Octane

[Greg__S]

I have a couple new 5.4's in my work trucks. I was at the Lincoln Dealership this week and their 5.4's in the Lincolns need 91 octane gas. Its the same motor and same specs. The salesperson said its because the 3 valve motor will burn out around 60-70k miles if you use 87 octane. My Ford dealer knows nothing about this.

Anyone know if the 300hp 5.4 needs better gas?

Greg

[BMcNab]

The 5.4 3 valve needs minimum 89 octane, NOTHING less. Use premium if you want the best performance and longevity.

[jimmy]

If you have high compression ratio and program PCM for Premium then you need premium fuel to reach the advertised HP/Torque figures. If the PCM is programmed for reg unleaded then the engine will use reg unleaded and make advertised HP/Torque in normal conditions and may not make any more HP/Torque with premium since PCM tuning is for regular.

The 5.4 3 valve has lower compression ratio than the 4 valve 5.4 Lincolns used before 2005. I think the 3 valve 5.4 navigators recommend reg also.
If you have an engine that recommends premium and you use reg there is a slight risk of over heating as a high compression engine using reg may have significantly retarded timing if driven hard or if used for towing. Knock sensors on modern engines report knock activity to PCM and PCM reatrds timing to suppress knock. Retarded ignition timing increases coolant temperature.
If you have an engine that should use premium and you use reg and run it hard or tow AND it is not equipped with knock sensors, then the timing NOT being retarded for the lower octane fuel can cause lowerd power and even piston damage from excessive knock.
So, some engines MAY be damaged from lower octane fuel and some will not.
Engines tuned to use reg will often have cold start issues when using premium as premium fuel has less volatility and is harder to ignite when cold.
Use premium in a premium recommended modern engine if you drive hard, tow, or climb long grades, use reg in a reg recommended modern engine as it will likely not run one bit better with premium. Reg engines may benefit from more octane in HOT weather towing situations though as those situations can make it have higher levels of knock and may trigger spark retard in hard use.

A few years earlier, fuel grades had greatly varying levels of detergents and lower octane fuel use could lead to more problems with deposits in injectors and in intake ports. Fuel companies upped the additives to give the different grades similar detergent levels as car manufacturers starting recommending the fuel companies with high detergent levels in ALL grades since that caused car makers less warranty expenses.

That is the facts, make your decision as to whether fuel company needs extra cash from you.