The 193 castings (suffix of the casting number) were used in GM light trucks and fullsize GM passenger cars during (approx) 1987-95. They are called "swirl port" heads because the intake port directly under the intake valve has an incline that causes the intake flow to rotate before it enters the cylinder.

This is done so that the air/fuel mixture (A) speeds aound the cylinder and that helps (B) shear the air/fuel mix as it passes the protrusion of the spark plug nose, and (C) causes the mixture to burn more smoothly and rapidly once the plug fires.

GM considered those heads (193 casting, as well as the 191 casting on some trucks, plus the 187 casting used 87-95 on 305 engines, and the v6 version of the same head used on the LB4 v6) to be fast-burn heads in the literature of the day. The fast-burn also allowed slightly (static) higher compression ratio to be used even on 87 octane fuel, and the fast burn also helped reduce emissions.

The major whine people have with those heads, a complaint that is repeated on GMHTP's website as well as on the web site of one of the major GM parts sellers (Sallee?) is that the heads don't flow well. Of course none of those critics post flow data showing how good/bad they are nor do they compare them with other contemporary small-port production heads. And the obstruction ramp in the port is almost entirely shielded by the pushrod wall, except for the region very close to the valve stem.... so the statement about obstructed airflow is an overstatement.

As it turns out, the 193 heads flow about as well (or as poorly) as the 083 iron heads used on the 3rd gen Fcar L98 engine. The 193 heads flow slightly less on the intake side, but much better on the exhaust side. See this thread, and others at the Thirdgen.org web site. The heads are not nearly as bad as nearly everyone says (without proof) and can be ported to decent flow levels especially for small-volume port GM production iron heads. The cited link to the TGO website gives several examples if you wade through the lengthy thread. HTH.