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| 1981 | C3 | Computer Controlled Carburetor | |
| 1982 | C3 | Cross Fire Injection | Throttle Body Injection (TBI) |
| 1983-84 | C4 | Cross Fire Injection | Throttle Body Injection (TBI) |
| 1985-89 | C4 | L98 - Mass Air Flow System | Tuned Port Injection (TPI) |
| 1990-91 | C4 | L98 - Speed Density System | Tuned Port Injection (TPI) |
| 1992- | C4 | LT1 | Tuned Port Injection (TPI) |
| * Note: these are base model engines. | |||
Throttle Body Injection (TBI) systems usually mount on top of the
intake manifold similar to a carburetor. The injectors release fuel into the intake above
the throttle/butterfly valves. This system is more reliable and efficient than a
carbureted system due to its ability to maintain proper air/fuel ratios when outside
influences change (such as temperature and air pressure.) This results in high fuel
economy, reduced emmissions, increased net horsepower, and starting reliability.
Tuned Port Injection (TPI) (also known as Multi-Port Injection, MPI)
systems differ from throttle body in that they release fuel directly above each intake
valve (i.e. one injector per cylinder.) Basically, the air and fuel are not mixed until
right before they enter each cylinder. Unlike TBI, this setup prevents fuel from
depositing on the throttle valve and intake walls during cold conditions and/or heavy
accelerations, allow for even better fuel economy and higher horsepower.
Mass Air Flow (MAF) systems utilize an air flow sensor to calculate
the amount of air that is reaching the engine. This sensor is usually placed in front of
the throttle valves. Since air flow is being measure directly, this type of system easily
adapts to Volumetric Efficiency (VE) changes due to changing cams, intakes, headers, or
exhausts. Although throttle response tends to be less than that of MAP systems, due to the
air flow sensor's ability to react to sudden changes in air tempurature, density, and
velocity.
Manifold Absolute Pressure (MAP) (also known as Speed Density) systems
use intake manifold pressure to calculate air flow. This requires the computer to store
Volumetric Efficiency (VE) tables based on RPM and Pressure (Kpa) inorder to make these
calculations. If modifications are made to the engine (such as cams, intakes, headers,
exhaust, etc) which affect VE, these tables will need to be "tuned" or modified
in order for the computer to accurately produce proper Air/Fuel ratios. Once tuned
properly, MAP systems have the added advantage of quick throttle response due to the
nature of the type of sensor used.
Volumetric Efficiency (VE) is a measurement of how well an engine
consumes air. It is calculated as follows: VE%
= 100 * Actual AirFlow / Theoretical AirFlow
Whenever modifications are made to the way an engine uses air, the VE will change. For
instance, if mufflers or an airfilter were added which increase airflow, then the VE would
normally increase. The addition of camshafts with different durations and lift will change
the VE. For more info on how a engine mods affect Volumetric Efficiency, see the following
sites:
chevythunder.com
- Discusses modifications made to Fuel Injected engines.
Air/Fuel ratios play an important role in the horsepower, fuel economy, and
emissions of an engine. The standard A/F ratio, at which all air and fuel entering and
engine is consumed, is 14.7 to 1. During normal driving conditions this
is ideal. Usually at Wide-Open-Throttle (or when accelerating) the A/F ratio is decreased
to allow more fuel to be added for increased horsepower. The disadvantage is less fuel
economy, but WOT conditions are less frequent in everyday use (as oppossed to dragstrip
and racing where fuel consumption is not a big concern.) Some Fuel Injected systems have
the ability to enter a "Highway Mode" which decreases the A/F ratio slightly
during long durations of constant speed. This allows for better fuel economy on
interstates and highways.
Most OEM Oxygen sensors utilize a zirconium crystal to generate the voltage read by computers. This range is from 0 volts (lean) to 1 volt (rich). The standard 14.7:1 A/F ratio is usually obtained when the oxygen sensor reads approximately 500mV. For troubleshooting purposes, these are good estimates at what is happening in the engine.
| All | Computer-Controlled-Carbureted Engines | Two-Pulse (2000 Pulse Per Mile) Square Wave, DC Current | |
| Thru 1992 | Throttle Body Injection (TBI) Engines | Two-Pulse (2000 Pulse Per Mile) Square Wave, DC Current | |
| 1985-1989 | Tuned Port Injection (TPI) Engines - MAF | Two-Pulse (2000 Pulse Per Mile) Square Wave, DC Current | |
| 1985-1993 | Camaro 3.1/3.4 V6 Engines | Four-Pulse (4000 Pulse Per Mile) Sine Wave, AC Current | |
| 1990-1993 | Tuned Port Injection (TPI) Engines - Speed Density | Four-Pulse (4000 Pulse Per Mile) Sine Wave, AC Current | |
| 1992-1993 | Tune Port Injection (TPI) Engines - LT1 | Four-Pulse (4000 Pulse Per Mile) Sine Wave, AC Current | |
| For more information on Vehicle Speed Sensors visit http://www.customconversion.com/Pages/SpeedSensors_Speedometer.html. | |||