Great as a stock replacement, our exhaust manifolds were designed with expansion joints and heavy webbing to prevent cracking. The exhaust runner design maintains exhaust velocity to drive the turbine wheel. Runner size does not go beyond the size of the exhaust ports on the head because when the runners are larger, the velocity to drive the turbo is lost due to the expansion and temperature in the exhaust manifold reduces the turbine drive pressure needed for turbocharger response.
On engines with enhanced fuel flow and larger turbochargers, the principal remains the same. Velocity and temperatures are needed to drive the turbine wheel which turns the larger compressor wheel that supplies the extra volume to attain proper air-to-fuel ratios.
BD’s 1-Piece Stainless Exhaust Manifolds do not require the expansion joints because of the ability of the Stainless Steel to withstand higher heat without cracking.