According to my 1981 parts manual, the E46 engine was essentially a run of the mill 318 with a 4 bbl intake on it. The E45 had a 2 bbl intake with some heavy duty features like double row timing chain. The E48 got all the heavy duty high performance stuff, special cam, special heads, timing chain, valves, 4 bbl intake, exhaust manifolds, bearings, and windage tray. It doesn't mention the E47 so I assume it was no longer available.