EFI THROTTLE BODIES BY JIM GIBSON The last carbureted passenger vehicles were manufactured almost 25 years ago, yet many performance engine builders still rely on carburetors. That’s changing, in part due to the availability of many fuel injection options. uilders of custom street rods and “resto mod” In simple terms, we have either throttle body injection vehicles are generally divided into two camps (TBI), where the injectors are mounted in the throttle body with regard to the engine’s fuel system. They assembly, providing a central point air/fuel delivery atop the either want to retain an old school carburetor intake manifold, or multipoint fuel injection (MPFI), where approach or upgrade to electronic fuel injec- one fuel injector is dedicated per cylinder, with each injector tion (EFI). Since an OE-type fuel injection providing fuel closer to each combustion chamber, at the system may be somewhat limited in terms of performance output end of each intake runner. Band definitely won’t cut it in terms of appearance, builders Aftermarket performance throttle bodies are current- are always looking for aftermarket options. ly available in a variety of configurations, to replace an Today’s offerings allow a choice between a system that fea- existing throttle body on an OE injection system tures a central plenum with a single “dry” throttle body (OE (mounted to a stock or aftermarket manifold plenum) or design approach but with added performance and custom ap- a throttle body that mounts to a 4-barrel-style intake pearance) or one that utilizes a carburetor-style intake mani- manifold. A throttle body that incorporates one or more fold that accepts either a dry throttle body and multipoint in- fuel injectors provides both fuel and air and can be re- jectors installed in the runners, or a “wet” EFI throttle body ferred to as a wet throttle body. A throttle body that with built-in injectors to somewhat mimic a carb setup. serves only to regulate intake air is usually referred to as a dry throttle body. A variety of styles and sizes of dry throttle bodies are de- signed to mount in the same manner as a 4-barrel carbure- tor and incorporate one fuel injector per cylinder, with the injectors mounted in injector bung locations on the intake manifold intake runners. This approach provides an option of a carburetor appearance with the advantage of electroni- cally controlled fuel injection. In addition to single-carb intake manifolds that accept a single throttle body, multiple-injector setups are also avail- able with multiple throttle bodies, providing the appearance of a multiple “downdraft” carburetor theme, with injector rails located at the center of the engine valley, in-between the two rows of velocity-stack-equipped throttle bodies. Photo courtesy Edelbrock courtesy Photo From a performance perspective, the options are varied to This photo shows Edelbrock’s hi-rise Pro Flo EFI system with suit individual taste. larger-than-stock throttle body and increased plenum. MPFI systems can be either group (bank) fired (with a continued on page 52 50 April 2014 cates a large-diameter single-throat the required amount of air can pass. throttle body offset toward the front of You can’t compare cfm ratings on an the engine. equal basis between carburetors and Another style features a series of EFI throttle bodies. Just because a throttle bodies with individual velocity throttle body is rated at 1375 cfm does stacks, resembling an exotic downdraft not mean that the engine will be forced multiple-carb setup. The FAST/In- to accept this volume of air; the throttle glese/Weber setup is a good example of body simply makes this amount of air this, with inboard fuel rails and injec- available if the engine needs it. The en- tors that are nearly hidden from view gine will draw only the amount of air it (looks very akin to an exotic “old needs. Fuel delivery is handled as a Photo: Jim Gibson Photo: school” downdraft Weber setup, but separate, controlled issue. FAST offers a composite low-profile intake and dry throttle body for direct with EFI efficiency and tunability). An replacement of OE systems. array of variants exists, allowing your Fuel Rails customer to obtain not only the perfor- Depending on the design of the sys- batch of four being triggered at the mance that he desires, but the appear- tem, external fuel rails may or may same time), or sequential (SFI), where ance that best suits his project as well. not be featured. If the system uses each injector is triggered exactly timed multipoint injection, with one injec- as needed, at a precise crankshaft angle Throttle Body tor dedicated to each cylinder, each per cylinder, near TDC overlap with One aspect that will undoubtedly con- bank of the engine will feature a fuel the intake valve opening. SFI is a more fuse many folks is the cfm rating of the rail that delivers fuel to an injector complex system, but it’s more precise EFI throttle body. As you begin to re- bank (on a V-type engine, one rail per and—depending on the controller sys- search the various throttle bodies and bank, with the rails secured to the in- tem—injector phasing can be custom EFI systems, you begin to notice that take manifold). A combination throt- programmed for fine-tuning. their cfm ratings are huge, compared to tle body unit that features built-in in- In terms of today’s performance af- those of carburetors. Typically, perfor- jectors may feature a short fuel rail termarket offerings, a wet-style throttle mance aftermarket EFI throttle bodies located on each side of the throttle body type injection setup can offer will be rated at 1000, 1200, 1375 and body (from front bowl to rear bowl). both simplicity and disguise. Two ex- even 2000 cfm. Looking at these num- EFI systems operate under much amples that readily come to mind in- bers with a carburetor mentality, these higher line pressure than a carburet- clude Holley’s new Terminator, which ratings will at first seem way off the ed system—generally in the 40- to looks like a dry performance billet charts. Here’s what you need to under- 60-psi range, as compared to 6 to 8 throttle body and includes an external stand: While a carburetor moves both psi for carburetion. Wherever a flexi- controller, and Professional Products’ fuel and air (and must be sized accord- ble hose or hose clamp is to be in- Powerjection III, which in outward ap- ing to engine size and horsepower), an stalled, make sure it’s rated for use pearance resembles a dual-feed 4-bar- EFI throttle body moves only air, es- with an injection system. Anywhere a rel carburetor. Actually, this is a throttle sentially serving as an air door. Fuel de- long run is needed (tank to pump to body that hides four injectors (two in livery is controlled by the electronically engine area), a steel fuel line that each bowl), with the ECU built onto operated injectors. EFI throttle bodies the body itself, so there’s no need for an are sized larger (in terms of available external ECU. airflow) to provide greater latitude for At the other end of the spectrum, a range of air delivery requirements. multipoint injection systems are avail- The engine will pull only as much air able that feature a throttle body (strictly through the throttle body as it needs. for air intake/control) mounted atop an If you place a 1000-cfm throttle body intake manifold that features dedicated on a mild small-block engine, the en- injector bungs at the end of the runners gine may need to pull only, say, 600 (one injector per cylinder). Depending cfm. A wild big-block 500-CID with on the system, the intake manifold (sin- healthy heads and a wild cam may re- Speed Retrotek courtesy Photo gle-plane or dual-plane) accepts a top- quire, say, 1000 cfm. The same size This is an example of a dry throttle mounted four-bore throttle body that throttle body can handle this wide body that has been mounted to a resembles a 4-barrel carb (minus the range because, again, the throttle body carb-style intake manifold fitted with bowls); or a ram-type manifold that lo- serves only as a door through which injectors and feed rails. 52 April 2014 your engine’s anticipated horsepower and the engine’s BSFC (Brake Spe- cific Fuel Consumption) number “ef- ficiency,” and the amount of fuel the engine will consume, divided by its power output—how many pounds of fuel will be used per hp produced in an hour (see the box on page 54). Injector size is approached in a somewhat similar manner to that of selecting a carburetor size; bigger is- FAST courtesy Photo Photo courtesy Professional Products Professional courtesy Photo n’t always better. If too large an injec- Wet throttle bodies can be mounted Performance aftermarket throttle tor is selected, you’ll waste fuel and onto conventional 4-barrel intake bodies that offer greater intake air manifolds with no injector ports, volume are available as direct re- hurt throttle response. Low-imped- since the throttle body has built-in placements for OE intake manifolds. ance injectors are available up to 160 injectors and short fuel rails. lbs./hr., so don’t simply jump to a big- meets SAE J526 standards should be ger injector because it sounds like a in different lengths, so between lower used. good idea. Remember, injectors alone O-ring diameter and overall length don’t make power; you must tune the differences, you can really get con- Fuel Injectors system with the most efficient sized fused if you’re piecing an MPFI sys- Fuel injectors are electronically con- injector for the application.