399501 Good low end and mid range torque and HP, smooth idle, daily usage, 5.4L towing, 2200-2600 cruise RPM, valve spring upgrades required. Also mild supercharged or mild nitrous.
399511 Good mid range torque and HP, good idle, performance usage, bracket racing, good w/supercharger or mild nitrous, 2600-3000 cruise RPM, valve spring and computer upgrades required.
399521 Good mid to upper RPM torque and HP, fair idle, performance usage, bracket racing, auto trans w2000+ converter, 11.0+ compression ratio advised, 3000-3600 cruise RPM, valve spring and computer upgrades required.
399531 Good upper RPM HP, rough idle, performance usage, bracket racing, auto trans w2500+ converter, 11.0+ compression ratio advised, 3200-3800 cruiseRPM, valve spring and computer upgrades required.
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