A Matter Of Height
Question: I have a 1971 Corvette with an LS5 454 and Turbo 400. It's not matching numbers, so I am modifying it for street performance. When I bought the car, it came with an Edelbrock Torker 2 intake, a 750 Holley, and a drop-base air filter that barely fit under the hood. I want the low end performance that comes with a dual plane, but I'm having trouble finding one that will fit. Edelbrock says its performers will not fit. The height of the 300-42 mid-rise Holley and the 8019 Weiand seem to be ok, but I have not been able to get any height measurement on a 360 Offenhauser. Is there another manufacturer I am overlooking?
Answer: Somewher in never-never land, someone is working on a dual-plane big-block intake manifold that will meet your requirements ~ I just don't know who that person is. In the meantime, you'll have to make do with what currently exists. The Torker 2 is the only "short" manifold in the Edelbrock catalog. If you were to put a straight edge across the carb mounting flange, it would be 3.050 inches to the block rail (sealing surface) at the front and 4.60 inches at the rear. By comparison, the Performer 2.0 measures 4.30 inches at the front and 5.40 inches at the rear. All the other Edelbrock manifolds are taller than the Performer. The height situation doesn't appear to be any better with the offerings from Holley and Weiand.
Offenhauser 360-degree manifolds tend to be on the short side, but as their name implies, they're of the 360-degree, rather than the 180-degree, design. The 360- degree configuration is essentially a single-plane manifold with a divided plenum. Since this design doesn't have the long runners associated with a 180-degree manifold, it may not meet your low-speed requirements.
The root of the problem you're facing is the law of physics. Long-intake manifold runners tend to enhance low-speed torque. Runner length is partially determined by the height of the carb mounting flange - the higher the flange, the easier it is to build long runners with gentle turns. Hence the "high-rise"design.
Unfortunately, you have to close the hoods of most cars before driving them, and when high rise meets low hood, you have a problem. That's why many Corvette hoods contain a bubble or scoop and some Corvette engines are equipped with low-rise intake manifolds. Your best bet may be a stock intake manifold. I know that's not what you want to hear, but it's not the only alternative unless you modify or switch hoods. Personally, since it's not a numbers matching car, I'd install an L88 or an LT-1 style hood, which will provide enough clearance for a high-rise, dual-plane intake manifold. Companies such as Eckler's offer a number of custom Corvette hoods that look very much like factory originals, yet have a higher rise for increased hood clearance.
