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Original Message
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| RE: Rebuild follow-up, rods and pistons |
| By Gerry Proctor - 07/16/2004 6:46:22 AM; IP 207.133.188.254 |
Shotpeening compacts the grain structure to make the rod stronger and when done in conjunction with polishing, does reduce stress risers.
The cast v. forged pistons is more a matter of how you're going to use the engine. Cast (or hypereutectic cast) is fine for engines that won't see rpm continually over 6k or so and no supercharging, chemical or mechanical. Hypereutectics can take a fair amount of hard use (including boost and detonation) and cover the gap between plain cast and forged. You're dealing with two issues on a cast piston: Strength of the piston pin boss in tension, and; durability under high-heat and stress from detonation.
If the engine is going to be abused then you'd want to look at a forged or at least a hypereutectic piston. If the engine will live a more reserved life then a hypereutectic or regular cast piston would be better. Hypereutectic pistons cover a lot of ground. They're much stronger than eutectic cast pistons but not quite as strong as a forged piston. There are also balance considerations to make when using a forged piston so if you're not going to have the assembly balanced you need to use a piston that is as close in weight to the previously balanced crank. Ultimately, you would like to put as light a piston as possible on the rod and you won't get there with a FM or TRW forging. You have to go to Ross, JE, and the others for a custom piston. There are also at least two alloys used in forged pistons depending on how it will be used. A real race piston is very light, strong, and just about unusable in a street engine due to clearance and noise issues. The TRW/FM off the shelf forgings are more suited to a street and occasional race engine, run quieter, but they are on the heavy side.
So use a plain cast piston if the engine will live a grocery-getter life and won't see any hard use. Use a hypereutectic if you're going to lean on the engine. Use a forged piston if you're going to be brutal on the thing. |
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