Aluminum Cylinder Head(s)
If the camshaft is the brains of your engine, then your cylinder heads are the lungs. Adding a high-flow cylinder head to an otherwise stock engine isn't going to net you that much gain. However, when used to complement other power producers-like a more aggressive camshaft, a better intake manifold, increased fuel, and a good flowing exhaust-huge power gains can be realized. Larger valves, smaller combustion chambers for higher compression, and better flowing ports are all hallmarks of aftermarket performance cylinder heads.
Look For: Aluminum heads have better heat-shedding capabilities and will usually allow pump gas use with a full point of compression more than iron heads. Engines with a cylinder bore size of 3.95 or bigger can usually go with a 2.02-inch intake valve. Smaller engines and lower performance engines can use 1.88- or 1.94-inch valves to maintain good cylinder-to-valve clearance and low-speed port velocity.
They Claim: Up to 70 hp at the crank.
We Say: With the right match of components, up to 70 hp at the crank is possible. On a stock short-block Jeep with the regular intake and exhaust mods, expect about 20 hp at the wheels.
For the stock crowd
If you're dealing with a Poindexter of an engine to start with, know when to say when. While many early Jeep L- and F-head engines offer better driveability with an electronic ignition conversion from points and maybe a 2bbl injection system or rebuilt carburetor, it's really not worth chasing fairies trying to eke out any real power.
Sadly, we've begun putting the ubiquitous 2.5L Jeep four-cylinder in this category. While they do respond well to turbo or supercharging, it's often an effort in futility trying to squeeze performance out of these engines with regular bolt-on parts.