A/F gauges reads the stock O2 sensor by splicing into the wiring and not very useful for tuning.
EGT's, or,exhaust gas temperature, tells you the temperature of the exhaust coming out the cylinder and requires drilling and tapping for a temp. probe, preferably in the manifold. It can tell you if your lean; When your too lean you get some knock, which leads to retarded ignition timing, which leads to late combustion, which drives up EGT's (makes sense?).
EGT's, or,exhaust gas temperature, tells you the temperature of the exhaust coming out the cylinder and requires drilling and tapping for a temp. probe, preferably in the manifold. It can tell you if your lean; When your too lean you get some knock, which leads to retarded ignition timing, which leads to late combustion, which drives up EGT's (makes sense?).