539521, 549521 Good low end and mid range and HP, good idle, daily and performance usage, auto trans w/2500+ converter, 2600-3400 cruise RPM, 8.75 to 10.5 compression ratio advised.
539531, 549531 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 w/supercharger
539541, 549541 Good upper RPM torque and HP, rough idle, performance usage, auto trans w/ 3000+ converter, 4000-4800 cruise RPM, 11.0 to 12.5 compression ratio advised.
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