What Pistons to Use With a Turboby Richard Rowe
You can think of a turbocharged engine as a sort of hybrid jet engine. The only real difference between the two is that a turbocharged car engine uses a rotating assembly to keep the power shaft spinning at low rpm, whereas a jet engine uses completely static combustion chambers and must continuously rotate at high rpm to keep the combustion going. The engine is really only responsible for containing the turbo pressure, so you must build it with this philosophy in mind.
Because the engine is essentially an anvil for the turbo pressure to beat against, everything in it needs to be as strong as possible. If you're building an engine with a turbo in mind, do not try to save money using inferior cast or hypereutectic (silicon-alloy) pistons. These are fine for most naturally-aspirated applications, but will severely limit your turbo engine's power potential.
Low Compression Pistons
Turbochargers increase pressure in the combustion chamber, making the mixture more prone to detonation. High compression pistons also increase the detonation potential, which makes engine compression the primary limiting factor where boost is concerned. One good rule of thumb is to drop compression by about one point for every 15 pounds of boost. However, keep in mind that dropping compression too far will reduce engine power when off of boost, increasing turbo lag and decreasing driveability.
Opinions vary whether turbo engines are best suited for dished or flat-top pistons, but the general consensus is that it depends on combustion chamber design and volume. Small combustion chambers tend to do better with a deep piston dish, and larger ones do better with a shallow dish. Ideally, you want a fairly small, open combustion chamber with little to no quench (flat) area and a deeply dished piston.
Optimized Ring Lands
While modern piston technology allows designers to place the piston rings closer to the piston crown for greater power, you'll want to err on the safe side by purchasing pistons with ring lands as far away from the action as possible. Distance will decrease stress on the piston rings and increase the amount of stress the pistons can take before cracking.
Turbo pistons typically have very thick tops and extra reinforcement around the wrist pin. These reinforcements do add some weight, but they'll increase the amount of pressure the pistons can take without breaking. This is a vital consideration for any purpose-built turbo engine.
- turbo image by Elijahu from Fotolia.com