Piston Tech Tips
All of the components of your engine will be subjected to additional stress when a power adder such as a blower are used. But taking the brunt of the “action” and the stock part most likely to fail is the piston. The increased temperature and pressure created by blower use are bad enough. But it is the increased tendency toward detonation that use of a power adder causes which is the worst culprit. Detonation is uncontrolled combustion that occurs away from the normal flame front. When detonation occurs, cylinder pressure spikes rapidly and very high temperatures may be produced in a local area. This may quickly lead to piston damage and eventually to catastrophic failure.
Piston Materials, Design, and Manufacturing Processes
Modern pistons are made from a variety of aluminum alloys and are produced using two basic processes. Most OEM pistons are made from alloys which contain a large percentage of silicon and are produced by a casting process. These high silicon alloys are known as “hypereutectic” alloys due to the silicon levels above 12%. A properly designed and manufactured cast hypereutectic piston offers excellent durability, light weight, and relatively low cost. However, these alloys are brittle and thus not very tolerant of detonation. High quality forged pistons are made from alloys such as 2618. These have a much lower silicon content (<1%)and this makes them less prone to cracking under extreme conditions. In addition, the forging process itself tends to produce a tougher piston.
The design of a piston plays as much or more of a role as the material and the manufacturing process in determining suitability for use with a power adder. The shape of the piston crown is a major determinate of compression ratio. The thickness of the crown and the design of the pin boss area are major determinations of strength, as these areas are prone to failure under extreme conditions. Another critical area is the thickness of the area of the crown above the top ring – this area (known as the “top ring land”) is particularly prone to failure due to high cylinder pressures. The last major area to consider is the wrist pin itself. Wrist pins are heavy. So, in the important effort to minimize piston weight, many high performance pistons for normally aspirated use utilize light weight wrist pins. These are often unsuitable for boosted applications.
The Bottom Line
Based on the above, the ideal piston for use with a power adder will first of all provide the desired compression ratio. It will be a high quality forging with a thick crown. The design will be beefed up in the pin boss area and the top ring will be moved down from the usual position. Heavy duty wrist pins may be needed, in spite of the weight penalty, if very high power levels are anticipated.
When do you need a high-quality aftermarket piston? With careful tuning to avoid detonation, stock parts may provide acceptable service life with a 30-40% power increase from a power adder. This is equivalent to 100-150hp on a typical V-8 and perhaps 1/2 as much on a 4-cylinder motor. If you plan to increase your power by 50% or more over stock, you will need a new set of forged “slugs”.
What Brand of Piston Should I Buy?
There are many good pistons available. We have has good results with JE and Ross among others. If you plan to purchase from us we can make a specific recommendation based on your needs and budget. It’s all part of the “Kennedy Advantage”
Any Other Piston Tips?
We have had good results with using thermal barrier ceramic piston crown coatings on hi-power power adder applications. By holding heat in the combustion space, there is a small increase in horsepower. But more importantly, the piston crown will be 100-150 degrees cooler when a barrier coating is used. This helps prevent piston failure. However, the application of the coating must be done properly. If too high a temperature is used this may adversely effect the hardness of the crown and lead to deformation and eventual failure. We have had good results with coatings from Swain Technologies. Be sure to talk to us about coatings if we are building a motor for you.