The practice of engine swapping is nothing new and dates back to a time when materials like rich mahogany and canvas were regulars amongst the automotive design arsenal. Stuffing bigger, more powerful engines into smaller, lighter-weight chassis is one of the most effective ways of generating a power-to-weight ratio that the factory won't. Your grandpappy did it; so can you. The number and variations of engine swaps are seemingly limitless, but the principles for one that doesn't end up in failure don't change. Sandwiching an engine in between a unibody is the easy part; getting ancillaries like electronics and power steering to work as well as properly retrofitting fuel and cooling systems into place can mean the difference between a fully functioning hybrid or 2,500lb worth of salvage stuck between you and your lawnmower. At the very least, the following considerations should be made before starting any engine swap.
Choosing the Car
Not every car deserves an engine swap, but that won't stop at least somebody from trying to shoehorn a 5.7L V8 into a Daewoo. That V8 is just one component of the overall package, though. No matter how much power it makes, the Daewoo's suspension, brakes and structural rigidity will always suck. Lightweight chassis and engine swaps go together like lists of cars that nobody cares about and Daewoos. Choosing something lightweight will make any engine swap a lot more gratifying, heightening that whole power-to-weight ratio. For lightweight chassis, look for something older that has less airbags than it does seats and sparse accouterments that you don't care about and only add to its heft. Finally, not every engine swap will make sense. Converting a Civic to RWD with the help of an S2000 powertrain might make you Internet famous until you realize that buying an S2000 costs about the same, is exponentially easier and can handle a whole lot better.
Choosing the Engine
Picking an engine that's more powerful than or has more potential than whatever's being ripped out is the obvious part. So is whether or not it'll actually fit. If the Internet can't tell you whether or not it'll slide into place, measure the engine's and transmission's overall dimensions and find out for yourself. Hood clearance, as well as territorial issues between the oil pan and the ground or subframe, are also worth thinking about early on. Radiators and batteries can be moved around, but if firewalls and shock towers are in the way, it's time to consider another powertrain. Engine rotation is also important unless you're particularly interested in five or six kinds of reverse. Whichever engine you pick, be sure to get the accompanying wiring harness, manifolds, ECU and ancillaries, like its alternator, starter, and distributor (if equipped). Finally, choosing an engine and transmission from the same manufacturer can make sorting out any wiring and electronics issues easier but isn't necessarily a prerequisite.
Mounting it Up
No matter which engine and transmission you choose, it's got to be held in place by some sort of brackets and mounts, most of which likely won't have anything to do with the ones you've already got. If you're lucky, the aftermarket will be there for you or some sort of OEM combination might work. In Honda land, for example, Hasport makes all sorts of mounts that allow almost any engine to fit into any chassis. Lexus owners, on the other hand, looking for 2JZ-GTE power can often use a mixture of factory pieces. In other extreme cases, the factory mounts must be cut from the unibody to make room for something entirely custom.
Axles and Drivetrain
Somehow the transmission's got to send torque to the wheels or some sort of differential. FWD swaps do this with axles while RWD transplants use a driveshaft. Unless the aftermarket offers a custom solution or some magical combination of OEM axles works, then custom shafts have got to be made. For front-driven swaps, measure between the front wheel hubs and the transaxle or intermediate shaft (if equipped). You'll need axles about this long with the corresponding joints that fit the hubs on one side and the transaxle or intermediate shaft on the other. For RWD swaps, a custom-length driveshaft can be made by chopping up or extending the car's original shaft using joints that are compatible with the new transmission's yoke and the existing rear end's U-joint.
Getting the gas and clutch pedals to communicate with the new engine and gearbox is as important as you think it is. Older engines with cable-driven throttle bodies need only a simple cable swap. Typically, whatever throttle cable goes with the engine works. Electronically controlled throttle bodies can be a whole lot messier. Getting these to work is a product of using the appropriate ECU, pedal assembly and wiring everything up right. As for the clutch pedal, there are two types of mechanisms: cable-operated and hydraulically controlled. When swapping a cable-operated gearbox into an older chassis that's native to that sort of transmission, the car's original cable can often be reused. When swapping a hydraulically controlled transmission into a newer chassis that's already outfitted for one, some sort of high-pressure adapter line will usually need to be made to connect the new transmission's clutch slave cylinder to the car's clutch master cylinder. A simple Teflon-based, steel-braided line along with the appropriate fittings are all you'll need. The possibilities don't end here, either. Hydraulically controlled gearboxes can also be dropped into older chassis that feature cable-operated transmissions. Such conversions require some sort of adapter kit and often the appropriate pedal assembly and hydraulic components, like a clutch master cylinder and reservoir.
Most shifter assemblies make things happen using a series of cables or a rod-and-lever linkage. Use whatever the new transmission was originally matched with. Often times, that means the car's shifter tunnel may need to be modified to allow the new shifter assembly to fit. Mixing and matching shifter components usually means trouble, so be sure to source the entire assembly from the same donor vehicle.
Bungling up a fuel line can turn your hatchback into a pile of ash, but properly modifying the fuel system for any engine swap isn't hard. Almost every fuel system is made up of an in-tank fuel pump, some line, a fuel filter, some more line, at least one fuel rail along with its injectors, and even more line. Most of the time the pump and filter can be recycled with lines made up that'll attach to the new engine's fuel rail(s). Be sure to use fuel injection hose that's able to withstand the higher pressure that regular old fuel line can't. Fuel injection hose clamps that don't tear up the hose they're clamping into like regular worm gear clamps do should also be used.
In most cases, the factory radiator will do, but bigger engines and limited space constraints often say otherwise. More room can be found by positioning an electric cooling fan on the radiator's opposite side. Be sure to reverse the fan's polarity and direction here, though; air must always pass through the radiator, whether it's being pulled or pushed. Radiator hoses and any heater hoses should be sourced from the engine but are more often than not something entirely custom.
A/C and Power Steering
A lot of times, A/C and power steering simply aren't compatible. Mechanical power steering pumps can interfere with the hood and A/C compressors and lines just won't fit. When they do though, a combination of parts scoured from the car and from the engine are often necessary. Generally, the engine's power steering pump and A/C compressor can be used and modified to connect to the car's remaining components. High-pressure lines will most likely need to be modified and welded to fit. Some swaps allow the car's A/C compressor to be reused and bolt up to the new engine block using some sort of aftermarket bracket. If this is an option for your swap, take advantage.
Intake and Exhaust
Unless whatever swap you've been eyeballing is already available in your chassis—perhaps from another country—then its intake and exhaust systems will be custom. Intake piping can vary a whole lot depending on make, model, and engine but it's typically best to start with a system that's either compatible with your engine or your car and chop, reroute and weld for a proper fit. That is, if the aftermarket doesn't offer something already. The exhaust is slightly different. Here, the engine's exhaust manifold(s) probably isn't going to bolt up to whatever exhaust system is underneath. An adapter pipe must be made with a flange on one end that fits up to the manifold and another at the opposite end that complements the exhaust piping or catalytic converter.
The electrical system is usually what separates a successful engine swap from an inoperable hunk of metal with a big, useless engine under its hood. It starts by selecting the right ECU. Typically, whatever ECU corresponds with the engine should be used. When cross-pollinating engine and chassis makes, though, like when installing a Chevy V8 into a Mazda RX-7, this isn't always possible. Here, a standalone engine management system is the alternative. Depending on the make, there can be as many as three separate wiring harnesses to contend with: one for the engine, one for underneath the dash and another for the chassis. You'll need shop manuals for both the engine and the car and the ability to understand electrical schematics. Finally, be prepared for unexpected complexities, like wire splicing and connector swapping and parts you never thought you'd need, like instrument clusters and vehicle speed sensors.
Things That Get in the Way
Just because an engine and transmission fit in between the unibody doesn't mean that's the last of any clearance issues. Often times, a taller hood must be used or, at the very least, its supportive under-skeleton trimmed. Notching crossmembers and subframes also isn't uncommon as is having to relocate an ABS pump, brake proportioning valve, fuel filter or anything else that might be in the way.
Suspension and Brakes
Whatever springs, shocks, rotors, calipers and pads your car was sold with were designed for a presumably lighter and less-powerful engine than what you're swapping in. The results can reduce shock travel and sacrifice braking abilities. Most any high-performance dampers or coilover system with stiffer rates take care of the first part. Braking upgrades can be made with something as simple as better-performing pads or, in some cases, a rear-disc conversion or larger brakes all around.
The process of making an engine swap emissions-legal varies depending on state and local laws and is the least glamorous part. The new engine must meet all of the emissions standards that the car is already expected to as well as those for itself. Often times, this means spending money on sensors and equipment that has nothing to do with making your car any faster but can keep you out of trouble come time for whatever testing your state might require. If you live in California, the requirements are even stricter. Here, the donor engine must be as new as whatever chassis it's going into, which can restrict engine selection significantly.