I have a '91 with the 4.5L and a '93 Eldo with the 4.9L. The Eldo gets 27MPG at cruise while the Allante and the 4.5 gets less mileage.
Here's the question: why the difference? The cam specs are the same, both use an OD trans, engines are similar xcept for the induction system, my Eldo uses sequential injection with a throttle body, speed/density system, whereas the Allante uses the mass-airflow system, I believe. Perhaps there may be a simple way to improve mileage on the 4.5? It CAN be done as evidenced with the 4.9.
Automobile(s): 1988 Allante' (sold), 1984 Eldorado, 84 Sedan DeVille
Re: gas mileage with the 4.5L
Both are speed density. My first question is what mileage does the Allante get? Also the Allante final drive ratio is numerically higher. Whats the static timing? calls for 10 degrees, but they get better mileage at 12 degrees BTDC. Is the area in front of the throttle body butterflies perfectly clean? Whats the state of tune on the Allante? plugs, fuel filter cap/ rotor.