Jeep 134-cube F-head
The candidate: Introduced for the '50 Willys trucks and put into the '53 CJ-3B, the 72hp/114lb-ft (high-compression versions made 75hp) 134-cube Hurricane F-head offered 12hp and 9 lb-ft more than the Go Devil L-head it replaced. With the intake valves moved from the block to the head, the F-head could breathe much more freely than the L-head.
The perception: It's nearly a bolt-in swap that will up the power over the older L-head and help a stock(ish) Willys maintain speed on grades. It delivers 20 percent more horsepower and nearly 10 percent more torque.
The reality: It's all true. However, nowadays there are other ways to get drivability out of your L-head and better swap candidates that don't use antiquated parts like the F-head and don't require a special carb or a hole to be cut in the hood for carb clearance.
The exception: We're totally down with an F-head swap if you're building a period-correct '50s/'60s-vibe retro wheeler.
We think: It's better to save the F-head for a resto job on a CJ-3B or early CJ-5 or CJ-6. If you want power in your L-head-powered flattie, why not be different and slap a 2.2L Chrysler turbo on your L-head, add a Pertronix electronic ignition, and a one-barrel TBI injection? Or, if you're really good with swapping in a different four-banger, why not run a 2.3L Pinto engine, or better yet, swap in a 2.5L TBI or MPI Wrangler or XJ engine? Use a 2.8L GM V-6 bellhousing and put an SM420 behind it and never worry for power or drivability off-road again.
Chrysler/Jeep 4.7L V-8
The candidate: The Chrysler 4.7L overhead cam first saw Jeep duty in the '99 Grand Cherokee and wormed its way into the Dodge Dakota and fullsize Ram platforms as well.
The perception: For under $3,000, I can pick up a thrashed Grand Cherokee with the 4.7L and five-speed auto tranny and put it in my trail Jeep.
The reality: Have you ever looked under the hood of a 4.7L-powered Grand Cherokee? Besides looking like a nightmare to extricate, the WJ's overhead cam 4.7L is super-wide, very complex, and only puts out around 235-265hp. And again, there's very little in the way of aftermarket harness and ECU support to make the swap easy.
The exception: We really can't think of any.
We think: If you want modern V-8 power, you'll be way better off finding a wrecked Silverado and nabbing the 5.3L Vortec engine instead.
Northstar V-8
The candidate: In '93, grannymobile Cadillacs began running GM's premier DOHC 4.6L Northstar engine. With dual overhead cams, four-valves-per-cylinder, and an output of either 275hp/300lb-ft (LD8) or 300hp/295lb-ft (L37), the all-aluminum Northstars combined light weight, high strength, and smooth power delivery in one iconic package. The catch was that up until the LC3 came along in '07, all Northstars were transversely-mounted in front-wheel drive applications.
The perception: I'll buy a '90s Caddy for $800, yank the engine, and slap it in my Wrangler and go!
The reality: The transverse-mounted architecture is a nightmare to overcome. For starters, the log-style oil pan can't be chopped into a rear-sump design. Then, the intake manifold must be turned 180-degrees on the engine and the water pump manifold and water pump removed from the rear of the engine and replaced with an electric water pump for firewall clearance.
The exception: The engine architecture works well for rear-engine sand rail applications and, indeed, these engines are supported with aftermarket wiring harnesses and stand-alone ECUs.
We think: Leave 'em to the buggies. If you really need a DOHC Northstar, go find yourself an Olds 3.5L Shortstar V-6, add a turbo, and pretend you're running the V-8.