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Cars powered by 2.5-liter engines, like the Subaru Outback or Nissan Sentra, tend to be zippy and responsive, even though these two don't belong to the same class. The Subaru Outback engine has quite a history and evolution . Formally introduced at the 1994 New York Auto Show, the car was launched initially with the DOHC 2.2-liter flat-4 engine, and was promptly given a 10-hp improvement in 1998. A flat-4 design, also called a horizontally-opposed-4, has two cylinders mounted on each side of the crankcase. This is the same design used for the engine of the classic Volkswagen Beetle. The pistons move in opposite directions, analogous to a boxer slamming his gloves together, thus earning the nickname boxer engine. The counteracting motion contributes to the substantial canceling out of vibration. This is how the Subaru Outback engine evolved: * First generation - 2.2L SOHC 135 hp (101 kW) H4 * Second generation - 2.5L SOHC 165 hp (123 kW) H4 * Third generation - 2.5L SOHC 175 hp (130 kW) H4 or 2.5L DOHC 250 hp (186 kW) H4 turbo * Fourth generation - 2.5i SOHC i-AVLS 170 hp (130 kW) H4 The 2.5i models produce 170 hp and 170 lb-ft of torque, coupled to a six-speed manual transmission that enhances superb performance features. Some owners of the 2.5-liter Outback engine have encountered head gasket problems or failures when these overheat too often, resulting in thermally warped heads, but Subaru has already redesigned the head gaskets with good success. The engine uses a timing belt that may have to be replaced every 100,000 miles (160,000 km) or so. The Nissan Sentra engine, on the other hand, was introduced during 1982 with the Nissan Sunny, and has gone through a series of incremental improvements. By 2002, a new 165-horsepower SE-R was in circulation, coupled with a 5-speed manual transmission or optional 4-speed automatic. The QR line of inline- or straight-4 piston engines from Nissan come in displacements of 2.0 liters to 2.5 liters. They share aluminum, dual overhead camshaft (DOHC) pedigrees with optional direct injection in four-valve configuration and variable valve timing. The Nissan Sentra engine for the SE-R Spec V sport compact variant is the 2.5-liter QR25DE, which was originally mated to the Nissan Altima. It could put out 175 hp at 6000 rpm and 180 lb•ft (244 N•m) of torque at 4000 rpm, sufficient to accelerate the car to 60 mph (about 100 kph) in less than seven seconds. The QR25DE comes out of the plant in Decherd, TN with aluminum intake manifold, connecting rods made of cast steel and vibration-reducing balance shafts. This 5th generations Nissan Sentra engine started production in 2000, and has served its role quite well through the years, despite its share of mechanical glitches that have led to service bulletins and recall episodes. Some owners have observed comparatively higher oil consumption as well as valve screws in the intake manifold becoming loose and leading to fluctuating idling or loss of power. About: Even as Nissan Sentra engine looks into the problems, owners of affected vehicles are being advised to check their oil as frequently as every 700 mi or 1,100 kilometers.
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