To those of you considering aftermarket forged pistons...

You guys are thinking wayyy to far into this, if your worried about piston slap on warmup...just never shut down the engine, problem solved! All joking aside...anybody consider running a block heater to offset the amount of time it takes for the car to warm up and reduce slap time?

Moar slap time!!!

Does all forged pistons slap on start up, or is it just a few that do?

During the warmer months, my motor sounded stock. In the winter, you'd probably think it was a diesel.

Since I'm in CA, I shouldn't have to worry too much then. Having my motor rebuilt next week and going with forged pistons this time.

I went with uncoated CP's.

Is there really a big difference with the coated ones?

Other than a coating that wears off after a couple thousand miles, no.

That's what I thought. Just wanting a reliable motor that will last me another 100k without major oil consumption, and make the step up to Stage 2 in the near future.

Does all forged pistons slap on start up, or is it just a few that do?

 

Not all forged piston motors slap. It depends on the piston pin offset, piston material and piston-to-wall clearance used by the builder.

 

That's what I thought. Just wanting a reliable motor that will last me another 100k without major oil consumption, and make the step up to Stage 2 in the near future.

 

Then stick to the lower expansion 4032 pistons (Mahle, Wossner, Cosworth) and pick a builder who is used to setting the clearances appropriate to your use. For reliability, blueprint the bearing clearances. Subaru doesn't do a great job in making them consistent from the factory which will affect the oiling, especially on the #2 & 3 rods which are fed by the same main bearing. I have 5 blocks sitting here, all of whom failed on either the #2 or 3 rod.

Edited March 11, 2013 by BlackHole

Just pulled the short block out today, found a respectable Subie builder near by. He only uses CP pistons, so I'm going to stick with that.

Cobb Plano uses the 4032 Mahle in their builds and they are really quiet. They have some really high horsepower cars running them.

 

If I were to do it again this is what I would go with.

 

I have JE pistons and they are noisy, and before anyone opens their mouth my motor was built by Watt's Shop, Watt Bingham, and Cobb Plano in Dallas Area

 

TOP NOTCH BUILDER but it is noisy with the JE's

Cobb Plano uses the 4032 Mahle in their builds and they are really quiet. They have some really high horsepower cars running them.

 

If I were to do it again this is what I would go with.

 

I have JE pistons and they are noisy, and before anyone opens their mouth my motor was built by Watt's Shop, Watt Bingham, and Cobb Plano in Dallas Area

 

TOP NOTCH BUILDER but it is noisy with the JE's

 

I plan on using the Mahle 4032 pistons in my build. They seem to be the quietest forged piston; I understand they aren't capable of huge hp numbers and don't want that on my daily driver anyways.

My builder just pulled the block apart and said every thing looks good. He's going to polish up the rods, replace the bearings with ACL race bearings and put in CP pistons. He only uses CP pistons, since he had nothing gone wrong with them. So, I'm going to take his word on it.

This is a great summary for those thinking or going forged (like me). although I already knew about possible piston slap and needing correct bore/honing.. not all do! Important info Very nicely written OP.

I went with the Mahle pistons. There pretty quite even on start up on a cold day. Louder yes slightly. But I think it is a very good compromise between oem and forged pistons. Some people complaine the Mahle postons brake, but I think it's all in the machining and tune.

I ended up w/ wiseco pistons, yes on cold start up the slap around a bit, but when everything is warmed up you can't even tell. I'd say it's quieter than the stock setup (but i new new exhaust valves too.. that might help a little)

I've read through all of this and I'm grateful for everyone's input. My 05 is running well with the AVO380 and 21 psi. However, I'm also at 175k and I want to be prepared down the road.

 

I'm leaning towards this setup eventually with lots of questions and gaps:

 

EJ257 SB with CP or Weisco Pistons (are those 100mm bore?)

Heads? Is there a better option than stock?

Not sure about any valve work or rods or bearings.

 

I would like the best of both worlds with hp capacity though my inference is that I can't necessarily have my cake and eat it, too. Perhaps a stock 257 would be wiser then?

I also have a question (not to take attention away from doobaru above though).

If we choose to go with an OEM piston when rebuilding, or something similar that isn't forged, would it be advantageous to throw in forged rods on top, or would that create uneven pressure/wear?

The weak part of the motor are the pistons, getting forged rods is not going to make the pistons any stronger or last longer. You'll only need the forged rods if you plan to go stage 3 and beyond. Otherwise, forged pistons is your better option.

I'm about to be installing an HTA68 and supporting fuel, so I am going stage 3. It is my DD so i'm going to be tuning conservatively. Would forged be in my interests then?

Forged pistons for sure, and forged rods would be a good investment. May want to check with your builder for advice, just to make sure you need them or not. Otherwise, if money is not of a concern then I say do it.

So in your opinion, the drawbacks of forged don't outweigh the pros to doing them on a DD?

So in your opinion, the drawbacks of forged don't outweigh the pros to doing them on a DD?

 

Pretty much. If you're not planning on doing a high-boost, high-HP build, forged isn't going to help you. Spend the extra money on a good tuner; the OEM shortblock is just fine.

As you've probably read, anything beyond stock, and even stock on the poor factory tune, you are running into issues of breaking the ring lands. Tune does have a lot to do with this as a bad one will wreck any engine, but for our cars the pistons and quality control on them seems to be a big issue as well.

 

With a good tune, 4032 alloy pistons should be significantly better than stock Subaru ones, while keeping the piston to bore clearances close to factory piston spec.