Pistons

Pistons

The piston plays an vital part in the component mix for our engines.

The specification for the piston is defined by:

  • application the engine is intended for (Rally, Race, street, drag etc)
  • the rev range/piston speeed we are intending to use
  • the flow we achieve through the head
  • the target compression ratio we require
  • weight, shape, metallurgy, coatings.
  • 3rd party manufacturer reputation.

The primary variable for the piston specification is the compression ratio. Compression ratio, as a measure, is only useful when other factors accompany it. Compression pressure is what the engine actually sees. High compression pressure increases the tendency toward detonation, while low compression pressure reduces performance and economy.

Compression pressure varies in an engine every time the throttle is moved. Valve size, engine RPM, cylinder head, manifold and cam design, carburetor size, altitude, fuel engine/air temperature and compression ratio all combine to determine compression pressure. Type of fuel used and even altitude also play a part in the decision. Supercharging and turbocharging can drastically alter compression pressures.

The goal of most performance engine designs is to utilise the highest possible compression pressure without causing detonation or a detonation-related failure. A full understanding of the interrelationship between compression ratio, compression pressure, and detonation is essential if engine performance is to be optimised.

Getting the compression ratio correct can make a big difference to the overall performance of the engine in terms of maximum power and power delivery. On forced induction engines it is particularly important to match the compression ratio to the to the proposed maximum boost.

One further important variable is weight. The lower the weight of the piston the less reciprocating mass is acting within the engine. This will reduce power loss and improve responsiveness. Weight must also be balanced against strength and reliability. Theses are also key factors in selecting a piston.

In order to optimize the benefit from fitting the right piston we use years of experience and simulation software to select the right piston. Simulation is verified during the test stage of the engine build using our dyno.

If an engine does not perform as we expect on the dyno, we find out why and review the simulation algorithm. The algorithms used include head flow, cam timing, stroke length, rod ratio, fuel used, engine size and max revs used.

When we select a 3rd party supplier of pistons we take meticulous care to select the right product, looking at every aspect of the piston design and the integrity of the manufacturer. We are fully aware of the need to source pistons from trusted suppliers.

In many cases we carry out minor modifications to the piston where we think it is required. Each piston used is also matched in weight to others in the engine to obtain optimum engine balance.

The pistons we use are selected as an end result of precise measurement and simulation in the context of each individual engine and its characteristics. We apply our experience to select the right manufacturer and modify it to our needs.

There is no guesswork involved here.

No rule of thumb is used.

It’s the right piston for the engine.