NATURALLY ASPIRATED PETROL ENGINES

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1 ADDING CAPACITY

(power gain proportional to capacity gain)

The old saying of “there is no replacement for displacement” is true for naturally aspirated engines. If the capacity of an engine is increased by 10%, tuners are generally able to unleash 10% more power when the fuelling is increased for the added volume of airflow at the ideal 14,7:1 air-fuel ratio. A popular method is to enlarge the cylinder bores and fit oversized pistons to increase capacity.

 

2 PERFORMANCE AIR FILTER

(typical power gain of 2%)

As mentioned, the airflow determines the amount of fuel that can be added. Any restrictions to the air path through an engine limit the maximum possible airflow. An air filter catches particles that can damage the engine, but that also hampers airflow. A performance air filter offers less resistance to airflow and allows more air into the engine. However, this filter removes fewer particles than an OEM-spec unit. Adding fuel to the extra airflow increases the power of the engine.

 

3 FREE-FLOW EXHAUST

(power is typically increased by around 8%)

The same argument of restriction to airflow applies to the exhaust system. Silencers and catalysts are restrictors of airflow. Removing these items and fitting straight-through pipes improves the exhaust flow and in turn ups the rate of fresh air entering the intake. Then you add more fuel to get more power. (Take note, however, that noise and emissions pollution increase.)

 

4 PERFORMANCE CAMSHAFTS

(5% more power)

Valves are a big restriction of airflow to the combustion chamber. Fitting higher-lift camshafts with longer duration lowers the resistance and increases the airflow. Another technique is called valve overlap, where the exhaust valve is kept open during the first part of the intake stroke, and utilises some of the exhaust energy to suck more fresh air into the chamber. More fuel is then added. This method puts more strain on the valvetrain and negatively impacts part-load driveability and idle stability.

 

Hardware changes to withstand the forces related to the higher piston speeds are required

5 INCREASE MAXIMUM ENGINE SPEED

(gain proportional to engine speed increase)

View the engine as an air pump and it makes sense to turn the powertrain faster in order to pump more air. But air has mass, which leads to inertia and, at higher engine speeds, the cylinders aren’t able to fill to their capacity. Close to the redline, it is quite common on standard engines for the volumetric efficiency to drop to below 75%, so any extra air owing to a maximum engine speed increase is offset by the loss of volumetric efficiency. Hardware changes, such as special con-rods and pistons to withstand the forces related to the higher piston speeds, are required.

 

6 FUEL-OCTANE BOOSTER

(typically, a 5% gain)

The octane number of petrol is the resistance to auto ignition (or knock) that can destroy an engine. If a higher-octane fuel is used, the ignition timing can be advanced (and the compression ratio raised), which leads to an increase in power without knock occurring. If the ignition timing is kept standard (fixed compression ratio), there is no increase in power.

 

7 ECU AND CHIP TUNING

(5% increase)

Manufacturers develop a complete calibration of their engines that takes care of fuelling and ignition timing across the entire engine-speed and load range. The calibration takes into account various inputs, such as atmospheric pressure and engine temperature, and the end result is optimum performance, fuel economy, emissions and reliability. If a tuner wants to alter the values (such as increasing fuelling and advancing ignition timing), they can either tap into the existing engine-control unit (ECU) and change critical maps, add a “chip” which is essentially a multiplier on the fuelling and timing signals, or fit an aftermarket ECU and calibrate the maps from scratch. Gains are possible, but compromised reliability becomes a concern because the engine is run outside its original performance envelope. Tuners tend to run an engine “rich” at high loads to lower exhaust-valve temperatures (to protect the engine) and achieve more power thanks to charge cooling. Fuel consumption generally suffers.

 

NOS ANYONE

That red button on the steering wheels of modified performance cars in movies is linked to a gas called nitrous oxide (NOS) that is sprayed into the intake manifold. It’s not a fuel, but a compact form of oxygen. In the combustion chamber, oxygen molecules are released and are available to sustain combustion of extra fuel – the source of the extra power. Depending on the amount of N2 O that is used, fuelling can be dramatically increased and so too the power output at that instant. It sounds ideal, but it’s of cardinal importance to remember added stresses on the engine will spell disaster for standard internal components … and the fun lasts only until the NOS bottle runs dry…

BUT KEEP THIS IN MIND

Professional tuning is a (costly) skill that’s the result of in-depth knowledge and experience, and it’s important to remember that an engine functions as part of a larger system and to modify one component may not necessarily achieve the expected power gain. For example, the effects of a performance air filter alone may hardly be noticed but, in combination with a performance camshaft, free-flow exhaust and the correct fuelling, healthy power gains can be achieved (or even exceeded) and smiles broadened.