400 breathing

Are you talking about a 400 or a 400i?

For the 400, make sure that the carbs are well synched before you are worried about breathing.

 

This is what's under the 'cam cover' with the throttle on it. Looks like the front and rearmost cylinders are going to struggle to get their fair share of mixture?
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I've yet to see a redesigned intake housing, there isn't much room under the hood on a 400, hence the original side carb setup.

I do believe there is a high flow filter available.

 

#33-2570, same filter for a 1973 Volkswagen K-70

3 1/2" Dia. cold air intake for per bank @ 147 c.i. each I think is pretty generous. The only starving point is inside cam covers as pointed out(wondering if those stacks could be milled down a bit for better breathing).