Actually the cylinder walls are not coated in either case. The technique is called hypoeutectic cooling/casting. The concept is to disolve as much silicon dioxide (sand/glass) in the matrix of aluminum. As I recall this was about 17%. The amount that can be held in solution varies the freezing point and changes the amount that can be disolved. By controlling the freeze an artificially high level of SiO2 is held disolved. What this all means is that even though the whole thing is 17% the first part to freeze is about 35% and the last about 8%. Sort of the opposite of drinking frosen kool-aid. The first to melt is real sweet and the last is real weak.
Anyway the cylinder bores are made to freeze out first and they are very rich in the SiO2. This is very high for only a small depth.
After three step boring with ever finer cut. The honing is intended to polish out all mechanical deformation. After looking like glass (it is 35%) a siliconizing procedure is done. This sounds like its adding something, but it actually involves leather swabs dipped in an acid etch being drug through in a honing like procedure that removes the aluminum on the surface. This leaves a surface that IS glass and wears like it.
Bosch Master, ASE Master, L1
33 years MB technician