Specifically, the VERY short air cleaner elbow on a ZJ was due to the low, sloping hood height in the Grand Cherokee in comparison to the trucks using the same engine. The aftermarket spacers take up all the available space, usually causing the elbow to rub on the hood insulation. I suspect it helps the airflow straighten above the throttle body bores a bit.

The ZJ 5.2l exhaust system also uses more restrictive exhaust manifolds, due to the tighter package and relocated starter (passenger side on a Jeep, driver's side on a Dodge) As an aside, the '92-'93 "Magnum" truck exhaust manifolds are about the best flowing small block Mopar V8 stock manifolds, only sightly bettered by those from the old 340 six pack.

Next time you visit a junk yard, compare the differences!

BigJim, I don't have pictures of the Dodge vs. Jeep dyno readings to show the differences in the respective power curves, but I suspect they would show up more at the higher RPM range, at WOT, where the max horsepower readings are taken.

Air cleaner designs are a compromise of power/fit/Govt. sound level restrictions/filtering efficiency/cost. Sometimes the aftermarket can come up with improvements because they don't face the same limitations as the factory does. Rarely do they actually filter the air or fit better, sometimes they actually improve breathing. Usually, they add to the engine noises. Most times, the added cost isn't worth the gains.

Engine pumping losses are a serious consideration with more modern engines being designed to lower the power used to turn the engine. New designs coming out of Detroit etc. are using computer controlled cam, ignition and injection timing, and electronic throttles to lower part throttle cruise engine vacuum to lower these losses. The PCM will control a wider than needed overlap of the cams, open the throttle blade wider and using direct cylinder fuel injection make the engine run smoothly at cruise with very low engine vacuum. A new engine design may only have 5" vacuum at cruise where the old engines were efficiently cruising at 15", this increases fuel economy and power for the same amount of fuel burnt, through better efficiency.

Pumping losses can be decreased in older engines with more efficient intake and exhaust systems.