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relies on an intake vacuum to pull fuel from the fuel bowl through the metering circuits into the air stream. So most TBI systems can tolerate radical cams with durations up to about 250 degrees with much more finesse than a carburetor. The engine will run smoother, idle better, start easier and provide better throttle response and cold drivability with TBI than a carburetor. Idle speed is not controlled by an adjustment screw on the throttle linkage, but by an idle air control bypass valve operated by a solenoid or electric motor and the controller. On most of the bolt-on TBI systems, the throttle is still mechanical and connects the same as a carburetor with a cable or linkage and return spring. A TBI system uses closed loop feedback fuel control to constantly fine tune the air/fuel mixture. The TBI controller looks at the O2 sensor signals to see if the engine is running rich or lean, then compensates by adding or subtracting fuel as needed to balance the fuel mixture. This is done by increasing or decreasing the on-time (duty-cycle) of the fuel injectors. This approach saves fuel when extra fuel isn't needed (as when cruising or decelerating), makes the engine run Circle 232 for more information 32 March 2014 | High Performance Buyers Guide
cleaner (lower emissions), and makes for a much smoother running engine. This can make many vehicles much more enjoyable and practical for everyday driving. The controller can also compensate for changes in temperature and altitude that change air density. So the engine is always in tune regardless of what the weather is doing. That's a hug advantage over a carburetor with fixed jetting. There is also less danger of "over-carbureting" an engine with a TBI system. For a carburetor to perform well, its Cubic Feet per Minute (CFM) rating has to match the engine's displacement and breathing characteristics. A bigger carb that flows more air can make more power at higher RPMs, but if the carburetor is too big, it won't respond well at lower RPMs, which hurts throttle response and low-speed torque. A typical street performance small block Chevy can usually handle a 600 to 750 CFM carburetor. By comparison, most of the bolt-on TBI systems have a throttle body that flows up to 930 CFM -which means it can be installed on almost any engine that is capable of making 250 up to 600 or more horsepower. That's a lot of latitude from a "onesize-fits-all" throttle body.