Page organisation in this revision
- Page type
- Technical article
- Subject
- Performance
- Parent
- Section level
- Applies to
- No ECU-specific applicability
- Tool groups
- None
- Manufacturers
- None
Changes from revision 1 to revision 1
Removed text is struck through; added text is highlighted.
More volume per stroke
Displacement is the volume the pistons sweep. For a given cylinder it is the area of the bore times the stroke, and the engine displacement is that volume times the number of cylinders. At the same volumetric efficiency and the same engine speed, a larger displacement takes in more air mass per minute. That is the performance reason to change it. The extra air still has to be fuelled, and the extra torque still has to be carried by the pistons, rods, bearings and transmission.
Volumetric efficiency is not automatically the same after the change. A larger bore needs a port and valve that can fill it in the time available. A longer stroke at the same engine speed raises the mean piston speed, so the same port has less time and a faster piston to keep up with. The runner and valve timing that suited the original speed range can be in the wrong place afterwards.
Bore and stroke are different changes
Increasing the bore increases the flame path across the chamber and the area of the piston the cylinder pressure acts on. It requires a block that has the wall thickness, and a head gasket and piston that match the new diameter. Increasing the stroke changes the crankshaft, usually the rods, and the piston speed. It also changes the compression ratio unless the combustion-chamber volume is altered to suit, because the swept volume has changed while the clearance volume may not have.
Either change can make the piston meet the head or a valve. That is a geometry check, with the gasket thickness, deck height and camshaft timing that will actually be used, including any variable-timing travel. A stroker kit's catalogue compression ratio is only true for the chamber, gasket and deck it was calculated with.
The calibration and the limits
An ECU that estimates load from the original displacement, or a rev limit chosen for the original piston speed, does not update itself because the crankshaft changed. Remapping after a displacement change is about the new air mass, the new compression and the new safe engine speed, not about adding a percentage to every table.
The bottom end sets the ceiling. A longer stroke can increase the load on the rod and the side load on the piston. A larger bore increases the force on the rod for the same cylinder pressure. Bearings, the oil supply and the head fasteners are part of whether the extra air can be used continuously. So is the cooling system, because more fuel burned per minute is more heat. None of those parts increase displacement. They decide how much of it is usable.
Sources
Internal combustion engines, lecture 8: intake and exhaust processes. MIT OpenCourseWare, course 2.61, spring 2017. Volumetric efficiency as the fraction of the swept volume that is actually trapped. Accessed 2026-11-11.Isentropic compression or expansion. NASA Glenn Research Center. Clearance volume and swept volume as the compression ratio. Accessed 2026-11-11.
Rendered revisions
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Revision 1: Engine displacement
More volume per stroke
Displacement is the volume the pistons sweep. For a given cylinder it is the area of the bore times the stroke, and the engine displacement is that volume times the number of cylinders. At the same volumetric efficiency and the same engine speed, a larger displacement takes in more air mass per minute. That is the performance reason to change it. The extra air still has to be fuelled, and the extra torque still has to be carried by the pistons, rods, bearings and transmission.
Volumetric efficiency is not automatically the same after the change. A larger bore needs a port and valve that can fill it in the time available. A longer stroke at the same engine speed raises the mean piston speed, so the same port has less time and a faster piston to keep up with. The runner and valve timing that suited the original speed range can be in the wrong place afterwards.
Bore and stroke are different changes
Increasing the bore increases the flame path across the chamber and the area of the piston the cylinder pressure acts on. It requires a block that has the wall thickness, and a head gasket and piston that match the new diameter. Increasing the stroke changes the crankshaft, usually the rods, and the piston speed. It also changes the compression ratio unless the combustion-chamber volume is altered to suit, because the swept volume has changed while the clearance volume may not have.
Either change can make the piston meet the head or a valve. That is a geometry check, with the gasket thickness, deck height and camshaft timing that will actually be used, including any variable-timing travel. A stroker kit's catalogue compression ratio is only true for the chamber, gasket and deck it was calculated with.
The calibration and the limits
An ECU that estimates load from the original displacement, or a rev limit chosen for the original piston speed, does not update itself because the crankshaft changed. Remapping after a displacement change is about the new air mass, the new compression and the new safe engine speed, not about adding a percentage to every table.
The bottom end sets the ceiling. A longer stroke can increase the load on the rod and the side load on the piston. A larger bore increases the force on the rod for the same cylinder pressure. Bearings, the oil supply and the head fasteners are part of whether the extra air can be used continuously. So is the cooling system, because more fuel burned per minute is more heat. None of those parts increase displacement. They decide how much of it is usable.
Sources
Revision 1: Engine displacement
More volume per stroke
Displacement is the volume the pistons sweep. For a given cylinder it is the area of the bore times the stroke, and the engine displacement is that volume times the number of cylinders. At the same volumetric efficiency and the same engine speed, a larger displacement takes in more air mass per minute. That is the performance reason to change it. The extra air still has to be fuelled, and the extra torque still has to be carried by the pistons, rods, bearings and transmission.
Volumetric efficiency is not automatically the same after the change. A larger bore needs a port and valve that can fill it in the time available. A longer stroke at the same engine speed raises the mean piston speed, so the same port has less time and a faster piston to keep up with. The runner and valve timing that suited the original speed range can be in the wrong place afterwards.
Bore and stroke are different changes
Increasing the bore increases the flame path across the chamber and the area of the piston the cylinder pressure acts on. It requires a block that has the wall thickness, and a head gasket and piston that match the new diameter. Increasing the stroke changes the crankshaft, usually the rods, and the piston speed. It also changes the compression ratio unless the combustion-chamber volume is altered to suit, because the swept volume has changed while the clearance volume may not have.
Either change can make the piston meet the head or a valve. That is a geometry check, with the gasket thickness, deck height and camshaft timing that will actually be used, including any variable-timing travel. A stroker kit's catalogue compression ratio is only true for the chamber, gasket and deck it was calculated with.
The calibration and the limits
An ECU that estimates load from the original displacement, or a rev limit chosen for the original piston speed, does not update itself because the crankshaft changed. Remapping after a displacement change is about the new air mass, the new compression and the new safe engine speed, not about adding a percentage to every table.
The bottom end sets the ceiling. A longer stroke can increase the load on the rod and the side load on the piston. A larger bore increases the force on the rod for the same cylinder pressure. Bearings, the oil supply and the head fasteners are part of whether the extra air can be used continuously. So is the cooling system, because more fuel burned per minute is more heat. None of those parts increase displacement. They decide how much of it is usable.
Sources