Test to Determine the Relative Engine Load
With or Without a Camper Shell
(and with the tailgate up or down)

With a desire to finally (we hope) determine the best configuration for fuel mileage, my oldest son and I decided to run a few tests with our Ford F150 in three different configurations:

1)  The Camper Shell On

2)  The Camper Shell Off and Tailgate Down

           
3)  The Camper Shell Off and Tailgate Up

The basic specifications for the truck:
                Make:   Ford
                Model:  F150 XLT Lariat
                Year:    1994
                Engine:  5.8L MFI
                Trans:   4-Spd Automatic E4OD
                Bed:     8 foot

To conduct the tests, we built the simple circuit shown below, with the GM Manifold Air Pressure Sensor connected to the vacuum tree on the intake manifold. We purchased our sensor from a local junk yard for $5.00.

Circuit Schematic

Here is another drawing (I don't have a good picture) of the MAP sensor giving its pin-outs and some electrical characteristics:

For a very good photograph of a MAP sensor identical to the one we used, check out Robie Blair's web page at  GM MAP Sensor Identification. Robie has a page full of detailed information on MAP sensors there.

Each truck configuration was tested over the same 9 mile stretch of interstate highway, in the same direction, and at the same speed of 65 mph -- very carefully set using cruise control. Once the speed was set the readings were taken from the digital voltmeter at 15 second intervals until cruise control was disengaged. This procedure provided us with about 8.5 miles of constant speed driving for each configuration.

As for the results . . . the data we recorded during these tests is shown in the two charts that follow. In both charts the lower the voltage (the vertical axis) the lower the engine load.

The first chart plots the MAP sensor voltage data we recorded for each configuration in solid lines and the averages for each configuration in dashed lines.

The second chart plots the average voltages of the MAP sensor data for the three configurations on the MAP sensors transfer curve, giving us the average intake manifold vacuum.

A little study of the above charts will reveal, much to our surprise, that the configuration that creates the least load on the engine and the highest vacuum is with the camper shell off and the tailgate up -- represented by the green lines in both charts.

The configuration with the camper shell off and the tail gate down -- represented by the blue lines in both charts -- caused a slight increase in engine load.

However, the camper shell on configuration -- represented by the red lines in both charts -- caused a significant increase in engine load.

Well, there you have it. Our results indicate:
                    Attaching a camper shell increases engine load by a significant amount.   
                    The shell off and tail gate down decreases the engine load significantly.
                    The shell off and tail gate up provides the greatest decrease in engine load. (slightly better than with the tailgate down)

Hope you Ford truckers found this useful.

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