Clear all

Compression differences JL vs. JK

Member Admin
Joined: 2 years ago
Posts: 31
Topic starter  
In 2016, FCA updated the 3.6 Pentastar motor for the first time since 2009 to improve power and fuel efficiency across the Chrysler, Jeep and Dodge lineup. This resulted in a 6% gain in fuel economy and a 14.9% gain in torque below 3000 RPM. Enhancements such as two-step variable valve lift (VVL), cooled exhaust-gas recirculation (EGR) and innovative weight-reduction strategies boost the engine’s efficiency and performance, all while preserving its smoothness.
The additional torque on the is due in part to the numerous internal changes that better harness the combustion event. The most notable being the engine’s compression ratio jumps to 11.3:1 from 10.2:1, compared to the previous gen 1 version. Notice in the picture above how the pistons are nearly identical, but the left piston lacks the additional material found on the top side of the right piston. That little bit of extra material is all that is needed for a compression increase. 
But how? How does an increased compression ratio translate into more power? Efficiency. Efficiency is defined as the ratio of fuel input to the power output. Since all internal combustion engines work by extracting energy derived from an expanding heated medium in a confined space, it follows that the largest difference between the volume of the unexpanded cold charge and the fully expanded hot charge needs to be as large as possible. Increasing the compression ratio is a way to maximize this. 
The balance of boost versus compression ratio have been a tuner’s challenge for years. Today, the average high-performance street or strip turbocharged four-cylinder race engine typically has  a compression ratio of 9.5:1, with some even running compression ratios as high as 11.5:1 or more on alcohol or E85. Years ago this sort of combination was not possible, typically the higher the boost pressure, the lower the compression ratio of the engine had to be. Modern technology allows our generation to experience the best of both worlds, high boost pressures with high compression ratios.