if one day i decide to make my bottom end stronger.
what will i need to do?
honestly, i dunt know that much about engine building. so some help will be appeciated.

 

Im on my second engine build now, and frankly this is a very subjective question, because it all depends on what your your goals/expectations are. Our bottom ends are proving to be quite robust, even in stock form(when properly tuned!) With my GT30 I was pushing close to 400 Dynojet whp on 91 oct on my stock motor for about 25K miles with no problems at all, though this is probalby the most you want to push the stock bottom end. We have a nice forged crank and beefy forged rods but our weak point is our hypereutectic cast pistons. Car makers like HyperE pistons because they are cheap, good on emmisions, quiet, and are very reliable in the range that they are designed to operate in, but pushed past that limit they can easily break. kind of the example of a scalple that stays very sharp but can break when pushed to hard, forged pistons on the other hand are an axe

adding a set of good forged pistons to a stock bottom end is the best all around bang for the buck mod you can get, this alone with the stock forged crank and rods should be good to get you safely over 400 dynojet whp if the rest of your setup and your tuner are doing their job right. The best way to do this is start with a FRESH 2.5 block from Subaru, even if your motor is fine, just dont bother rebuilding it if all you plan on is a piston upgrade. Our 2.5 turbo shortblocks can be found for around $1500 +/- brand new, BRAND NEW!!! EVO guys WISH they could get asembled shortblocks that cheap from Mitsu! You couldnt send out your shortblock to an engine builder and get it rebuilt with all new rings and bearing for this much and still hope to have factory build quality, so dont bother, besides, you can pull out your block still in good shape and sell it to a 2 liter guy for $7-800 and make half your money back on the new block(w00t!). Installing the new pistons is pretty simple, you dont even pull apart the block, theres side plates on the block that you can access the piston wrist pins through so you can slide out the pistons and slide the new ones in. Theres a quite a few companies now that offer drop in pistons for the STI, you just reuse the stock rings in the same location you took them off the stock pistons so everything maintains the factory tolerances, be sure to measure and match the aftermarket pistons to the closest cylinder on your block. Since your not experienced with engine building this part you should probably be left to an engine shop, they should be able to do it for $2-300, its really just a 2-3 hour job. I would also STRONGLY suggest using side and top coated pistons or if you buy pistons that are not coated send them out to Polydyn (www.polydyn.com) for coatings. Moly side coatings have proven to reduce internal friction and ceramic top coatings have proven to reduce piston temps and help resist detonation. Car manufacturers use it and engine builders recommend it, so theres no reason you should not put them in your block if you alrady have them out.

Drop in pistons do have their drawbacks, since the drop in pistons are made to a single size, they may not match idealy to the cylinder bore, this can make for excessive piston to wall clerance that can cause increased piston noise and oil consumption. I got lucky with my last engine build with drop ins but I have known those with motors that sounded like diesels when cold and ate an extra quart of oil between changes

if you want more the next step would be to buy a set of oversized pistons and have them bore matched to each cylinder, this way you can machine them to their specified clearance and get the best wear and performance from them. This will though cost you quite a bit mor eat the machine shop, they now have to disassemble the entire short bock to bore the cylimders. The upside of going this far now you can have the rotating assembly balanced, this is relativly cheap and does wonders for making a smooth reving motor and can help you safely stretch your RPMs out a bit further. On a big bore 4 cylinder like ours balancing can make a BIG difference!

Well I hope that gets you going in the right direction, If you want to go bigger than this it starts getting REAL expensive, so decide what your goals are first. Dont even get me started on what to do with your heads! Let me know if you have any other questions

 

ooh. i didn't see someone actually replied to this topic.
thank you for all the information!

so basically what you are saying is, get a new block, forged rods, i will probably get new pistons too. how about the bearings or the valves? leave those alone?

I heard changing the bearings out makes a big difference too.

I just want something that could rev a bit higher and can withstand at least 400-450whp.

any other advices?
Thank you so much!

 

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