Oversized throttle bodies are one of the hot spots for many EFI tinkerers. A bigger hole can suck in more air, but the extra air is only useful if you have the extra fuel to go with it. At lower rpm, you don't necessarily gain anything but you won't get the "soggy bottom" conditions found when you install a larger carburetor. The larger throttle body does offer enhanced upper-end performance, but if the injectors run out of flow first, you can create a deadly lean condition, so higher-flow injectors should be considered if a substantially larger throttle body is installed.
To what degree, if any, will larger throttle bodies on a port-injected engine cause "soggy" bottom-end performance? Do OEM-size throttle bodies limit upper-end performance?
With low-rpm performance, throttle-body size is not critical, since it's not responsible for atomizing the fuel. Intake runner size and design is much more an issue. In the high-rpm ranges, a larger throttle body may offer a performance increase, but only if the fuel rate is increased to match. Injector fuel flow rates and engine airflow rates are closely related. Intake valve size is more of a limiting factor than throttle-body CFM rates on the engines I've tested.
Same question for throttle-body fuel injection?
The answer is largely the same. There are more issues with keeping the fuel atomized when you have more distance for the air and fuel to travel, but the injectors in a TBI system do a better job than a carburetor. Also, intake manifold design, port location, or spray targeting are so poorly engineered in some port-injected systems that their advantage over TBI is largely negated.
What's the best way to compensate for the effects of a "hotter" cam profile?
The best way is to use a stand-alone Electronic Control Module (ECM) and build a fuel and spark map commensurate with the cam profile and power curve. There really isn't another good way to accomplish this. As I mentioned previously, some MAF systems will compensate for small cam changes quite well, but injector sizing becomes a major factor fairly quickly here.
Alright, how do you know when you don't have large enough injectors, and how do you choose new ones?
Generally, if you modify your engine beyond a very low level, you will out-strip the injector performance. Frequently, you can't tell if your injectors are too small, and that can be a major problem. The only chance you have to "feel" the problem is at WOT (Wide Open Throttle). If it feels like you're running out of gas, serious damage is near. Lower-rpm, heavy-load situations are more insidious because, unless you can electronically detect detonation from a lean condition, your first indicator will be when you "toast" the engine. Ideally, you choose injectors based on dyno test and flow-bench evaluations. Outside the lab, there are some formulas that can get you into the ballpark, but it's important not to exceed an 80-percent injector duty cycle below WOT or you can overheat the injector.
The other important consideration is being able to control the injector for low-speed operation. A high-flow injector may supply too much fuel for the engine at low speeds, even at its shortest pulse width. That's why the OEM injectors are sized very close to maximum flow for the base engines because that allows for the closer calibration parameters needed to meet emissions requirements. Again, a stand-alone ECM with careful calibration can go a long way towards controlling large injectors at lower speeds to make a tractable engine.
Can't you increase fuel pressure to compensate for inadequate injector flow rates?
Bad, bad science! There are pressure spikes that occur in most fuel rails due to the opening and closing of injectors. They usually equate to about 40 percent over the rated injector-operating range. That means a 40psi injector might be operating at nearly 80 psi, but it will probably work OK in most cases. If you run your fuel pressure up to 60 psi to increase injector flow, your pressure spike may now reach over 100 psi. Some injectors won't open against that kind of pressure, and you end up running some cylinders lean. Lean mixtures burn valves and cause detonation-bad! Better to keep the designed pressure and install higher-flow injectors.