EFI-based engines are most commonly tuned using a dynamometer; however, they can also be tuned through trial-anderror testing on the street and racetrack. To simplify tuning and reduce testing time, Harley-Davidson Screamin' Eagle and aftermarket companies offer a variety of pre-tuned software calibration modules for various engine combinations. The calibration modules are software tables containing new fuel flow and ignition timing parameters, which are loaded into the ECM. Similar to a carbureted engine requiring jetting changes to maintain proper air/fuel mixture when modifications are made, an EFI engine requires ECM changes to alter fuel flow and ignition timing for a modified parts combination.
To obtain maximum performance from free-flowing exhaust systems, performance cams, high-flow heads, and bigbore kits, more air and fuel is required. As with carbureted engines, the stock EFI air cleaner should be replaced with a high-flowing unit. A variety of performance air cleaners are available for injected engines. Be sure to match a new air cleaner to your year and model of injected engine.
Once you have installed a high-flow air cleaner, a larger throttle body can be considered to further increase airflow through the intake tract. However, you cannot treat the intake tract as a bunch of individual parts. Instead, you should look at it as a complete system of interdependent components, from the air cleaner and TB to the manifold and cylinder heads, which must satisfy the airflow requirements of the engine for a given application. Here's the point: Upgrading parts separately or in a physically sequential order may not provide the best results. For example, it may be wiser to upgrade the cylinder heads before the TB or doing both at the same time. As one bottleneck in the system is eliminated, another impasse becomes the next restriction. In other words, you end up moving the bottleneck somewhere down the line. But the new performance limitation isn't necessarily located immediately upstream from the old impasse. The true bottleneck must always be identified.
Anyway, getting back to the TB - it is the EFI equivalent to the carburetor, with one major difference: A TB only flows air. Injectors downstream of the TB supply fuel to the engine. TB design varies depending on the year and model of the EFI system. Magneti-Marelli systems were used from 1995 through 2001 and Delphi systems from 2001 to present. However, periodic updates were made to these systems, so make sure that a new high-flow TB fits your engine and EFI electronics.
The stock Delphi TB throat is about 46mm. Performance throttle bodies ranging between 48mm and 60mm and even larger are available. Since a fuelinjected system is only flowing air from the air cleaner inlet to about the intake port (where the injector is located), TB sizing is not as critical as carburetor sizing, but it is still important because it can affect idle quality and acceleration. Ideally, you should match throttle body size to the engine's horsepower level and not displacement, because horsepower is in part determined by airflow. However, since many engine builders think in terms of engine displacement, here are few very general TB sizing guidelines, with emphasis on the word "general": 48mm TBs work well for mildly modified engines, 50mm TBs do well on 88-ci to 95-ci engines, 54mm TBs for 103ci to 113ci engines, and 54mm and larger TBs for 113ci and larger engines. Consult with your engine builder and tuner for more specific information. Additionally, make sure your original equipment EFI parts and electronics are fully compatible with a new TB.
4. On the right is a stock Delphi EFI 46mm TB; on the left is a 57mm Kryakyn TB and manifold ready for installation on a big-inch Twin Cam engine. | 
5. A big-bore Screamin' Eagle Pro 50mm EFI TB and manifold (left) sits next to a stock 46mm Delphi body. New for 2008 touring models, Screamin' Eagle introduced an even larger one-piece 58mm TB and manifold (not shown). Be sure to match the TB's airflow to the engine's requirements. Going too large can affect idle quality and acceleration. | 
6. BC Gerolamy can port stock TBs up to 3mm to 4mm larger for increased airflow, or they can sell you a new larger body. Clockwise from left are stock TBs modified by BC Gerolamy: Delphi TB bored to 48mm, dual Magneti-Marelli bored to 42mm, and ported Buell TB. |