In a world once obsessed with downsizing with turbochargers to enhance volumetric efficiency, there are early signs that manufacturers are leaning back towards larger displacement engines to reduce costs and simplify the engineering process. In the realms of car modification however, engine capacity has always been linked with power output.
Before forced-induction became popularised, the route to power was always a large displacement. This would leave lots of room within the cylinders for plenty of air and fuel to enter for combustion, creating a powerful downward stroke to rotate the crankshaft.
With aftermarket power gains generally being found through forced-induction, the art of increasing the displacement of an engine can often be overlooked. Thankfully, stroker kits can be purchased that simplify the process of engine enlargement by lengthening the stroke of the piston, avoiding any expensive machining needed for boring-out a cylinder.
A stroker kit increases an engine’s displacement by lengthening the stroke of the piston. In a kit will be bespoke engine components altered from OEM parts to achieve this increase in stroke. They allow the piston to travel further up and down the cylinder thanks to the use of a different crankshaft. The journals along the crankshaft are lengthened by a given distance depending on the severity of the stroke enlargement. An increase in the height of the journal makes for double that increase in stroke length. This is due to the relative increase at TDC (top dead centre) and BDC (bottom dead centre).
To stop the piston crashing into the cylinder head, other aftermarket modifications are also present. Firstly, the piston pin can be set higher up the connecting rod, or the rod itself can be shortened. This avoids any need for complicated CNC machining of cylinder heads or walls to incorporate the longer stroke.
A stroking kit can be justified through the governing equations of an internal combustion engine. Let’s start with the volume of the engine’s cylinders:
Here you can see the relationship between the stroke and swept volume of the engine. From these equations you can see that as stroke length increases, volume also increases. So how does this relate to power? Firstly, let’s look at the effect an increase in stroke has on torque:
As the stroke increases, the torque output of the engine will increase due to the additional leverage created by the ‘distance’ that the force is acting from. In the same way that using a longer socket wrench to remove a wheel is easier than using a smaller stock tool, the extended stroke means a large twisting force is applied to the crankshaft.
Power is related to the torque output at a specific engine speed so an increase in horsepower will be had with an increase in stroke.
As with most ventures away from an OEM powertrain, there are going to be some drawbacks. As the piston is having to travel further during its reciprocation, the amount of stress induced can increase. Horizontal load on the piston also increases during each stroke, with both of these outcomes leading to excessive wear. An engine’s ability to rev will also be hindered due to the increased travel in the piston’s movement. This means a stroker kit can decrease the responsiveness of an engine, potentially stripping it of character. The new crankshaft will also need to be fully balanced for the engine at hand, with unwanted vibration becoming an issue.
Obvious dangers come with the smaller tolerances introduced with a longer stroke. Although the precautions mentioned earlier can be undertaken, the risk of collisions within the engine are inherently increased. A common issue is the crank journals contacting oil rails lying between the crankshaft and the oil pan. To counteract this, many stroker kits will provide modified sumps/pans to allow for a smidgen more clearance.
A stroker kit is a relatively complex modification intended only for the most handy of petrolheads. The ability to disassemble an engine, replace the key components and then put it all back together is not for the faint of heart, but the power gains associated with a stroker kit are thoroughly respected in the muscle car world. So if you have the mechanical knowledge and the hardware to go about it, a stroker kit may be the key to a whole new stable of horses.
Have you ever thought about a stroker kit for your car? Or does your car feature such a kit and if so, how much more power has it led to? Comment with your experiences below!