Jay Dickens tells us all about...
Jay Dickens tells us all about the various technologies involved with racing pistons.
The modern racing engine has several parts that are tunable as to design and configuration. The piston is one of these parts. We are required to hold to certain dimensioning for bore and stroke, crankshaft size, rod length, head and intake design, carburetors, and valvetrain geometry. Any tweaking we do has to be done to very few parts. Here, we will explain the parts and functions of the modern racing piston.
Of the few engine parts we can freely design, the piston, the cam, and our valvetrain geometry are the most popular. Since the cam design is best left to the manufacturer's professionals, and valvetrain geometry is often quite complicated, we have the ability to choose our piston design without possessing a college degree.
Not to oversimplify the process of choosing the right piston for your needs, the final say should be left to an engine professional. But with a little knowledge, we can influence the choices that are made by our engine builder. When we build our own motors, it is our own choice. Either way, it doesn't hurt to gain a little insight into the process of choosing the right piston for your application.
The choices involve the following:
Racing pistons come in all...
Racing pistons come in all shapes and sizes, and the design you need for your application is specific to the heat cycles and rpm you will experience.
The choice of the final weight and the alloy used in the production of the piston are determined in part by the overall strength needed for your application. Life expectancy is a major consideration. The two most common materials used in the aftermarket today are 2618 low-silicon and 4032 high-silicon.
The range of horsepower output of the engine, the intended rpm range, and the duration of the running time per cycle of the piston are all important considerations in determining the correct piston for your engine based on its use. Running one race cycle of 500 miles, an entire 12- or 24-hour endurance event, or a certain number of laps per weekend over an entire season are examples of different engine run time cycles.
The cut for relief of the...
The cut for relief of the valves should be to a minimum. This piston is designed for the Nextel Cup motor Jay built. It is quite radical from what short-track racers are used to.
For overall piston design, we will need to know maximum power and how high an rpm we will reach to determine, among other things, skirt length, the heat range or determine head thickness and the compression ratio for strength design.
A 350hp two-barrel Late Model motor topping out at 5,800 rpm and running 35-lap features would require a much different piston design than say a 650hp Super Late Model motor turning 7,200 rpm and running a 200-lap race.
The forces acting on the 350hp motor would be much less than that of the higher horsepower motors, and so the piston could be much lighter in design, which could help reduce wear and tear on the bearings, oil, and rings.
The height of the wristpin,...
The height of the wristpin, the length of the skirts, the dome shape, and the height of the ring grooves are all variables in the design of the piston. Each area is built specific to the application.
Piston weight, for example, can be reduced in several ways. One way is by the use of pin squirters that are machined into the block. These spray oil onto the bottom of the piston to cool it, thereby allowing us to run a thinner crown, thus saving weight.
Reduced skirt length for lower rpm applications saves weight over the use of pistons designed for higher rpm. The use of lighter pistons will result in less rotational weight and quicker acceleration of the drivetrain. Be careful in your choice of piston. Just because a piston is right for a high-horsepower motor does not mean it is the best choice for your 350hp two-barrel engine.
The pin diameter and length are also relative to the horsepower and rpm range your engine will experience. These determine the size of the pin boss, which affects the weight. Barrel and skirt design must not interfere with the crankshaft or connecting rods.
The valve reliefs should be cut to the minimum clearances for the lift and diameter of the valves to be used.
Different chamfers and radii here can be beneficial to fuel mixture and exhaust flow. And the dome or dish shape should match the combustion chamber exactly.