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Engine tuning is the adjustment, modification of the internal combustion engine or modification to its control unit, otherwise known as its ECU (Engine Control Unit). It is done to yield optimal performance, to increase an engine's power output, economy, or durability. These goals may be mutually exclusive, and an engine may be detuned with respect to output (work) in exchange for better economy or longer engine life due to lessened stress on engine components.
It has a long history, almost as long as the development of the automobile 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 overhauls. 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.
Setting the idle speed, fuel/air mixture, carburetor balance, spark plug and distributor point gaps, and ignition timing were regular maintenance items for all older engines and the final but essential steps in setting up a racing engine. On modern engines, equipped with electronic ignition and fuel injection, some or all of these tasks are automated, although they still require periodic calibration.
Engine tune-up 
A tune-up usually refers to the routine servicing of the engine to meet the manufacturer's specifications. Tune-ups are needed periodically as according to the manufacturer's recommendations to ensure an automobile runs as expected. Modern automobiles now typically require only a small number of tune-ups over the course of an approximate 250,000-kilometre (160,000 mi) or a 10-year lifespan.
Tune-ups may include the following:
- Re-fastening of cylinder head bolts
- Adjustment of the carburetor idle speed and the air-fuel mixture
- Inspection and possible replacement of ignition system components like spark plugs, contact breaker points, distributor cap and distributor rotor
- Replacement of the air filter and other filters
- Inspection of emission controls
- Valvetrain adjustment
Chip tuning 
Modern engines are equipped with an engine management system (EMS)/Engine Control Unit (ECU) which can be modified to different settings, producing different performance levels. Manufacturers often produce a few engines which are used in a wider range of models and platforms, and this allows the manufacturers to sell automobiles in various markets with different regulations without having to spend money developing and designing different engines to fit these regulations. This also allows for a single engine to be used by several different brands, tuned to suit their particular buyer's market.
Performance tuning 
Performance tuning focuses on tuning an engine for motorsport, although many such automobiles never compete but rather are built for show or leisure driving. In this context, the power output, torque, and responsiveness of the engine are of premium importance, but reliability and fuel efficiency are also relevant. In races, the engine must be strong enough to withstand the additional stress placed upon it, and so is often far stronger than any mass-produced design on which it may be based, and also that the automobile must carry sufficient fuel. In particular, the transmission, driveshaft and any other load-transmitting powertrain components may need to be modified in order to withstand the load from the increased power.
In almost all 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 all essentially operate to increase the rate (and to a lesser extent efficiency) of combustion in a given engine. This is achieved by putting more air/fuel mixture into the engine, increasing compression ratio (requires higher octane gas) burning it more rapidly, and getting rid of the waste products more rapidly - this increases volumetric efficiency. In order to check the amount of the air/fuel mixture, air fuel ratio meters are often used. The weight of this fuel will affect the overall performance of the automobile, 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 automobile.
The specific ways to increase power include:
- Increasing the engine displacement by one or both of two methods: "boring" - increasing the diameter of the cylinders and pistons, or by "stroking" - using a crankshaft with a greater throw.
- Using larger or multiple carburetors, to create a more controllable air/fuel mixture to burn, and to get it into the engine more smoothly. In modern engines, fuel injection is more often used, and may be modified in a similar manner.
- Increasing the size of the poppet 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 effect – it is often more difficult to fit several small valves than to have larger single valves due to the valve gear required. However, it is difficult to find space for one large valve in the inlet and a large valve on the outlet side. Sometimes a large exhaust valve and two smaller inlet valves are fitted for improved flow. As the pressure generated during combustion provides more force to exhaust the waste gasses than the force available to inlet clean charged gas, a larger inlet valve area is needed to provide easier flow. The two smaller inlet valves' total area is larger than that of the single exhaust valve and thus provides that easier inlet flow. This is why exhaust valves are typically smaller in area than the inlet valves.
- Using larger bored, smoother, less contorted inlet manifold and exhaust manifolds. This helps maintain the velocity of gases. Similarly, the ports in the cylinder head 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 efficiency of the modifications. Manifolds with sharp turns force the air–fuel mix to separate at high velocities as fuel is more dense than air.
- 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 and high-flow, high-efficiency air filters. Muffler modifications will change the sound of the automobile's engine, usually making it louder; for some tuners this is in itself a desirable effect.
- Increasing the valve opening height (lift), by changing the profiles of the cams on the camshaft, or the lift (lever) ratio of the valve rockers (overhead valve, or OHV, engines), or cam followers (overhead cam, or OHC, engines).
- Optimizing 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. This can usually be achieved by fitting a differently profiled camshaft. See also valve timing, variable valve timing.
- Raising the compression ratio by reducing the size of the combustion chamber, 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 gaskets, or by using a milling machine or "shaving" the cylinder head. High compression ratios can cause engine knock unless high octane fuels are used.
- Forced Induction; adding a turbocharger or a supercharger. The air/fuel mix entering the cylinders is increased by compressing the air. Further gains may be realized by cooling compressed (and thus heated) intake air with an air-to-air or air-to-water intercooler.
- Using a fuel with higher energy content and by adding an oxidizer such as nitrous oxide.
- Reducing losses to friction by machining moving parts to lower tolerances than would be acceptable for production, or by replacing parts. A common example of this is, in overhead valve engines, replacing the production rocker arms with replacements incorporating roller bearings in the roller contacting the valve stem.
- Reducing the "rotating mass", which comprises the crankshaft, connecting rods, pistons, and flywheel. Doing so can improve throttle response due to lower rotational inertia, as well as reduce the automobile's overall weight. This may be achieved by using alloy parts instead of steel. However, a heavy crankshaft can void the need for a flywheel (which is common on V6 engines).
- Changing the tuning characteristics electronically, by changing the firmware of the EMS. This chip tuning often works because modern engines are designed to produce more power than required, which is then reduced by the EMS to make the engine operate smoothly over a wider RPM range, with low emissions. This is called de-tuning and produces long-lasting engines and the ability to increase power output later for facelift models. Recently emissions have played a large part in de-tuning, and engines will often be de-tuned to produce a particular carbon output for tax reasons.
- Lowering the underbonnet temperature, which has the effect of lowering the engine intake temperature, therefore increasing the power. This is often done by installing a type of thermal insulation (normally a heatshield, thermal barrier coating or other type of exhaust heat management) on or around the exhaust manifold. This ensures that more heat is diverted out and away from the underbonnet area.
- Changing the location of the air intake, moving it away from the exhaust and radiator systems to decrease intake temperatures. Additionally, the intake can be relocated to areas that have higher air pressure due to aerodyamic effects, resulting in effects similar to (though less than) forced induction.
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 smoothness at low engine speeds for improvements at high engine speeds.
Furthermore, tuners may also use analytical tools to help evaluate and predict the effect of modifications on the performance of the vehicle.
An overhauled engine is an engine which has been removed, disassembled (tear down), cleaned, inspected, repaired as necessary and tested using factory service manual approved procedures. The procedure generally involves honing, new piston rings, bearings, gaskets, oil seals. When done by a competent engine builder the engine will perform as new. The engine may be overhauled to 'new limits' or 'service limits', or a combination of the two using used parts, new original equipment manufacturer (OEM) parts, or new aftermarket parts. The engine's previous operating history is maintained and it is returned with zero hours since major overhaul.
Many times aftermarket part manufacturers are the OEM part suppliers to major engine manufacturers (e.g. Ishino manufactures both the OEM and the aftermarket cylinder head and valve cover gaskets for the Nissan VG30E. Often the Nissan logo is imprinted on the OEM part while the OEM suppliers brand will be imprinted on the same exact part when offered aftermarket.)
A top overhaul only covers the replacement of components inside the cylinder head without removing the engine from the vehicle, such as valve and rocker arm replacement. It may or may not include a valve job. A major overhaul however covers the whole engine assembly, which requires the engine to be removed from the vehicle and transferred to an engine stand. An overhauled engine refers to a major overhaul. By comparison, a major overhaul costs more than a top overhaul.
'New limits' are the factory service manual's approved fits and tolerances that a new engine is manufactured to. This may be accomplished by using standard or approved undersized and oversized tolerances. 'Service limits' are the factory service manual's allowable wear fits and tolerances that a new limit part may deteriorate to and still be a usable component. This may also be accomplished using standard and approved undersized and oversized tolerances.
A 'rebuilt engine' is an engine that has been overhauled using new and used parts to new limits by the manufacturer or an entity approved by the manufacturer. The engine's previous operating history is eradicated and it comes to you with zero hours total time in service, even though the engine may have had used components installed that have many hours of previous operating history. Production rebuilders or marketing material sometimes use the word 'remanufactured' to describe these engines.
Remanufacturing is a term to mean an engine put together to match factory specifications e.g. "as new". Although often a buyer may take this to mean all-new parts are used, this is not always the case. At the very least, the cylinder block will be used, as may most other parts. High-quality rebuilds will often include new pistons and line-boring of the crankshaft and camshaft bores.
Engine blueprinting means specifically to create an engine that reflects the design intent through the removal of manufacturing tolerances insofar as that is possible such that the engine literally reflects the actual blueprint created by the original engineers who designed the engine. While many people confuse blueprinting with high-performance engine building or tuning, these are in fact different goals.
Because few have the capability to actually blueprint, and because of the monetary incentive of claiming one has performed the work, many people have come to believe blueprinting only means that all the specifications are double-checked. Serious efforts at blueprinting result in better-than-factory tolerances, possibly with custom specifications appropriate for the application. Common goals include engine re-manufacturing to achieve the rated power for its manufacturer's design (because not all mass-production engines put out the rated power), and to rebuild the engine to make more power from a given design than otherwise intended (because custom engines can often be redesigned to different specifications). Blueprinted components allow for a more exact balancing of reciprocating parts and rotating assemblies so that less power is lost through excessive engine vibrations and other mechanical inefficiencies.
Ideally, blueprinting is performed on components removed from the production line before normal balancing and finishing. If finished components are blueprinted, there is the risk that the further removal of material will weaken the component. While it has nothing to do with blueprinting per-Se, lightening components is generally an advantage provided balance and adequate strength are both maintained, and more precise machining will in general strengthen a part by removing stress points, so in many cases performance tuners are able to work with finished components.
For example, an engine manufacturer may list a piston ring end-gap specification of 0.003 to 0.005 inches for general use in a consumer automobile application. For an endurance racing engine which runs hot, a "blueprinted" specification of 0.0045" to 0.0050" may be desired. For a drag-racing engine which runs only in short bursts, a tighter 0.0035 to 0.0040 inch tolerance may be used instead. Thus "blueprint" can mean tighter or looser clearances, depending on the goal.
The 'Igniscope' electronic ignition tester was produced by English Electric during the 1940s, originally as 'type UED' for military use during World War II. The post-war version, 'type ZWA' electronic ignition tester, was advertised as "the first of its kind, employing an entirely new technique".
The Igniscope used a cathode ray tube, giving an entirely visual method of diagnosis. It was invented by D. Napier & Son Ltd., a subsidiary of English Electric, and British Patents 495478, 495547 and 563502 applied. The Igniscope was capable of diagnosing latent and actual faults in both coil and magneto ignition systems, including poor battery supply bonding, points and condenser problems, distributor failure and spark plug gap. One feature was a "loading" control which made latent faults more visible.
- MR, MR. "Engine Overhaul Terminology and Standards". Mattituck Services, Inc. Retrieved 20 August 2011.
- Instruction manuals published by The English Electric Company Ltd., Industrial Electronics Department, Stafford.
- Advertising brochure, page 2
- Edit by Mr. J. B. Roberts, May 1948, to note on page 7 of the brochure for the Model ZWA
- Early military and later commercial instruction manuals
- Manual for the "Igniscope" UED tester, Appendix 1
See also 
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