H.O. 4.2L
Q What parts do I need to put a 4.0L head on the 4.2L?
A The H.O. 4.0L head found on '91-'06 TJs, XJs, ZJs, and WJs is drastically better than the factory head found on 232 and 258 engines.
Hesco (hescosc.com) offers a kit under PN HES4240CHC that includes a modified H.O. 4.0L head for use with the 4.2L block, gaskets, and a valve cover. You'll still need to obtain a 4.0L exhaust manifold or header.
If you choose to go the low-buck junkyard route, you'll need to fill in some of the water passages on the 4.0L head with epoxy or JB Weld. Most machine shops we've spoken with recommend using water-soluble packing peanuts. You shove the peanuts into the hole, leaving roughly 1/2- to 1/4-inch of room for the epoxy. Then fill it up and grind it smooth. The peanuts will dissolve into the coolant and won't affect flow within the engine.
The coolant passages that need filling will be evident, when you compare the 4.0L head to the 4.2L block, but they're on the intake side and flank the head bolt holes.
Forcing the Induction Issue
Q I'm considering adding either a supercharger or a turbo system to my 2.5L/4.0L for more power. Is one better than the other?
A Both a supercharger and a turbocharger do the same thing. They compress the air from the atmospheric pressure of 14.7-psi at (sea level), allowing the engine to ingest more air than it could when naturally aspirated and thereby allowing a proportionate amount of fuel to be introduced to the engine. It basically makes the engine think it's bigger than it is.
However, the way in which each accomplishes this compression differs. A supercharger is driven off the crankshaft of the engine. Since it's mechanically driven, it contributes a bit to the overall parasitic power losses the engine incurs. A turbo, on the other hand uses the flow of exhaust gasses to spin the compressor. Although it's not exactly free power, a turbo doesn't accrue the same parasitic losses as a supercharger. Both methods require some form of intercooler to realize their full potential since the intercooler will combat the inescapable byproduct of compressing air - heat.
That said, at the modest boost levels of 4- to 7-psi that most Jeep forced induction systems run, these differences between turbocharging and supercharging are minimized. Furthermore, the aftermarket is very adept at matching turbo sizes to their intended aftermarket application to do away with excess turbo lag and decreased drivability.
Which do we prefer? Based on its sheer capacity for power building, we'd be more inclined to go turbo. In the Jeep world, 505 Performance (505performance.com) has some pretty complete kits whipped up that make some real power. Just check our recent Insane Inline series that made an astounding 693 hp with a 5.0L stroker inline-six. On the other hand, supercharger companies like Avenger Superchargers (avengersuperchargers.com) and Adventure Innovations (adventure-innovations.com) both have very complete, well-designed supercharger kits for Jeeps. In the end, it's up to you to pick your own poison.