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4-1 or 4-2-1 headers?
4-1 (tuned length) aren't all that great without an open system, but they are still better than the old single down pipe manifold.
4-2-1 (interference design) are the best choice when used with mufflers, but they need to be kept small if your engine is nearly stock. 1.5" primaries & 1.75" secondaries would be good for anything from a B18B to a B20E.
Volvo's dual downpipe manifold is strange & a little interesting because cylinders 2 & 3 dump in to a big chamber that tapers down before entering the engine pipe. This is a little less than ideal, but it hardly makes any difference on stock to mild motors. One advantage that the dual downpipe manifold has is that it's quieter than tubular 4-2-1's & it gives a unique note to the exhaust due to 2 & 3 being siamesed together.
My preference would be the Volvo dual downpipe manifold with 1.75" diameter engine pipes extended to 24" long before merging together into a 2.125" exhaust system. Use only decent straight through mufflers, the B20 doesn't like turbo style mufflers.
If you did some head work, fitted a mild to medium cam, increased the capacity up to 2030 or 2130 & used a pair of 45mm DCOE's, I'd use 1.875" engine pipes & a 2.5" exhaust system. You will need to port the outlet of the manifold a little for 1.875" downpipes.
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