Engine tuning or engine building is the adjustment, modification or design of internal combustion engines to yield more performance, either in terms of power output or economy. It is a popular pastime with amateur mechanics or "gearheads" and "petrolheads". It has a long history, almost as long as the development of the car in general, originating with the development of early racing cars, and later, with the post-war hot-rod movement.
Tuning can describe a wide variety of adjustments and modifications, from the routine adjustment of the carburetor and ignition system to significant engine modifications. On older engines, setting the idling speed, mixture, carburetor balance, spark plug and distributor point gaps and ignition timing were both regular tasks on all engines and the final but essential steps in setting up a racing engine. On modern engines some or all of these tasks are automated.
At the other end of the scale, performance tuning of an engine can involve revisiting some of the design decisions taken at quite an early stage in the development of the engine.
Performance tuning focusses on the tuning of an engine for motor sport, although many cars built by hobbyists never compete but are rather built for display at motor shows or the simple pleasure of owning and driving such a car. In this context (and depending on the particular event), the power output, torque and responsiveness of the engine are of premium importance, but reliability and economy are also relevant. To win, a car must complete the event. This means the engine must be strong enough to do so, often far stronger than the production design on which it is based, and also that the vehicle must carry sufficient fuel. The weight of this fuel will affect the overall performance of the car, so fuel economy is a competitive advantage.
This also means that the performance tuning of an engine should take place in the context of the development of the overall vehicle. In particular, transmission, suspension and brakes must match the performance of the engine, otherwise the car will be unreliable, uncompetitive, and perhaps extremely dangerous.
In most cases, people are interested in increasing the power output of an engine. Many well tried and tested techniques have been devised to achieve this, but essentially all operate to increase the rate (and to a lesser extent efficiency) of combustion in a given engine. This is achieved by putting more fuel/air mixture into the engine, using a fuel with higher energy content, burning it more rapidly, and getting rid of the waste products more rapidly - this increases volumetric efficiency. The specific ways this is done include:
- Increasing the engine displacement. This can be done by "boring" - increasing the diameter of the cylinders and pistons, or by "stroking" - using a crankshaft with a longer stroke (in combination with pistons of shorter compression height, to maintain the original compression ratio), or both.
- Using larger or multiple carburetors, to create more fuel/air mixture to burn, and to get it into the engine more quickly. In modern engines, fuel injection is more often used, and may be modified in a similar manner.
- Increasing the size of the valves in the engine, thus decreasing the restriction in the path of the fuel/air mixture entering, and the exhaust gases leaving the cylinder. Using multiple valves per cylinder results in the same thing - it is often more practical to have several small valves than have larger single valves.
- Using larger bored, smoother, less contorted intake and exhaust manifolds. This helps maintain the velocity of gases. Similarly, the ports in the cylinder can be enlarged and smoothed to match. This is termed "Cylinder head porting", usually with the aid of an air flow bench for testing and verifying the efficacy of the modifications.
- The larger bore may extend right through the complete exhaust system, using larger diameter piping and low back pressure mufflers, and through the intake system, with larger diameter airboxes, high-flow, high-efficiency air filters. Muffler modifications will change the sound of the car's engine, usually making it louder; for some tuners this is in itself a desirable property.
- Increasing the valve opening height (lift), by changing the profiles of the camshaft or the lift (lever) ratio of the valve rockers (OHV engines), or cam followers (OHC engines).
- Optimising the valve timing to improve burning efficiency - usually this increases power at one range of operating RPM at the expense of reducing it at others. For many applications this compromise is acceptable. Again this is usually achieved by a differently profiled camshaft. See also Four-stroke cycle#Valve Timing, variable valve timing.
- Raising the compression ratio, which makes more efficient use of the cylinder pressure developed and leading to more rapid burning of fuel, by using larger compression height pistons or thinner head gasket or by milling "shaving" the cylinder head.
- Forced Induction; adding a turbocharger or supercharger. The fuel/air mass entering the cylinders is increased by compressing the air first, (usually) mechanically.
- Using a fuel with higher energy content or by adding an oxidiser such as nitrous oxide.
- Changing the tuning characteristics electronically, by changing the firmware of the engine management system (EMS). This chip tuning often works because modern engines are designed to give a great deal of raw power, which is then reduced by the engine management system to make the engine operate smoothly over a wider RPM range, with low emissions. By analogy with an operational amplifier, the EMS acts as a feedback loop around an engine with a great deal of open loop gain. Many modern engines are now of this type, and are amenable to this form of tuning. Naturally many other design parameters are sacrificed in the pursuit of power.
The choice of modification depends greatly on the degree of performance enhancement desired, budget, and the characteristics of the engine to be modified. Intake, exhaust, and chip upgrades are usually amongst the first modifications made as they are the cheapest, make reasonably general improvements (whereas a different camshaft, for instance, requires trading off performance at low engine speeds for improvements at high engine speeds), can often actually improve fuel economy, generally do not affect engine reliability too much (because no moving parts are modified), and are in any case essential to take full advantage of any further upgrades.
- Manufacturer Detuned Engines - Changing the tuning characteristics electronically, by changing the firmware of the engine management system (EMS). This chip tuning also works because many manufacturers produce one engine which is used in a range of models and the power and torque characteristics are determined solely by the engine management system software. This allows the manufacturers to sell cars in various markets with different tax and emissions regulations without the huge development cost of designing different engines. Cross platform engine sharing also allows for a single engine to be used by different brands, tuned to suit their particular market.
Examples of models using one engine with different ECU software providing varying specifications:
Volvo V70 D5 Euro IV available as 126 bhp, 163 bhp, 185 bhp, all sharing the same 2.4 turbo diesel engine. Mini One and Mini Cooper available as 90 bhp and 127 bhp respectively, both sharing the same 1.6 normally aspirated engine. Ford Focus ST225 and Volvo S40 T5 both sharing the Volvo 2.5 turbo petrol engines, with different power outputs controlled by the engine management system.
- TuningMag.net - Tuning magazine
- HispaRecambios - Car accesories and spare parts in Spain
-  - Vehicle Standards and Modification Information
- Tuned-Belgium.com - Belgisch grootste nederlandstalige tuningsite
- Custom Auto - General information on tuning and customizing
- Honda Civic Engine Tuning FAQ - Information on general engine tuning issues and concepts
- 450 Performance - North America tuning events coverage
- 2fastlane.com - Car tuning blog