There are a number of different upgrades for TBI systems. These modifications can range from mild to wild, and can make a considerable difference in power and economy. The throttle body spacer is generally a beginner's modification due to its low cost, ease of installation and minimal change in how the engine operates.
Although it does not sound exciting, the phrase "minimal change" is a positive statement. To understand why this is true, we need to understand the basics of how Throttle Body Injection works.
The TBI system is basically a middle ground between carburetors and multi-port fuel injection systems. Carburetors allow air to be drawn in, and this passing air draws fuel with it. Fuel injection of any sort instead relies on a throttle body to regulate air flow drawn in by the engine, as well as injectors to spray the fuel into the air. Throttle Body Injection relies on two injectors atop an intake manifold (of a similar design to a traditional, carbureted manifolds) to spray fuel into the incoming air stream. Multi-port systems rely on a number of injectors (one per cylinder) to spray fuel closer to the cylinder head.
The TBI system relies on a computer (ECM) and multiple sensors. The ECM can calculate the engines need for fuel and spark adjustments based on readings such as throttle position, exhaust oxygen content and other information furnished by the systems many sensors. The adjustments the ECM makes are based on the assumption that all parts and settings of the engine are stock as they left the factory. To make any major changes in the system, tuning of the ECM must be manually performed, which is a time consuming process that requires specialized skills and equipment.
This is a major reason why many small modifications, such as spacers, are popular among TBI enthusiasts. While the gains are minimal (a spacer may yield one horsepower), they do not require that the ECM be reprogrammed. They may not deliver immediate gains, but may help down the road when modifying your engine with parts that require better intake flow.
While the theory of a spacer, whether used under a carburetor or a throttle body, is to increase plenum size, it does include some drawbacks. Depending on what size spacer you use, you may not be able to coax the existing fuel lines into the new, higher position of the throttle body. Any adjustment over a half an inch will probably require you extending your fuel lines in some way. Adjusting the height of the throttle body assembly will also warp the geometry of the linkage attached to it. This is particularly a concern on vehicles with automatic transmissions, as poor adjustment of the TV cable can lead to very serious transmission issues including eventual transmission failure. Manual transmissions do not use a TV cable, as you do all the shifting yourself.
If you have weighed the risks and advantages and have decided that you would still like to proceed, you should obtain a throttle body spacer for your application. Chances are, if you buy it new, it will come with fairly detailed instructions. It is always best to faithfully read and follow manufacturer's instructions. However, here is an overview of a basic installation in a General Motors 4.3L/5.0L/5.7L vehicle:
1. Set your parking brake and block the wheels.
2. Remove your air cleaner assembly (remember to label any wires you disconnect from the air cleaner base).
3. Observe what holds the entire throttle body in place, including wiring harnesses and fuel line straps as well as actual bolts.
4. Remove any straps that secure the fuel lines.
5. Remove the three bolts that hold the throttle body in place (take care to label these, they need to go back in the exact place they came from).
6. Remove any pressure in the fuel system, and carefully remove the fuel lines from the throttle body (only if necessary to reach old gasket material).
7. Remove the old gasket material from the base of the throttle body and the mounting surface on the intake manifold. If this step is improperly performed, a vacuum leak is very likely to develop.
8. Lay down a new gasket on the intake manifold.
9. Set the spacer on the new gasket.
10. Set another new gasket on the top of the spacer.
11. Replace the throttle body. DO NOT over tighten the throttle body bolts past 12lb/ft.
12. Re-install the rest of the assembly from here, reversing the removal process. DO NOT INSTALL AIR CLEANER YET. You may also need to fabricate new securing straps for the fuel lines.
13. Double check your fuel lines for leaks, especially if you had to disconnect them.
14. Adjust your TV cable if applicable. Follow vehicle specific instructions to avoid any damage to your transmission.
15. Install the air cleaner assembly.
These instructions can vary, depending on your specific vehicle application. But this is the basic process of installing a throttle body spacer. While you are performing work in this area of the vehicle, you may also want to change your air intake assembly to a more open design as well. Many times a great performance gain can be had cheaply by replacing the restrictive stock air cleaner assemblies on TBI engines. Better airflow will compliment other, more major modifications like exhaust work. By starting small and working diligently, the TBI system can make very good power, while retaining the longevity it is famous for.