I've never seen or heard of a peformance downdraft manifold for A-series. At least not an off-the-shelf one. I've seen some home-made ones.
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What effect does heating the intake gas have on performance, emissions, or efficiency?
Good for emissions and fuel economy (when the carb jetting is tuned to match), but bad for performance. For hp, you want cool intake charge, not warmed up ...
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Will I have problems if I dont use this intake?
The only performance problem is during cold weather. When the temperature gets close to freezing, around 35 degress F and colder your carburetor can ice up. There are two solutions when using a tubular exhaust header:
1. Create a sheet-metal box that goes from the header tubes up to the bottom of the manifold. Some aftermarket headers had this but it's rare
2. Use the newer water-heated B310 intake manifold. This is the one that has the small coolant hoses to the manifold.