As an unsophisticated gender that's easy to please, males dig things that are stacked. It doesn't matter if it's a cheeseburger, potato chips in a can, or something far less politically correct. There are much better magazines for studying up on that last one, so we won't even go there. For boys and their toys, the same goes for induction systems. No one gawks enviously at a dual-plane intake manifold, or goes gaga over the plastic monstrosity that is a modern EFI intake. Instead, it's the tunnel-rams and sheetmetal manifolds that captivate our imagination, and perhaps the King of visual induction system punch is an individual-runner, eight-stack intake. They certainly have an undeniable raciness to them. Style alone is reason enough for some people to bolt these intakes onto their motors, but do they actually improve horsepower and torque output over a carb, or are they merely eye-candy that lacks any real substance? To find out, we swapped a dual-carb intake manifold for a Hilborn EFI system on a 482 ci W-engine, then hit the dyno.
Although 1964 is often considered the unofficial beginning of the muscle car era, Chevy was cranking out plenty of sweet rides before then. The '61 and '62 Bel Air and Impala are among the best looking Chevys ever built, but by today's standards, the W-series big-blocks that powered them aren't quite up to snuff. To modernize the W in his '61 Bel Air, Jeff Cameron of Dooley and Sons Rods and Customs gave it a complete modern makeover. The 409 was stroked to 482 ci using a 454 crank, and then topped with out-of-the-box, unported Edelbrock aluminum cylinder heads, a dual-quad intake manifold, and a pair of 500cfm carbs. With a Comp 254/254-at-0.050 hydraulic roller cam actuating the valves, Jeff was pleased with the overall performance of the motor, but sought to tame it down a bit for around-town cruising.
"I wanted to improve the drivability and fuel mileage of the motor by upgrading to EFI, but there aren't a lot of options out there for these motors. I thought a Hilborn eight-stack induction system would look great on the Gen I big-block, so I was eager to test it out," Jeff explains.
Hot rodders these days are smart enough to know that the horsepower potential of an engine is most heavily influenced by cylinder head airflow. What many people neglect to realize is that the best heads in the world will never reach their full potential if they're choked up by a junk intake manifold. The problem is that intake manifold design, by nature, is a compromise from the get-go. If you design an intake manifold for maximum airflow, it's going to take up a lot of space. This universal truth is why NHRA Pro Stockers—or just about any race car for that matter—have such massive hoodscoops. Air doesn't like to change directions, which is why it's incredibly difficult to convince speeding air molecules to negotiate a 90-degree turn from the intake plenum into the runners, followed by another 90-degree turn from the runners into the intake ports. Tunnel-ram intakes address these issues by positioning the plenum as high above the motor as possible, thereby straightening the induction path between the plenum and cylinder heads. The long intake runners of a tunnel-ram greatly enhance air speed as well, boosting airflow through the intake ports and into the cylinders.
Unfortunately, big scoops you can hardly see over aren't an option for street cars. The ugly truth is that the most important yet most uninteresting factor that goes into intake manifold design is often underhood clearance. Even if it was possible to tastefully fit a tunnel-ram intake in a street car, the design still has its flaws. Whether the intake manifold in question is a dual-plane, single-plane, tunnel-ram or a factory EFI design, all conventional intakes feature runners that share a common plenum. This isn't an issue with conservative factory-spec camshafts, but with long-duration performance cams that hold the intake valve open well past BDC, the result is a reversion pulse that shoots back through the intake port and runners, and into the plenum.
"In a high-rpm race motor, intake reversion isn't an issue, but it's very detrimental at lower rpm in a street car. When you have reversion pulses from eight different cylinders coming into a single shared plenum, it pushes the air around into different directions, which reduces flow," explains Andrew Starr of Hilborn Injection. "Since an individual-runner intake manifold doesn't have a common plenum, it prevents the reversion pulse from one cylinder from adversely affecting the airflow in another cylinder. The benefit is extremely smooth part-throttle drivability, which is a function of not having a reversion pulse in the intake tract to deal with."
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."