Forged pistons vs. Cast pistons?
Here we are going to address the options of a forged piston, compared to a cast piston, and stock castings versus aftermarket hypereutectic castings.
Are forged pistons good? Yes, they’re the best for most turbo applications, as they are manufactured from a stronger material, and the forging process inherently improves the grain structure, versus casting. Even the machining is better on forged pistons, in particular, the oil return holes/slots are of a better design with a forged piston, which is an area that will often crack when a piston is exposed to detonation.
So what’s the down side with a forged piston? It’s simply a cost factor. A set of forged pistons is typically north of $1500, whereas a good cast piston set is more like $300.
So can you get by with a set of cast pistons instead of spending $$$ on a set of forged pistons? The short answer in a lot of cases is yes.
Structurally, for most Isuzu G or 4Z series builds in the sub 300hp power levels, a cast piston will hold together from the forces generated from those power levels. It’s not the power that will hurt a piston, it’s detonation. And if the tune is spot on, meaning air/fuel ratios and correct, ignition timing is within limits, air temps aren’t too high, fuel octane rating is sufficient, etc, etc, then a cast piston will survive. However there are a lot of variables to consider, if one or more of these variables is off, then you will get detonation and likely crack or melt a piston. And the higher your power levels, the more likely some detonation will cause a problem.
Depending on your head gasket setup, what type of gasket, whether you have o-rings, etc, ideally your head gasket would blow first, and act like a fuse in the system. After all, a head gasket is easier to replace than pistons. But if you have a strong head gasket setup, then the piston is next in line.
This is where a stronger forged piston will be more resilient. Even a forged piston will be damaged by heavy or continuous detonation, but they will survive in situations where a cast piston will not. Plus they can handle the stresses of higher power applications.
So for our customer who want to go from stock power levels, to double that, with a turbo kit, such as 75hp to 150hp, yes stock cast pistons, or aftermarket cast pistons would be ok as long as the tune is good. For those guys modifying their engine combo further, pushing 200hp or more, a hypereutectic cast piston will be ok, but the tune becomes critical.
