Forged pistons vs factory cast? What is the right option for your build!
Hey!... Welcome to another #modmonday! Where I give you my take on a mod on one of my Subarus, today we will look into aftermarket forged pistons vs factory hypereutectic, what fits your build?
Factory on the left and Wiseco forged on the right. Slight visual differences.
If you are in the United states and many other areas your Subaru is most likely equipped with hypereutectic pistons. These pistons at stock limits can definitely make it through plenty of daily driving due to all the high mileage Subarus out there. But what if you want a bit more power or you are fearful of a self destructing engine.... Are forged pistons your answer? Let's take a look at what you gain and lose with each piston.
Number four piston… we hardly knew thee. If you didn’t know what a broken ringland looks like, here you go.
The factory hypereutectic piston is the choice from Subaru for it's higher silicone content over typical cast pistons to help handle the factory boost. They can also have a tight piston to wall clearance which helps keep blow by to a minimum, engine wear and noise on start up. What it gives up is with time and heat it can become quite brittle and with a bit of detonation and it's so long ringlands!
From up top they look very similar.
With those thoughts in your mind you may think that forged pistons are the answer to all your Subaru related problems! Remove those nasty stock pistons and get to the boosting! While forged pistons can take the abuse what do you give up for these attributes?
A little bit thicker ringlands on the forged units to help withstand detonation.
Forged pistons have a much larger thermal expansion and so you will need larger ring gaps and clearance, this can cause piston slap on start up until things heat up. With this you can have a bit of smoke before everything is warmed up and as little more wear as the piston is actually rocking in the cylinder bore. It is also said that this can cause forged engines to have a shorter lifespan with more wear being accumulated.
My choice for my STi was Wiseco’s standard compression pistons. Very stock looking with minor upgrades to improve on the design.
After having my Sti succumb to ringland failure I thought "Forged pistons in all the Subarus!" Why take the chance of breaking another...? Well.. what are you using the car for? Racing on the weekend, daily driving? Daily driving and racing?
It’s days like these that make you think… why not put forged pistons in everything..?
Now with this are you armed to make a decision? Not really, you can start to see the path that will work best for you but now is the time to talk to your tuner and see if your goals will work in your reality.
Things I have learned from my forged piston Sti are, I don't really care for the piston slap on warm up. I really don't like the extra blowby before warm up that lightly coats the intake tract with oil and smokes a bit. But I don't want another ringland to crack and I don't daily drive the car so those are small annoyances versus blowing the motor at a race event.
On the other side of the coin my Forester runs the factory cast pistons because for a daily I believe they can get the job done. The annoyances of a forged piston are something I wouldn't want to deal with everyday and that is why I own the Forester. I do have a few performance mods on the car but I keep up on maintenance to keep it running well.
On the other hand pistons much like these have resided in the Forester for 216k and never cracked. The new used motor in the Forester has 120k on it and they are still rolling strong too. So they aren’t all bad… right?
Do as much research as you can and be sure to ask your fellow owners to see how they feel after switching to forged pistons or why they stuck with stock units!
Questions, comments, thoughts? Leave them below!