The Cast vs. Forged piston debate should not really be over cost, or piston weight, or performance but more so if it is a REAL Street car or a RACE car.
IF this car is your daily driver, or you drive this thing a lot ( 10k+ miles a year) and in stop and go, want to start the car up and drive off a few minutes later, then this is something you really should think about. Cast Hypereutectic pistons have a very very low expansion rate and thus can have a very tight piston to wall clearance, and thus very little blow by or piston rock when the engine is cold. This makes for a great every day daily driver motor.
Everyone is fast to go with shelf forged pistons because they are available and cheap. All pistons have a spec'd piston to wall clearance to have the cylinders bored to spec so that the piston has the correct skirt clearance. This is of course as everyone knows, very very important. Now the part no one thinks about is at what engine temperature is this clearance? If you have your block bored at say 70deg F with a .0025" clearance, then how hot does the motor have to be to get the piston closer to .0008-.001"? On many many forged pistons this temperature is around 190-200 deg F. On most cast pistons, this temp is actually much lower around 150-160deg F. MOST cast Hypereutectic pistons also have a very tight COLD piston to wall clearance of usually less than .001". Most honda and nissan motors are around .0008" COLD and when at normal opperating temp, closer to .0006".
A cast piston will have much much less piston rock, slap, and cylinder scoring due to those effects, because of the much tighter piston to wall clearance. A Forged piston will have a lot more piston rock, slap, and so on when cold because the piston is rocking in the bore A LOT more than cast piston. This is why it is so very critical to let your engine warm up before driving or "smashing on it" most people will wait until the engine temp gauge starts rising, which is good, but you need to know how warm that is. A forged piston at 160 deg is still not "fully expanded" to its spec'd clearance as it should be. At 160-165 deg ( which many built street car setups run while cruising down the road) There is still a bit of extra piston to wall clearance which means excess piston slap and rock, which means excess piston and cylinder scoring...
For a REAL Street car I will try to choose FACTORY cast pistons anyday over a forged piston..Race car, Forged piston anyday ( unless have to use cast)
For turbo setups, that is a somewhat different argument...
Happy Boosting!!!:beer:
