This has a knock-on effect, with the biggest impact on low-speed driveability. The only way to get more air into the engine is fitting a larger-capacity turbo (bigger pump). The standard turbo can supply just a limited amount of extra air before it reaches the end of its operating spectrum (think of a small pump). Good examples are the 1980s 1,5-litre turbopetrol Formula One engines that produced more than 1 000 hp (746 kW) in qualifying trim. The capacity of a turbo engine is less relevant than that of an NA one because, by upping the boost, you can achieve similar airflow (and accompanying power output with the correct fuelling) as that of a much larger NA engine. Even two-valve-per-cylinder engines can produce prodigious power outputs when turbocharged. – are still relevant, but have only a minor influence when compared to what can be accomplished when upping the boost. The methods of increasing airflow in naturally aspirated (NA) engines – performance air-filters, high-lift and duration camshafts, free-flow exhausts, etc. Therefore, the only time air, at higher-than-atmospheric pressure, can be effectively utilised is at full-throttle conditions. Turbo-petrol Turbopetrol engines still have to run close to the ideal 14,7:1 air-fuel ratio (stoichiometric ratio), so the incoming air needs to be throttled (using a throttle/butterfly valve) during part-load conditions. Therefore, boost pressure is the pressure over and above atmospheric pressure. If the intake air pressure of an engine is increased to 200 kPa (absolute pressure), it is referred to as to one bar of boost. What is boost? The standard atmospheric pressure at sea level is 101,325 kPa, which is close to one bar pressure (100 kPa). By raising the intake pressure of air above atmospheric conditions, it is easy to force more air into the combustion chambers (volume above the piston in the cylinder assembly), which allows more fuel to burn. This is limited by the available air (oxygen) to sustain combustion. The more fuel that can be burned efficiently, the more power an engine will produce. The theory behind power production The general theory (as well as CAR’s stance on tuning) was covered in an earlier article, but to recap: an internal-combustion engine converts the chemical potential energy of fuel into motive power. A fitting anecdote that perfectly illustrates this feature … we investigate increasing the performance of a forced-induction engine. With a loud turbo whoosh, the M3 disappeared as if it had selected reverse gear. It was an aftermarket boost controller that upped the boost to 1,5 bar for “emergency situations”. With the words “we will need this”, he pressed the red button next to the gearlever.
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