The stock block F20c/F22c is well-known for being very stout. Many S2000s have been turbocharged to over 600whp without ever being opened up. However, our base target is 500 crank horsepower on Cali 91 octane while going through a catalytic converter with a mild exhaust. The only way to achieve that goal is to dive into the engine to lower the compression ratio.
In the 1990’s, Honda developed their Fiber Reinforced Metal (FRM) liners which is also known as a Metal Matrix Composite (MMC). The goal of the FRM liner was to eliminate the need for the cast iron (i.e. heavy) liner typically required for use with aluminum pistons. Aluminum on aluminum is a bad mix when it comes to a sliding interface; tribology is the study of materials sliding relative to each other in case you were wondering. One of the solutions was the introduction of the FRM liner. The downside of the FRM liner is the requirement for coatings on the piston to be compatible.
Enter Mahle with their Gold Series piston for use in FRM liners. The Gold Series Anti-Friction Skirt Coating is the secret sauce allowing the piston to work with the FRM linear. For the Manufacturing Engineering geeks, you can see the time/date/lot info etched into the top of the piston. Mahle is a huge supplier to the OEM automotive industry, so they do these OEM type things. Of course, they do motorsports things too like helping win the 24 Hours of Le Mans many times in boosted applications. For our turbo F22c, the compression ratio is a forced induction friendly 9.5:1 compared to the stock 11.0:1. We should have no worries at all with knock on 91 octane pump gas at our initial 15psi target.
To handle the boost we’re going to throw at the engine, the Mahle pistons start out life as a forging of 2618 alloy aluminum. The generous fillet radii you see in the forging are excellent at reducing stress risers.
The top ring groove is hard anodized to handle the high stress of high cylinder pressures and high piston speeds. With the Hondata K-Pro, we bumped up our redline to 8500rpm for those instances where we need to wind it out just a little more than the stock 8200rpm of the F22c. Mahle uses a phosphate coating on the piston which gives the piston the grey appearance. Phosphate is a dry lubricant and the application in the pin bores and ring grooves protects against galling and micowelding during a cold start before the oil gets moving. There’s that tribology stuff again.
The Mahle PowerPak includes all the piston rings and piston pin, so you’re good to go. I had the opportunity to work on some piston pin design a while back and this pin design is pretty stiff. It has a thick wall and no tapering/chamfer of material at the ends of the pin. Mahle has an excellent series of books you can buy on engine component design where I learned about this stuff.
Transferring the combustion pressure to the crank are Carrillo PRO-H connecting rods. According to the crew at Real Street Performance, these rods are good for around 275hp each. We’re only looking to push about 125hp a rod, so we have plenty of safety margin. Yes, torque is the more proper unit for strength, but everyone is more use to thinking in terms of horsepower. Anyway, we will have tons of fatigue life as the rods will be significantly under-stressed. The rods get their strength through a number of material processing features. The first process being forging of the material. The rods are heat treated to reduce internal stresses. Lastly, the rods are shot peened to improve fatigue strength. Check out the small end bushing which has holes to help feed oil to the piston pin.