Max pipe diameter for a 318?

2" OD pipe flared to fit the 1-7/8 manifolds is what is used for the flange pipes on a 318. After about 18-24" and near the bell housing , a larger intermediate pipe can be made to continue downstream to the muffler. Anything above 2.25" would be pointless
and ineffective. Larger than 2.25" can cause more interior resonation , loss of low and mid range torque.