Undersquare
From Wikipedia, the free encyclopedia
A piston engine is undersquare or longstroke if its cylinders have a smaller bore (width, diameter) than stroke (length of piston travel). This can be a negative trait, since a longer stroke usually means greater friction, a weaker crankshaft, and a smaller bore means smaller valves which restricts gaseous exchange; however, with the aid of modern technology, these are not the large problems that they used to be. An undersquare engine usually has a lower redline than an oversquare one, but it may generate more low-end torque. In addition, a longstroke or undersquare engine can have a higher compression ratio with the same octane fuel compared to a similar displacement engine with a much shorter stroke ratio. This also equals better fuel economy and somewhat better emissions. An undersquare engine does not overheat as easily as similar oversquare engine. Engines can be modified with a "stroker" crankshaft, which increases an engines stroke from stock, increasing torque.
Undersquare engines are typically shorter in length, but heavier and taller than equivalent oversquare ones, which is one of the reasons why this type of engine is not generally used.
Many British automobile companies used undersquare designs through the 1950s, largely because of a motor tax system that taxed cars by their cylinder bore. Therefore, many of the most famous cars of that era use this design. This includes the Austin A-Series engine and many Nissan derivatives.
The Chrysler Slant-6, in the most common 225 cubic inch (3.7 L) version, is a massively undersquare engine with a 3.40" (86 mm) bore and a 4.125" (105 mm) stroke, producing most of its power right on the peak of its torque curve. The achilles heel of this engine, otherwise known for its exceptional durability, is being over-revved by inexperienced drivers. Red line for a factory engine is under 4,500RPM; red line with aftermarket connecting rods is about 5,500RPM. On the other hand, a well-maintained Slant-6 can be made to idle as low as 75RPM (though this is *not* a recommended speed, neither the alternator nor the oil pump will function adequately). In some circles, the Slant-6 is nicknamed "The Stump-Puller" for its diesel-like low-speed torque. Appropriate gearing and driving skill is required for performance use.
GM's popular 2.5L I4 "Iron Duke" motor, used in cars ranging from the Pontiac Fiero to the Oldsmobile Cutlass and Chevrolet S-10, is also undersquare. In the Fiero, it was a particular issue, since the car's styling tended to attract drivers who would over-rev the motor at stoplight confrontations - a skilled driver would shift around 3,700RPM, just after the engine's peak torque, long before the mellow power gives way to the whine of a high-revving motor. Compounding this, many Iron Dukes had defective connecting rods, leading to several recalls of the Fiero and a few other cars.
Willys also used mostly undersquare engines, in fact the L- and F-134 engines, with their fairly small 3.125 inch (79.4 mm) bore and outrageous 4.375 inch (111.1 mm) stroke , are probably the most undersquare engines ever built.
Nowadays, undersquare engines tend to be quite rare, but this form of engine is still used in some applications. For example, a modern 8.4 liter Valmet 645 inline-6 tractor diesel engine is a longstroke/undersquare engine, but has an output of over 300 horsepower (224 kW) with turbocharger and intercooler.
