Oversquare Engine Design

[slewinson]

The actual article is about operating an oversquare engine "oversquare" in aviation parlance. To the non flyer, the term oversquare is foreign when referring to how an engine is operated.

Simon

[jenko]

Over square design is about the stroke Vs bore ratio, in particular an over square engine means the bore is greater than the stroke as per the example slewinson posted above.

Interesting about the plane engine ,
but getting into car engines an oversquare engine particually 3'' stroke v 4'' bore will give you the best torque curve for the fuel you can buy at the pump ( I mean best torque curve in rising fastest and staying flat for longest) given that your valve sizes are also in the 3/4 range ending with best cylinder fill
Now i am sure this shall be debated

[immortality]

Yeah, it becomes debatable which design is better over-square or under-square.

Over square is supposed to rev better, bigger bore means bigger valves, short stroke so less wear, less piston speed etc.

A lot of engines are square though, like our favourite 3800 V6 having a 3.8" bore and 3.8" stroke.

[delcowizzid]

v6 is 3.4 stroke 3.8 bore .in all reality you wont feel the difference from under over or square when driving

[antus]

http://en.wikipedia.org/wiki/Stroke_ratio

Oversquare engine examples

Oversquare engines (a.k.a. "short stroke engines") are very common, as they allow higher rpm (and thus more power), without excessive piston speed.

Examples include both Chevrolet and Ford small-block V8s. The BMW N43 gasoline engine has a bore/stroke ratio of 1.083.

Boxer engines (such as those built by Volkswagen, Porsche, and Subaru) typically feature oversquare designs since any increase in stroke length would result in twice the increase in overall engine width. This is particularly so in Subaru's front-engine layout, where the steering angle of the front wheels is constrained by the width of the engine. Although oversquare engines have a reputation for being high-strung,[citation needed] low-torque machines, the Subaru EJ engine EJ181 develops peak torque at speeds as low as 3200 rpm.

Nissan's SR16VE engine found in Nissan Pulsar VZ-R and VZ-R N1 is an oversquare engine with 86 mm bore and 68.7 mm stroke, giving it an impressive 175-200 hp but relatively small torque of 119 lb.ft-134 lb.ft

Extreme oversquare engines are found in Formula One racing cars, where strict rules limit displacement, thereby necessitating that power be achieved through high engine speeds. Stroke ratios approaching 2.5:1 are allowed, enabling engine speeds of 18,000 RPM while remaining reliable for multiple races.[4]

[jenko]

found an interesting bore stroke ratio table
http://users.erols.com/srweiss/tablersn.htm

[festy]

The BMW N43 gasoline engine has a bore/stroke ratio of 1.083.

Pfft, may as well be square!
My bike has a 1.63:1 bore/stroke ratio - not much torque until about 7k rpm, but it really comes alive from around 9k on

[jenko]

A bit dissapointing that the wankell engine wasn't listed

[vlad01]

A bit dissapointing that the wankell engine wasn't listed

are they even an engine? they seem to be as reliable as the $89 big W china made line trimmer motor lol.

Everyone I knew of back in the day when they were popular were forever rebuilding them every other weekend. Now you never hear anyone use them anymore. Maybe they ran out of 2nd hand engine to rebuild?

[festy]

Considering their appetite for seals, this seams like asking for trouble

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