Low-Tension Piston Rings Provide Serious Benefits For Automakers

Almost every new car since the early 2000s has been using low-tension piston rings. Automakers have their fair share of reasons for doing so, including reduced frictional losses, improved efficiency, and better emissions. While stats can vary between engines, a decent chunk of their total output is lost as internal friction. Roughly 20% of that output loss is just from the piston rings moving up and down, with the oil ring being the biggest contributor. All told, you can reasonably expect ring friction to rob at least 15% of total brake horsepower.

Further confirming the power loss argument comes from a test done by Hot Rod. When using thinner low-tension piston rings, the engine in that test (a small-block Chevy) saw its overall output jump by 6.8 horsepower and 3.8 pound-feet of torque. Perhaps that's why many manufacturers moved to low-tension ring packs, with Chevrolet in particular using much thinner rings on its LS and LT series. But it's not just high-performance Chevy V8s; YouTube channel driving 4 answers compares piston ring sizes of various run-of-the-mill Toyota engines from the past and present. Unsurprisingly, the piston rings have witnessed a noticeable decrease in size over time.

Although the many pros — including better fuel economy and emissions — are certainly welcome, there are practical disadvantages to using low-tension piston rings, with the most notable downside being oil consumption. As much as we love horsepower, reliability and longevity are just as important, if not more. And thinner piston rings have been around long enough to see themselves become the villain.

Because most modern-day engines are capable of running high compression ratios (and cylinder pressures), a lot of force is applied to the piston rings. Even though they're typically stronger (material-wise) than they were in the past, thinner rings don't necessarily equate to better sealing, especially as time goes on. This is primarily due to the tension itself being relatively low (from its low spring force), as demonstrated in driving 4 answer's video. Over time, this can result in blow-by and excessive oil consumption, which reduces engine efficiency and may affect emissions. An average consumption of 1.1 quarts of oil every 600 miles (or one liter per 1,000 kilometers) is considered acceptable today, but this would've been a huge red flag not too long ago.

As explained in the video, a Toyota 2ZR-FXE engine, seen across many hybrid vehicles from the brand, has a theoretical potential to consume all of its oil (at a four-liter capacity) in about 5,000 kilometers or 3,100 miles. The example scenario involves the engine consuming oil at a rate of 0.8 liters per 1,000 kilometers, which the industry considers acceptable. Although Toyota does specify in the owner's manual that some oil consumption is expected, the rate has been left out — almost strategically so. And it's worth pointing out that this is not exclusively a Toyota thing. According to Consumer Reports, cars from Honda, GM, Subaru, and other major brands have all experienced excessive oil consumption. If two-stroke engines are so environmentally unfriendly for consuming oil, how is this fine?