors are pumps, and pumps create flow. When the turbo creates
more airflow than the engine is consuming, then the air becomes
pressurized. So boost pressure will rise and fall as the turbo
output increases and decreases. Thats why the wastegte controls the
speed and airflow of the turbo. And pressure and flow are
directly related. That means you can not get "more flow at the
same pressure..."
THE WASTEGATE ACUTATOR
The wastegate actuator is simply a can with a rubber diaphragm on
one end, and 2 ports with hose fittings on the front end. Looking
into the engine compartment form the driver's side, it can easily be
seen just to the right of the turbo compressor inlet. The hose on
the left is connected to the turbo, and the hose on the right is
connected to the T/VSV. (more on that later) There is a spring that
holds the diaphragm in place that has a yeild value of about 7 psi..
As pressure builds in the turbo, air begins to fill the actuator and
pushes against the diaphragm. When the pressure exceeds the spring
value, ~ 7 psi, the actuator moves out, pushing a rod, and opening
the wastegate.
THE T/VSV <--- Click for picture.
The T/VSV (Turbocharger Vacuum Switching Valve) is a
"factory-installed boost controller". It is simply a
solenoid-operated valve that, when energized (open) allows air to
escape from the actuator, lowering the pressure in the actuator, and
allowing the wastegate to modulate closed. This vented air actually
flows back into the intake airstream between the airflow meter and
the turbo intake so it does not distort the ECU's air/fuel ratio
calculations.
The T/VSV is a normally closed-energized open valve. The ECU will
energize (open) it whenever you boost IF 1) engine coolant temps are
up to normal , and 2) ambient (intake) air temps are above ~ 32 deg
F. (There may be some variation with that number). If the T/VSV is
open (high boost mode) and the ECU gets a signal from the knock
sensor, it will de-energize (close) the T/VSV. Likewise, if there is
an over boost signal from the boost pressure sensor, it will
de-energize the T/VSV as part of the "fuel-cut" sequence.
NOTE: Unless you have an aftermarket boost controller, disabling
the T/VSV will lock you into low-boost mode!! It WILL NOT
increase boost.
The normal range of max boost ranges from 7.1 to 11.8 psi, according
to the Factory Repair Manual (BGB). Low boost problems are almost
always a result of some problem in the boost control system and
rarely with the turbo itself..
BOOST PRESSURE SENSOR AND FUEL-CUT
The boost pressure sensor is simply a pressure transducer that
monitors manifold pressure and reports it to the ECU as a voltage
value. If the voltage exceeds about 4.4 volts (~12 psi on '91, 92'
and early 93's, ~16 psi on late 93's and up) the ECU initiates the
fuel-cut sequence. It de-energizes the T/VSV to lower boost, retards
ignition timing, and restricts fuel delivery by limiting the
injector cycle. The "check-engine" light will come on for about 20
seconds, and a code 34 will be stored in the diagnostic memory. You
will be in "limp mode" and unable to boost again until you shut off
the engine and restart it. No other reset is required to return to
normal operation. You do not have to pull any fuses or disconnect
the battery (Internet legend!)
For a complete schematic diagram of the turbo and boost control
system, go here:
http://member.newsguy.com/~gtfour/technicl.htm#Turbo
Click on "Boost Control"
BOOST CONTROLLERS
Since the rate and amount of airflow to the wastegate actuator
controls its operation, we can increase the boost by either
restricting the airflow into the actuator chamber, or by increasing
the bleed rate of the air escaping the actuator. Boost controllers
range from simple orifice plugs inserted into the hose connecting
the turbo the the actuator, to complex multi-valve electronically
operated devices, but again, the all do the same thing: they
manipulate the amount of air (pressure) in the actuator.
FUEL-CUT DEFEAT SYSTEMS
Since the FC response is programmed in at ~ 12 psi*, you must
somehow eliminate it or alter it to boost above ~12 psi. Fuel-cut is
initiated when there is a 4.4 volt signal to the ECU from the
sensor, so ALL fuel cut defeat (FCD) system either prevent that from
happening, or delay it . Some methods simply eliminate FC completely
by preventing any thing over 4.3 volts from ever reaching the ECU.
This includes the Greddy BCC, the Zener diode, disconnecting the
hose, etc. The HKS FCD is an adjustable FCD that has one setting
that raises fuel cut to ~17 psi without eliminating it.
*(late 93's and later have FC set at ~ 16psi from the factory)
Here is some further information about the HKS FCD installed on the
MR2:
http://www.mr2.com/ARTICLE/HKSFCDat.html
Or I have designed a simple fully adjustable FCD that allows you to
set FC at any value you want, from stock up to the system max of ~
17.5. It will cost about $8 for parts and should take less than an
hour to build and install. No, I won't build you one.
Here is a discussion and instructions for the $8 BlackBox FCD:
http://toymr2.tripod.com/fcd.html
Hope this is helpful!!
bill
Last edited by
billwot on April 7th, 2002 at 07:07 PM |
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