EFI Basics
The TL1000 (and most other EFI Suzukis) use an EFI system that operates on as an alpha-n system at
lower throttle openings, and a speed-density system at larger throttle openings and engine speeds above
4000 RPM. Say what?
OK, first a few basics. Any EFI system operates on a very simple principle - measure the amount of air
entering the engine, and add the appropriate amount of fuel. The Engine Control Unit (ECU) is the "brain"
that "looks" at the various inputs in order to determine the air volume entering the engine. The operator
determines the amount of air by opening or closing the throttle. The ECU adds fuel by opening a
fuel injector for a given amount of time.
The Suzuki system uses these sensors and inputs:
Manifold Air Pressure (MAP) - this measure the air pressure under the throttle blades. More pressure
(less vacuum) under the throttle blades means more air enters the combustion chamber.
Throttle-Position Sensor (TPS) - this simply measures the opening of the throttle blades and sends the
appropriate voltage (between 0 and 5 V) to the ECU.
Intake Air Temperature (IAT) - this measures the air temperature, so the ECU can determine the outside
air density. The sensor changes resistance with varying temperature (a chart can be found in the
service manual).
Barometric Pressure Sensor (BARO) - this measure the ambient barometric pressure, so the ECU
can determine the outside air density.
Crankshaft position sensor - the ECU determines the crankshaft position and speed by reading a
toothed wheel on the crankshaft. It can fire the injectors at the appropriate time in the intake
cycle (typically, just before the intake valve opens) using this sensor and the...
Cam position sensor - this sensor allows the ECU to determine where each cylinder's progress in
the combustion cycle. Without this sensor, the ECU can't determine if a piston at top-dead-center
(TDC) has just completed the compression or exhaust cycle. With knowledge of the cam position,
this can be easily determined.
The engine speed, along with the MAP reading, calculated air density, and knowledge of the engine's pumping
efficiency (volumetric efficiency; discussed later), allows the engine to determine the necessary
fuel. Since the ECU is using engine speed and air density, this mode of fuel calculation is refered to as
speed-density.
So what is this "alpha-n" stuff that was mentioned earlier? At low engine speeds, the big V-twin (along
with its aggressive camshaft timing) causes some really large variations in MAP readings. Apparently,
Suzuki couldn't figure out a way to filter this into a realistic average MAP reading, so they choose to
ignore the MAP sensor entirely and just use the TPS reading to determine the amount of air entering
the engine. "alpha-n" refers to the fact that the engine is using throttle-blade angle.
Volumetric efficiency (VE) describes how much of the cylinder volume is filled during a combustion cycle.
At WOT, we'd like to fill the cylinder completely, as this gives us more mixture to burn. The torque
curve of an engine follows the VE curve very closely (torque is directly related to combustion-chamber
pressure, which is related to cylinder filling - simple, eh?). If you tune an engine for more upper-end
power, the VE curve will tend to shrink a bit at lower revs and increases at higher revs. The VE curve
changes with throttle position, as the throttle's purpose is to restrict the air entering the engine.
The EFI system uses a table of VE curves at various TPS and MAP readings to determine how much air
filled the combustion chamber during a given cycle of the engine. If you alter the engine's breathing
(through airbox mods, exhaust swaps, or internal engine work), you alter the VE curve, and therefore
you'll alter the fueling of the engine unless the VE curve is altered.
Roughly speaking, this is what the computer does when it determines how long to fire a fuel injector:
1: Determines air density by reading the IAT and BARO sensors.
2: Determines engine speed by reading the crank position sensor.
3: Looks up amount of air entering the engine by reading the MAP or TPS (depending on engine speed),
and then using the VE table to determine % cylinder filling.
4: Calculates injector pulsewidth with the following formula:
Injector pulsewidth = base pulsewidth x VE value x air density factor
5: Uses the crank position sensor to fire the injector at the appropriate time in the combustion cycle
quote from
Eric@bryantperformance.com