GM’s variable compression engine isn’t quite magic, but it’s still an impressive trick

Increasing an engine’s compression ratio to extract more performance is nothing new to hot rodders. High-comp heads were some of the first hop-up parts available for Ford’s Model A four-banger and iconic flathead V-8 engines, but what if you could increase the piston’s squish factor without swapping hardware? In the latest episode of Engineering Explained, Jason Fenske explores how new tech from the OEMs is making this possibility a reality.

The concept of a variable compression ratio is that automakers are looking to optimize both power and efficiency by changing how much fuel and air mixture is squeezed in the engine’s power stroke. Higher compression typically yields greater fuel efficiency and power, but in modern turbocharged engines, there is a limit to how much squish can happen before knock occurs. The ability to control the compression ratio allows for more squeeze at lower engine speeds (when better mpg is needed) and reduces that pressure at higher rpm, allowing the engine to produce horsepower safely.

While the concept is certainly cool, it’s proven difficult to master. Currently, Nissan/Infiniti is the only manufacturer to offer variable compression in a product (the QX50 and the Altima), but GM is looking to enter the fray with a design of its own. Fenske does an excellent job breaking down how the new system works, highlighting the changes to the crankshaft and connecting rod design to accommodate these on-the-fly adjustments.

EV technologies may be accelerating at a blistering pace, but breakthroughs like variable compression ratios, Koenigsegg’s camshaft-less Freevalve wizardry, and Mazda’s Skyactiv-X compression ignition gasoline engines prove that there’s still plenty of life left in the future of internal combustion.

What do you think about variable compression engines? Is the added complexity worth the power and efficiency gains? Let us know your thoughts in the Hagerty Forum below.

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