538491, 548491 Good low end and mid range torque, good idle, daily and performance usage, 2600-3400 cruise RPM, 8.75 to 9.75 compression ratio advised.
538501, 548501 Good mid range RPM torque and HP, good idle, daily and performance usage, also mild supercharged, auto trans w/2000+ converter, 2800-3600 cruise RPM, 9.0 to 10.5 compression ratio advised, or 8.0 to 9.0 with supercharger.
538511, 548511 Good mid to upper RPM torque and HP, fair idle, daily and performance usage, also mild supercharged, auto trans w/2500+ converter, 3200-4000 cruise RPM, 9.5 to 11.0 compression ratio advised, or 8.5 to 10.0 with supercharger.
538521, 548521 Good upper RPM torque and HP, rough idle, performance usage, also supercharged, auto trans w/3000+ converter, 3600-4400 cruise RPM, 10.0 to 11.5 compression ratio advised, or 8.5 to 10.5 with supercharger.
