Let’s take the Elantra’s NA 2-litre engine that produces 138HP, as an example. If this engine is to make 274HP by turbocharging, the boot should be more than 14psi, since engine output varies in a quasi-direct relationship to the amount of fuel burnt in a given time. In other words, you have to make the engine in question burn 100% more gas, which is only possible by feeding 100% more air (oxygen) by force. Fine, the turbocharger can do it. However, problem is that by feeding 100% more air means the effective compression ratio becomes about 20 to 1, which is well into the diesel territory. Higher compression ratio means higher temperature in the combustion chamber, which means the air/fuel mixture may ignite spontaneously. If the flame front (front of the combusting pressure wave) collides against the flame front from the sparkplug ignition that’s called knocking. Take a look at the following video:
http://www.geocities.jp/bequemereise/knocking.htmlTo reduce the chance of knocking you can do several things:
1) Use the gas less prone to spontaneous ignition (premium gas).
2) Lower temperature inside the combustion chamber by squirting (injecting) gas directly against the cylinder wall (still hot from previous combustion) so that evaporating gas cools the combustion chamber.
3) Time the fuel injection so that the sparkplug-ignited flame front reaches the cylinder wall before unburnt fuel/air mixture ignites spontaneously. This may require two- or three-staged injections in a combustion cycle.
4) Place the sparkplug in the top centre of the combustion chamber so that the sparkplug-ignited flame front reaches at the cylinder wall in the shortest time possible evenly. (Flame front travel speed is same for regular or premium gas.)
There may be other tricks to reduce the chance of knocking, but at any rate Hyundai should be commended for squeezing out 274HP from a 2-litre engine. BTW, fuel economy of turbocharged engine should be looked at with the understanding that turbocharger (as well as supercharger) is a device to burn MORE fuel. The published fuel economy is measured with rather easy gas pedal modulation. IF you step on it to the metal when starting and passing, certainly gas mileage goes down quite remarkably. Hyundai must be using all the known (and some unknown, maybe) tricks to reduce fuel consumption such as, when load is low, delaying intake valve closure and mixing oxygen-depleted exhaust gas in the intake air.