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."