There's no denying that EFI boasts drivability, fuel economy, and emissions benefits over a carburetor, but when pitted head-to-head on a dyno, the venerable carb usually comes out on top in the horsepower department. This has less to do with how the fuel is administered into the induction system, but rather the location where it's administered. By atomizing fuel at the top of the intake runners, and thereby increasing the density of the intake air charge, carburetors enhance the inertial ram effect of the induction system. This increases cylinder filling, volumetric efficiency, and horsepower. On the other hand, modern factory EFI systems position the fuel injectors at the very end of the intake runners, directly in front of the intake port entrance. "When the fuel injector sprays at the intake valve, the air/fuel mixture can't burn as efficiently because it doesn't have enough time to fully homogenize. In a carburetor, the fuel is introduced at the top of the plenum, which nets a nice shearing effect for improved atomization and increased charge density," says Starr.
That's not to say that a carb is an inherently superior fuel-mixing device. Simple tweaks to injector positioning enable combining the enhanced precision of EFI with the horsepower benefits of a carb. "In Formula One and IndyCar engines, the fuel injectors are placed at the very top of the intake ram tube. As a result, they benefit from the increases in air speed of an individual-runner intake manifold, as well as the improvements in fuel atomization and intake charge density," Starr explains. "At Hilborn, that's why we place the fuel injector nozzles very high up in the intake runner. This gives you the anti-reversion and air speed advantages of an individual-runner intake manifold in addition to improved fuel atomization."
Hitting the RollersAs no surprise, with the Hilborn EFI system installed and the FAST engine management computer dialed in, Jeff noticed an immediate improvement in the big-block's drivability, idle quality, and cold startup performance. While the big Bel Air felt snappier to throttle input, the best way to gather some real empirical data was to run it on a chassis dyno. Consequently, we headed over to the School of Automotive Machinists (www.samracing.com) in Houston, Texas, to strap the car down to its Dynojet. Prior to the Hilborn installation, the 482 laid down 236 hp at 4,900 rpm, and 289 lb-ft of torque at 4,200 rpm. With the new eight-stack induction in place, the numbers jumped to 256 hp and 290 lb-ft. An improvement of 20 rwhp is certainly impressive, but the real story here is the new shape of the torque curve. While the 482 picked up a measly 1 lb-ft at peak, the improvements in the low- and mid-range are truly astonishing. At 3,600 rpm, the Hilborn injection is good for an extra 74 lb-ft over the dual carbs. Likewise, from 3,600-4,000 rpm, the injected 482 puts out an average of 275 lb-ft to the carbs' 238 lb-ft.
Just as advertised, the Hilborn individual-runner induction system paid large dividends in low- and mid-range torque, as well as a few bonus horsepower up top. "By eliminating the reversion pulse issues that plague a common plenum intake manifold, an individual runner intake flattens out the torque curve dramatically. The increase in torque output on Jeff's 482 is a reflection of how much more efficiently the induction system is working at low rpm," Starr explains. "The more aggressive the cylinder heads and camshaft are on an engine, the more it will benefit from an individual-runner induction system. As this test proves, even in an engine with a milder camshaft that doesn't create as much reversion, the increase in low-end torque can be dramatic."