An engine swap is the process of removing a car's engine and replacing it with another. This is done either because of failure, or to install a different engine, usually one that is more powerful or more modern and maintainable.

An engine swap can either be to another engine intended to work in the car by the manufacturer, or one totally different. The former is much simpler than the latter. Fitting an engine into a car that was never intended to accept it may require much work – modifying the car to fit the engine, modifying the engine to fit the car, and building custom engine mounts and transmission bellhousing adaptors to interface them along with a custom built driveshaft. Some small businesses build conversion kits for engine swaps, such as the Fiat Twin cam into a Morris Minor or similar.

A common anecdote among tuners in the United States is that the easiest way to make a car faster is to drop in a small block Chevrolet engine as used in the Corvette. The Chevrolet Vega, a '70s economy car not weighing much more than a ton was a popular Chevy V8 small block swap then, and its popularity continues. Most surviving Vega vehicles today have a swapped V8. Chevrolet engines have been used in such cars as Toyota Supras, BMWs, RX-7s, Mazda Miatas, Jaguar sedans, Datsun 240, 260 and 280Z's, Chevrolet Corvairs, and others. A common myth is that a Porsche engine will fit in a VW Beetle, it simply doesn't and is not a common swap as people mistakenly think. The confusion arises from the early Porsche 356 engines which were basically a tweaked type 1 Beetle engine. The only "Porsche" engine that will fit into a Beetle is from a Porsche 914, but these engines are built from a VW parts bin, and are the same engine as fitted in many VW vans, albeit with different cylinder heads and pistons. One popular engine swap for the Beetle is the Subaru flat 4, as these do physically fit without too much difficulty (but liquid cooling and the need for a radiator and a cabin heater are additional complications). Other popular engine swaps are a turbo Subaru engine into an older Porsche 914, or a turbocharged engine from a Saab 900 into a Saab 99 or BMW. The low weight Suzuki Swift GTI engine mated to the Suzuki Samurai gearbox is a popular choice for smaller, British sportscars. The engine mount and gearbox connection similarities make the Ford Taunus V4 engine to Ford Cologne V6 engine a popular swap.

In the world of sport compact enthusiasts, engine swaps are common with Nissans. A stock Nissan 240SX produces 155 hp (116 kW) with its torquey KA24DE 160 hp (119 kW) engine. Swapped out with a Japanese turbo SR20DET, a Nissan 240SX will produce over 200 hp right out of the box. Mild modifications can increase power to a range of 250 hp to 300 hp (190 to 220 kW). Another common engine for swapping is the RB26DETT found in the Nissan Skyline GT-R. It produces 280 hp (210 kW) out of the box, and can be easily modified to produce higher powers. V8 swaps have also become popular in the Nissan 240SX. Common V8 engines to swap into the 240SX include the Nissan VH45DE from the Infiniti Q45, the first and second generation GM smallblock V8, and the GM LS-series V8. The LS series V8 is particularly popular due to its light weight and its performance potential.

As the price for turbocharged Subarus (such as the WRX) comes down, the popularity of turbo engine swaps into non turbo Subarus has gone up. The Subaru turbo 4-cylinder engine takes especially well to Porsche 914's, and also VW bugs and busses.

In the Honda world, popular engine swaps include the Civic Si (B16A), Integra GSR(B18C), and the Integra Type R (B18C5) engines. Swapping them into a lightweight 88-00 Honda Civic chassis can achieve greater performance.

Chrysler made many turbocharged vehicles in the 1980s, and these engines share much in common with their naturally aspirated brothers. It is quite common to obtain an engine from a vehicle such as a Dodge Daytona and swap it into such an unassuming-looking vehicle as a Dodge Aries. The Mopar Performance arm even offered a kit to upgrade the Dodge Daytona to rear wheel drive with a Mopar V8.

Engine swaps are also somewhat common within the Volkswagen tuning scene, often placing Mark III and Mark IV engines such as the VR6 and 1.8 T into the Mark II GTI, Jetta and Corrado. Less common is the swap into a Mark 1 Golf or Cabriolet, giving an amazing power-to-weight ratio, even with minimally modified powerplants.

In some jurisdictions with strict, arbitrary "smog" rules it may not be possible to register a late-model vehicle with an engine swap, even if it can be proven that it produces less pollution than the original engine (owing to "visual inspection" rules).

In the Super GT racing series, engine swaps can be considered a way of life for the upper tier GT500 cars, most of which are provided with specially modified racing engines from the manufacturers. GT500 class rules themselves allow for any engine to be swapped into a car as long as it is from the same manufacturer. Notable examples include Toyota swapping in highly tuned 4-cylinder engines originally from the Toyota Celica into their Toyota Supra GT500 race cars.

British sports cars (such as MGs and Triumphs) from the late 1960s and early '70s were attractive light-weight cars that had excellent suspensions, but were known for troublesome electrical systems, barely adequate power levels and unreliability. It is popular to take one of these classic sports cars and add a more powerful engine. The all-aluminum Buick and Oldsmobile 215 (3.5L) V8 engines are a traditional choice for these cars. Swapping an MGB all-iron 1.8L 4-cylinder engine and 4-speed transmission for a Buick 215 V8 and a modern 5-speed transmission actually improves both cornering and acceleration because it reduces the overall weight of the car by about 40 pounds. Power is approximately doubled. Derivatives of that classic General Motors engine, the Rover 3.5L, 3.9L and 4.2L V8's are also often used. Although more recent narrow sixty-degree Ford and GM V6 engines are more compact, they usually don't equal the Rover engine's power-to-weight ratio. They can, however, be very cost effective and an easier fit, notably the Chevrolet 3.4L. The cast-iron-block Ford 302 (5.0L) V8 in particular results in spectacular power-to-weight ratios for straight-line acceleration. With aluminum heads, intake, and water pump fitted, the Ford 302 only adds about 40 pounds (18 kg) to the front of an MGB, and is substantially more powerful and lighter weight than an MGC or Triumph TR6 iron-block six-cylinder engine. An aluminum 302 performance block is available that weighs 60 lb (27 kg) less than the common iron version, as is displacements of 331 and 347 ci, but they are significantly more expensive. The Nissan SR20DET is an all-aluminum fuel-injected DOHC turbocharged 4-cylinder. This compact engine, along with the very compact Mazda "13B" rotary engine, have both been transplanted into too many different cars to list.

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