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Intake Manifold Running Design - Ask Marlan

Tech Q + A

Zany Idea?
I was holding up an Edelbrock Performer intake manifold, about to bolt it on to my car's small-block, when I noticed the runner design: It has four short runners and four long runners. That is typical of a dual plane intake manifold made to give the engine a broader range of power. I was just thinking (a dangerous thing, most people tell me), "Why isn't the rest of the engine designed this way?" If power under the curve is everything, why not design the whole engine like a dual-plane intake manifold? Why not have valve sizes, cam specs, and header primary tubes all different sizes to optimize their respective function? For example, a stout street engine would start with a 9.5:1 Chevy 383 and a Performer RPM intake feeding some Dart heads with both 2.02- and 1.94-inch intake valves (depending, of course, on which runner led them there). The cam would be a custom-ground roller with 210-degrees duration on the torque side and 235-degrees duration on the horsepower side (both measured at 0.050-inch tappet lift) with 0.500-inch valve-lift on both sides. The headers would also have two different-size primary tubes-1 3/4 and 1 5/8 inches, respectively.

How would this engine run? Would it have the broadest, flattest powerband, or would it rip itself into several large pieces? It's about time to rebuild my engine and I am considering some crazy ideas!
Rick Bradshaw Jr.
Zanesville, OH

The legendary Jim McFarland built an engine like this back in the early '80s. The first version used two different intake and exhaust runner sizes, port volumes, valve diameters, and cam lobes. The two configurations alternated in a regular sequence corresponding to every other cylinder in the firing order so the engine would still run smoothly. Essentially, the result was an engine that behaved like two V-4s with two different, pronounced torque peaks-but unfortunately, there was a large dip between each peak. To compensate, "Phase 2" varied pipe, runner, and cam timing even more, changing the combination for each individual cylinder, not just every other cylinder. Cam timing changes eventually evolved beyond lobe-to-lobe lift and duration-per-cylinder variations to also include lobe centerline and intake/exhaust lobe displacement angle variations as well. It was tedious, cut-and-try work (back then tuners didn't have today's computer modeling and simulation techniques available), but eventually McFarland wound up with (as he puts it) "a torque curve as flat as a billiard table." The engine had great off-idle torque and "it stayed there all the way through the top-end." The first "proof of concept" engine found its way into a street-driven Camaro. Later McFarland built this type of engine for one of Junior Johnson's short-oval track stock cars where it "kicked ass." Some successful short-oval track and road-racers may be building engines using these techniques today, enhanced even further by modern computer modeling; but if they are, no one's talking! Lunati reputedly ground the special cams for McFarland.

Source
Lunati Cams, Inc.

Dept. CC
4770 Lamar Ave.
Memphis, TN 38118-0021
901/365-0950
www.lunaticams.com

Shedding Some Light
I would like to know how much benefit there is in switching from cast-iron cylinder heads to aluminum heads on a small-block Chevy in general, and in my particular case as well. The car in question is a '69 Chevelle with a TH350 trans and 3.23:1 rearend. My car's engine is a Goodwrench 350 with the stock heads and an Edelbrock Performer intake manifold, cam, and 600-cfm carb. It has an advertised 8.0:1 compression ratio with the standard heads.

What gain in power could I expect by switching to aftermarket aluminum heads? I know I could gain more power through smaller combustion chambers (to provide a higher mechanical compression ratio), bigger valves, better heat dissipation, and improved port shape and flow characteristics

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