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Petrol Engine Tuning – Basics

The goal for the majority of car modifiers is to make your car accelerate harder and travel faster. This is relatively easily achievable by spending money and changing some components in the engine. There is a big temptation to just open up a catalogue and order everything but realistically, this does not always work, there are some modifications that just do not perform well together.

For example, if you have a gas flowed cylinder head and add a high lift camshaft, if the two parts are not matched correctly, you could end up with a truck load of horsepower but a torque band that is narrow and unusable. If the parts are matched correctly, you can end up with more power than the sum of the individual parts. Say a high lift camshaft gives you 10 BHP on its own and a gas flowed cylinder head gives you 10 BHP, using the correct two parts together can give you an extra 30 BHP rather than 20.

The first thing you need to understand is how a 4 stroke petrol engine works. The piston moves up and down twice during each cycle creating the 4 strokes, Suck (down), Squeeze (up), Bang (down), Blow (up). A mix of fuel and air is drawn into the engine (suck), the fuel/air mix is compressed (squeeze), the mix is ignited by the spark plugs (bang), and finally the exhaust gas is emptied from the cylinder (blow).

The second thing you need to understand is what you’re trying to achieve which is get as much of the correct mix of fuel and air into the engine as quickly as possible (more fuel & air = bigger bang = more power), compress it as much as possible and when finished, get it out of the engine as quickly as possible.

Let’s look at some modifications you can try to achieve this:

Breathing modifications

These are modifications that help your engine breathe easier, i.e. air intake and exhaust. Your car comes from the manufacturer designed to pass emission and noise regulations which can strangle your engine. One of the first modifications most people do is to ditch the factory intake and exhaust and fit uprated items. Performance/sports air filter, induction kit with nice smooth pipes and less joins to help the air flow better, performance exhaust manifold, complete removal of catalytic convertor or fitting of a less restrictive item, larger bore exhaust pipe with free flow silencers.

Internal modifications

Items such as high compression pistons (do exactly what the name suggests), forged conrods (bigger bang means bent/broken conrods which can burst through the block and ruin your engine). However, when using a big turbo/supercharger, low compression pistons are used to prevent the effects of turbo lag and to reduce the chances of ignition problems. There are also other ways to increase the compression ratio such as decking the block and skimming the cylinder head. More compression can lead to pre-detonation of the air/fuel mix (pinking) which can damage the surface of the pistons and cylinder. This can be countered by using fuel with a higher octane rating.

Cylinder head

There are many modifications you can make to the cylinder head to increase power. The internal ports within the cylinder head can be polished and channeled to allow gases to flow more freely in and out of the cylinder. Larger valves can be used, again letting gases flow faster in and out of the cylinder. The angle of the valve and seat can be changed, again having the same effect as above. The camshaft can be changed for one which opens the valves to a different height and for a different length of time.

Engine rebore

As the Americans say, there’s no replacement for displacement. An engine rebore can give you a few extra hundred CC which effectively means you have a bigger engine in a block the same size as the original engine. This will however require you to use other parts such as larger diameter pistons and rings. Not all engines are suitable for reboring, such as aluminium ones with cylinder liners but most cast iron blocks are fair game. Another way of increasing your engines CC is to use a stroker kit which consists of special crankshaft, conrods and pistons.

Crankshaft grinding/polishing

Grinding a polishing a crankshaft has the effect of making it lighter which makes it easier to turn, resulting in a freer revving engine. It is also very important to make sure that it correctly balanced which will enable you to run a higher maximum rev limit without the engine shaking itself apart.

Forced induction

Forcing more air into the cylinder by using a turbo or supercharger means you can add more fuel, giving you that bigger bang you’re looking for. Both turbos and superchargers have their own advantages and disadvantages. Turbos produce more power than superchargers but only produce it when the engine is at speed, thus creating a noticeable surge of power when the engine produces enough exhaust gas to activate the turbo. This time delay is known as lag. Whereas a turbo only produces power some of the time because it is driven by exhaust gases, a supercharger produces power all the time because it is driven by a belt and pulley system. It is very easy to change the power output of the supercharger simply by changing the pulley.


Because you have more air going into the cylinder, you need more fuel. This is easily achievable on older cars by simply fitting a different carburetor. It’s still pretty simple on more modern cars by adding a larger capacity fuel pump, adjustable fuel pressure regulator and larger injectors. However, this is also the tricky part as you need to make sure the air/fuel ratio is perfectly balanced. If you run too lean, i.e. too much air, not enough fuel, the engine will run too hot and you can melt pistons and spark plugs. If you run too rich, i.e. too much fuel, not enough air, you will end up with black smoke from your exhaust, carbon build up on your spark plugs, carbon on the valve seats which mean they don’t close correctly.

ECU tuning

There are different ways to modify your cars ECU which can enable you to adjust things such as air/fuel ratio, spark timing, boost, and rev limits. Some ECUs and software will even allow you to add features such as launch control and run different engine maps for different situations such as one for everyday driving and one for racing. On most cars it’s relatively simple, just connect to car to a laptop with the correct software and off you go. On other cars the ECU is locked by the manufacturer. This can be overcome by using a piggyback ECU which sends false signals to the cars own ECU to allow control of the above functions. In some high performance applications where money is no object, it is possible to even throw the cars original ECU in the bin and use a special dedicated sports unit.


**DISCLAIMER** I accept no responsibility for any harm or damage caused by anyone modifying their car. This is just a simple guide on the basics of tuning. For more information on a particular topic, please ask on the forum or read a book. Always consult a qualified mechanic and read the workshop manual for your car before carrying out any work.

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