With stringent emission regulations, many subsystems that abate engine tailpipe-out emissions become a necessary part for engines. The increased level of complexity poses technical challenges for the quality and reliability for modern engines. Among the spectrum of quality control methodologies, one conventional methodology focuses on every component's quality to ensure that the accumulative deviation is within predetermined limits. This conventional methodology tightens the component tolerance during the manufacturing process and typically results in increased cost. Another conventional methodology that is on the other side of the spectrum focuses on tailoring an engine calibration solution to offset the manufacturing differences. Although the tailored engine calibration solution reduces manufacturing cost for components, it increases the development and validation cost for engines.Given the cost and time constraints, system integration plays an important role in engine development. This paper first reviews the traditional quality control methodologies such as Advanced Product Quality Planning (APQP) and Design for X, and then further explores Design for 6 Sigma (DFSS) applications in system integration. A case study is focused on controlling air fuel ratio that affects both engine performance and emissions. Both component level quality and system level quality are discussed in this case to demonstrate the benefits from Design for 6 Sigma applications in system integration.