The basic engine has been modified through the yearsto increase its overall efficiency. Many of the changeshave occurred inside the engine. Others involve thefuel and ignition systems. The result of these changesis improved performance and driveability and adecrease in exhaust emissions. Following is a summary of some of those changes.■ Better sealing pistons. Blowby gases are reducedthrough the use of better sealing piston rings andimproved cylinder wall surfaces. Many enginesare also fitted with low-friction piston rings. Thisincreases fuel economy and engine power.■ Combustion chamber designs. The primary goal indesigning combustion chambers is the reductionor elimination of the quench area. Another trendin combustion chamber design is locating thespark plug closer to the center of the chamber.Manufacturers have also worked with designs thatcause controlled turbulence in the chamber. Thisturbulence improves the mixing of the fuel withthe air, which improves combustion.■ Lower compression. By keeping the compressionratio low, combustion temperatures can be keptbelow the point where NOx is formed. However, newdevelopments have allowed the use of higher compression ratios on some high-performance engines.■ Decreased friction. Overcoming the friction of theengine’s moving parts reduces the power andenergy lost during engine operation. Improvedengine oils, new component materials, and weightreductions have had the biggest impact on reducing friction.■ Intake manifold designs. Thanks to the wide andsuccessful use of port fuel injection, intake manifold are designed to distribute equal amounts ofair to each cylinder. The use of plastic intake manifolds has allowed for smoother runners and better heat control of the air.■ Improved cooling systems. High engine temperaturesreduce HC and CO emissions. However, they alsomake the formation of NOx harder to control. Enginecooling systems are designed to run at high