rear of the casting. These details made swapping older blocks from passenger cars with front sump oil pans to more recent rear-sumped Mustang and LTD/Crown Vic Ford cars more difficult unless an oil pan had the dipstick mounted therein. In 1984, the rear main seal was changed from a two-piece component to a one-piece design. Introduced in 1969, it was initially rated (SAE gross) at with a two-barrel carburetor or with a four-barrel. When Ford switched to net power ratings in 1972, it was rated at 153 to 161 hp (114 to 120 kW), although actual, installed horsepower was only fractionally lower than in 1971. Around 8.6 million 351W engines were manufactured between 1969 and 1996 at the Windsor Engine Plant Number One. During the 1990s, motor enthusiasts were modifying 351 Cleveland 2V cylinder heads (by rerouting the coolant exit from the block surfaces to the intake manifold surfaces) for use in the 351W, resulting in the Clevor (combining Cleveland and Windsor). This modification required the use of custom pistons by reason of differing combustion chamber terrain (canted valves vs. straight valves) and intake manifolds. This combination yielded the horsepower potential of the 351C with the ruggedness of the 351W small block and was possible because more 351C 2V cylinder heads were manufactured than the corresponding engine blocks (the 351M and 400 used the same head as the 351C 2V). The 5.8 L, 351W, was changed during the '90s from speed density to MAF; performance gains were directly affected. Before 1994, the 5.8 L was equipped with speed density. This programmed coding was placed into the vehicle's computer to tell the motor how much air it should be getting, therefore supplying an appropriate amount of fuel. However, if modifications are made to increase air flow, the computer does not provide more fuel until the oxygen sensors register a lean burn and only then can the computer compensate fuel trim. After 1994, the engine was changed to mass air flow (MAF). This allowed the computer to directly read how much air the engine was receiving through the help of a sensor in the air intake as long as that exact sensor was used (MAF sensors cannot be upgraded for increased flow without using an aftermarket chip or ECU). The rest of the intake system can be modified in any number of ways, but generally the MAF sensor remains stock). Because the