The exhaust crossover passage plumbs a constant flow of exhaust gas right through the center of the manifold, often with a substantial area in the passage directly below the manifold’s plenum. The heat helps vaporize the fuel for better cold distribution, and actually warms the manifold to prevent part-throttle carb icing in very cold climates.
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Down here in the south we take and melt an old piston or two and fillin the exhaust runner before porting to get better exhaust flow and a cooler intake charge. Some performance intakes do not have provisions for the crossovers such as the Air-Gap series etc.
Wes