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post #1 of 3 (permalink) Old 09-03-09, 09:41 PM Thread Starter
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2 questions: Should I use 93 octane? What can I do for better fuel mileage?

I have a 98 Lincoln Navigator. Of course the 5.4L Engine. Im thinking it was actually an early 99 model with most of the 99 features..not really sure. Do I need to run 93 octane? It sounds like it is pinging a little. I think it had some paint work so there is no sticker in the fuel door to say premium fuel only.

What can I do to get better fuel mileage? I am going to get accell coil packs with new spark plugs, fuel filter, and Im looking at new exhaust. I was going to go for shorty headers and full new exhaust. Not sure what muffler would be just right, but I cant help but wonder how much it would help by gutting the cats. Of course I am planning on getting a cold air intake. I am going to get the upper and lower intakes ported and polished....maybe even go as far as porting and polishing the heads and getting an sct tuner. With all this done I should deffinatly see a boost in mpgs. Any ideas, suggestions, or comments?

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post #2 of 3 (permalink) Old 09-04-09, 01:36 PM
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Re: 2 questions: Should I use 93 octane? What can I do for better fuel mileage?

First off, you're wasting your money on a cold air intake. Even in those rare occasions when they do increase HP, it's only in the 5,000+ RPM range. Find the cold air intake model you're thinking about and then look up the power ratio chart to prove it to yourself. Follow the increased HP curve and match it to RPM. You'll be shocked at how little boost you get at RPMS lower than 4,000. How often do you drive in that range? Plus, don't confuse more HP with better gas mileage--the two are mutually exclusive. The entire theory behind a cold air intake is that cold air is more dense so you can pack more molecules into the cylinder. But since the MAF knows the density of the air, so it adds more fuel to get the perfect mixture. More fuel equals fewer miles per gallon.

One more thing about cold air intakes. The part the manufacturers don't tell you is that although cold air is less dense, it is also COLDER. Since raw gas doesn't burn (it has to be vaporized first) car makers intentionally use under hood (hot) air to assist in fuel vaporization. Put in a cold air intake and you forfeit all the advantages of vaporization. What's that mean? Poorer performance at low RPMS in cold weather.

Second, most "cost air" intakes are really nothing more that vroom-makers. If the air filter installs under the hood, that's NOT a cold air intake. To qualify, a cold air intake must take in OUTSIDE air from in front of the radiator, the wheel well, or under the front bumper.

Next, there is absolutely NO difference in power between 87 and 93 octane. None. Nadda. They both provide the exact same number of BTU's. The octane in 93 just increases the flash point. That's all it does. A higher flash point prevents pre-ignition in a high compression engine. Put 93 in an engine that specs 87 and at best you're just throwing money down the drain. At worst, you'll experience hard starting in cold weather (since you've no increased the flash point of the gas).

Read this about 87 versus 93 octane

87 versus 93 octane

Next, be very careful when you mess with the exhaust. These engines are designed to require a certain amount of exhaust backpressure. If you reduce backpressure, you can actually DECREASE power. It all has to do with valve overlap--that short period of time when the exhaust valve is closing but not yet closed, and the intake valve is opening. Reduce backpressure at that point and the exhaust that's rushing out of the cylinder will actually sweep some of the new air fuel mixture out with it.

Finally, gutting the cats is illegal--like $10,000 fine illegal. I'm reasonably sure (but will have to check my Alldata) to see if there's a post cat O2 sensor on this vehicle. Gut the cat and you'll be throwing trouble codes and the PCM will pretty much go nuts.

Want better gas mileage? But a different vehicle.

I'm a retired ASE Master/L-1 Technician. I still keep current with the latest automotive technology. Visit my blog for cool articles and TSB's: http://www.ricksfreeautorepairadvice.com/
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post #3 of 3 (permalink) Old 05-03-10, 12:01 PM
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Re: 2 questions: Should I use 93 octane? What can I do for better fuel mileage?

sounds like a pile of money to save a few coins at the pump.
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