An under square engine design features a piston stroke that is longer than the cylinder bore diameter. This geometry increases the crank throw, which generates more leverage and low-end torque. It limits the engine's maximum RPM due to high piston speeds, making it ideal for heavy vehicles and trucks that need pulling power rather than high-revving horsepower.
We went with an under square setup for the stroker motor so it can actually tow the trailer up a hill.
Also heard as
- long-stroke
Do not mix it up with
- Over square: Under square means a longer stroke than bore; over square means a larger bore than stroke.
Related terms
Over square
An engine design geometry where the cylinder bore diameter is larger than the piston stroke length.
Torque converter
A fluid coupling that transfers rotating power from the engine to an automatic transmission.
Destroke
Destroking an engine means reducing the piston stroke length to decrease displacement and alter the power band.
Overbore
To machine an engine cylinder to a larger diameter than its factory specification.
Compression ratio
The mathematical ratio of cylinder volume at the bottom of the piston stroke compared to the top.