I have a 1998 Lincoln Navagator that needs more power. It had BFG 285/75 R16 put on that really hurt its performance, what can be done to give it more acceleration?

 

Did those tires hurt performance b/c they spin easily and offer poor traction, or b/c they're too tall and effectively negate some of the rear gear ratio, or what?

There's a lot you can do with the potential of the 4v motors. I'd suggest a supercharger if you need substantially more power.

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Wes Tarbox
90 LX 5.0 (10.69 @ 134.7)--597rwhp/590rwtq
96 Cobra (12.34 @ 115.8)--392rwhp/433rwtq
99 Lightning (13.20 @ 103.8)--364rwhp/444rwtq
00 Expedition XLT 5.4
http://members.aol.com/Wa2fst/index.html

 

Hi Ford Rules All,

Well, what hurt was changing to those taller tires, as you have in effect changed your effective gear ratio to one that is numerically lower, and as a result, you lost torque multiplication. That's the *real* problem.

The 1998 Navigator doesn't have the 300 hp DOHC motor, it uses either a 235 hp or a 260 hp version of the 5.4 SOHC engine, most likely the 235 hp version.

To restore your lost power when you do a tire size change like this, the engine needs to be restored to turning the same rpms it used to for a given speed in Overdrive before you installed the taller tires, is the "correct" fix. Now, with those taller tires, you are turning less rpm for any given speed and consequently, the engine isn't up in it's power band anymore, and you get lousy performance and it also hurts your gas mileage. The load on the motor has been increased, while the torque available at the drive wheels has been decreased, all from this tire size change.

You can either change your gear ratios to compensate, which costs about $1200 - $1500 to do on a 4WD vehicle, or you can do a few aftermarket bolt-ons to give you more power.

The top 3 bang-for-the-buck power gainers on these vehicles are:

1.) Optimize your power train programming with the Superchip, this will add 23-26 more horsepower on your 5.4 motor, and will also give you better shift characteristics in the automatic transmission. You will have to use premium gas with the Superchip, as tuning the engine specifically for premium is how we are able to increase the power available at any throttle position. This is the single best performance improver for the money. This product is available from us, just contact us via email or phone, our contact info is below in my signatiure line, and we'll be happy to go over all the details with you so you can give it some thought.

2.) Reduce restriction on the intake side, using either a K&N air filter, or an airbox elimination kit. The drop-in K&N air filters are worth about 5-6 hp, and the airbox elimination kit in the Expedition body, which is basically what you have there from a powertrain standpoint, is worth about 14 hp on that 5.4 motor. These products are available right here on F-150 Online in the Online Store.

3.) Reduce restriction on the exhaust side, with a good cat-back exhaut system. These do not have to be louder than stock to give you more power, and a good cat-back system on that body is worth about 12-13 more hp. This too is available here in the Online Store.

The other issue is that your speedometer and odometer are now a bit off as well from the taller tires, registering slower than what you are actually travelling; up thru the 1998 model year in most FoMoCo vehicles, this can still be recalibrated inexpensively and easily, by changing the reduction gear in the back of your transmission, to correct this error.

Please feel free to give us a call if you'd liek to go over any of this in greater detail, and best of luck whatever you decide!

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Mike Troyer
Performance Products, Inc.
National Distributor of Superchips
(540) 862-9515
Email: mtroyer@compuserve.com
Performance Products F150Online Superchip ordering system: F150Online Superchip Ordering System
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