Advantages of Mahle Forged Pistons
MAHLE OEM Pistons vs Forged Pistons:
While MAHLE supplies OEM pistons for the HEMI 5.7 and the 6.1L engines, which are cast and designed only to accommodate the OE manufacturers original specifications, the ones supplied to Arrington Manufacturing / ShopHEMI by MAHLE Motorsports are the forged variety. The following information is useful to help distinguish between common cast type and stronger forged pistons that Arrington uses in its HEMI engines. It also points out why high performance engines need forged internals and some of the metallurgic differences between the alloys commonly used in piston construction.
1) The picture below is taken of a garden variety cast piston. Of special note are the parting lines visible in the casting. These lines are a result of the multiple pieces necessary in a casting mold to remove the finished piece from the mold itself. These lines are not present in a forged piston undercrown. As you might expect, cast pistons are weaker than forged units. As a result they cannot hold up to the demands of racing engines.
2) Below is a photo of one of MAHLE's forged pistons. Notice the lack of parting lines in the undercrown area. The forging process itself dictates that you have a punch and a die. A solid puck or billet of aluminum (preheated) is placed in the bottom die (basically a cylindrical can with the top side open). The punch is then pressed into the die forcing the puck of aluminum that is in the bottom of the die to form itself around the punch. This forging process is done at extremely high pressures (upwards of 50 tons). This forging process gives the alloy of the part a grain structure or flow versus the cast part making it (the forging) a stronger alternative. The end result is a stronger piston that can withstand the punishment of high performance HEMI engines.
You can see from the above information it is very easy to distinguish a "true" forged part from one that is not. Additionally to the forging process, race pistons are constructed from a variety of alloys. MAHLE psitons produced for ShopHEMI are of the 4032 variety alloy. This alloy is utilized for it's good strength characteristics and it's relatively low thermal expansion.
An alternative alloy called 2618 is sometimes utilized instead of 4032 as it does produce some increased strength characteristics short term over the 4032 alloy. The down side to the 2618 alloy is the significant increase in thermal expansion resulting in the need to run the pistons at a greater piston to cylinder wall clearance. This increase in clearance results in piston slap, which causes a very audible noise specifically during cold starts. The 4032 alloy is utilized to eliminate the noise issue especially geared towards your high end customer base. The 4032 alloy, although slightly less strong than the 2618, is still more than adequate for the horsepower and torque rating that these engines produce. We have great success with utilizing these 4032 alloy pistons in engines producing in excess of 1000 HP in a small block!
The chemical composition of the 4032 alloy versus the 2618 alloy differs mainly in the amount of silicon present. The 4032 is consider a eutectic alloy which contains in the 12 to 13 % range of silicon. This is not to be confused with a hyper-eutectic alloy which contains upwards of 17% silicon (typically resulting in a very brittle composition). The 2618 material is considered a hypo-eutectic alloy which contains less than 1% silicon. The presence of the silicon has the biggest impact on the thermal expansion of the pistons and does potentially have some negative effect on the strength as well. The 2618 alloy typically does have slightly higher concentrations of copper in the alloy to help further increase the strength characteristics.
Advantages of MAHLE Motorsports Pistons
Alloy alone will not necessarily suffice when it comes to making a piston of superior strength to withstand the operating characteristics of an engine. Good, solid design techniques along with the appropriate alloy combine to get the job done. Some competitor pistons fail due to inadequate design regardless of the alloy. For each piston design MAHLE Motorsports create a solid CAD model in order to ensure that all features of the piston are adequate for the intended application. They also take the extra effort to calculate additional information such as pin bore loading, inertial stress on the pin bosses, etc. to further ensure a superior part. With MAHLE's technical and engineering resources they stand behind their products and challenge anyone to produce a better component at a more competitive price.