Forged Internals - explained.
 

Just need some clarification.

I always hear the term "forged internals from the factory" for the WRX STI models. I understand these engines are built stronger. Is that so?

My question is ... does this apply to WRX models, or just WRX STI.

The reason I am asking: I'm looking for another Impreza and just saw a lovely 2003 WRX, looks clean, low mileage, and fair price at 7k, as I know some of the WRX STI are going for 10-12k

Only the STi tuned cars had forged/hypereutectic pistons.

alot of people get what is and isnt factory forged, reality is since the bug (2001) not many are, even jdm sti's arnt. some of the special editions are, but thats it.

Hypereutectic is NOT a forged piston, its a heat treated cast piston.

If you're thinking of buying a 17 year old Subaru then go in with your eyes wide open. There's a minefield of hidden corrosion you often can't see till it's too late. Way up high above the rear wheel arches where there's a double skin which hides the rot from the inside. Do some searching on this forum for horrific pics and comments.
There's no rear wheel arch liners on them, which doesn't help.

Obviously but they were fitted to STi tuned cars of that era.

Okay, thanks for input, but perhaps I can re-word my question and anyone who wants to add input, I would like to hear views.

The WRX STI engine is higher-revving and stronger according to what I hear. So would it be accurate and fair to say that the WRX STI engine is superior to the engine in a WRX model.

My main concern is that I don't want to buy a WRX if the engine is inferior / weaker compared to the WRX STI.

I know, it is not easy finding a gem. I had a version 2 import that was very clean, sold it many months ago, I am now seeking something newer i.e. 2003 onwards, but avoiding the 2.5 unless forged, as I know these engines are not as good from the factory.

2003 will be hypereutectic not forged iin STI's jdm or uk

can of worms opened,,,,, lol


depends what your wanting to do, wrx engine is designed for wrx power, sti engine is designed for sti power. In stock form (other than the 2.5) they are pretty reliable as long as you look after them and treat them right (warm them up before booting them, regular maintenance etc etc).

Sti is a higher spec out of the box (inc the engine) but the engine is pushed harder out of the box.


2003 wrx for 7k is about 2k overpriced by the way.


Lets go back a step, what is your budget and what are you looking for? weekend toy? performance over comfort? mix of both? etc etc

Read my reply again.

some of the the 2 litre jdm sti were higher revving than the same age u.k. 2 litre sti version , the downside the jdm often have a shorter 5 or 6 speed ratio so on a motorway the revs even at cruising are high , the sti 2.5 standard engine is probably more prone to failure as it’s running more power than a standard wrx 2.5 ,the 2.5 sti has a few more mods to it like better heads better cams variable cam timing , the 2 litre engines were stronger , the sti twin scroll or spec c are good cars , i found my sti jdm wagon great fun to drive but very laggy , my 2.5 fsti would destroy it in all road situations even if the wagon had more bhp , so i think you need to look at what years you want and which type of car , even with the 2.5 weakness i think they are a great engine especially if they have been forged already , or as above a 2 litre newer age sti jdm , the twin scroll reduces the lag the 2 litre often has

At the moment, I would be happy to spend £8k, or could wait until the new year, and have £10k, maybe a tad more.

I wouldn't say a weekend toy, would like to use it more than that, most important thing for me is, rust, service history, not too high mileage, fair power i.e. 300-350 bhp, and reliability.

The 2003 WRX has had many upgrades, and looks good, see link: what do you think?

https://www.autotrader.co.uk/car-det...us=1500&page=2

Always pays to do an MOT history check to see what it's failed on over the last few yrs, last MOT had a long list of advisories on it to be fair, minor or otherwise but quite a few.

Agree, always do. I believe those issues have been addressed.

over priced, its a wrx.

Haha, that's my point, it's like WRX is seen as inferior. Maybe I should hold out for a WRX STI

so options are wrx or sti , i much prefer an sti as a lot of the mods are already done 6 speed bigger brakes tougher engine with more power , a 2 litre is more reliable than a 2.5 , but a 2.5 is a very good engine , with more peace of mind if it’s a forged 2.5 , not all 2.5 fail but there is a chance of it so consider a budget to get it forged if you need too , or go jdm sti 2 litre but could be more £ , my sti wagon was 6000£ on a 2001 plate , fresh import and very clean

i had a few wrx before i went sti , all the wrx i had were good cars , but i prefer sti now after owning a few , i prefer the forester sti as it’s not your regular car it’s a quick sleeper that’s a great all rounder

Simon, Id like to advise

Many of us started with a WRX ( I still have mine ) but have ended up changing bits until its a Sti . No shame in that but 7k for the car on ad is OTT.

I reckon mine is a 7k car on a WRX300 MY05, but I have a EJ207 93mm bore 2.038 semi closed deck engine with Manley Rods / New Nitrided Crank / Mahle Pistons / etc , The engine work alone is £3500
In addition to porting the heads myself I have added a SC36 Turbo a 3 inch CAT Exhaust system/ Harvey uppipe /. 3 Port Control / 255 Walbro pump / Sti V8 TMIC / 550 cc Injectors and a 335mm Brake conversion and Coilovers to hold all this on the road.
As a new engine , Its running 1.3-1.4 bar Max 320lbft 330hp on a Twin E85 map from Plymouth Engine tuner,

However I'm stuck now as although this engine will take 500+ bhp, I do just have in my WRX a 5 speed box and Excedy Blue Clutch.
I have rebuilt the gearbox but , you really should not put more than 350/350Bhp thru this if you want it to last for a while.

As the Engine is now with 3k miles my last mod might be some RCM headers and a Map tweak to Boost manage the Power / Torque to 350lbft/350Bhp on E85 Fuel.
I am aiming to reach 250lbft at 3600 rpm (currently 200lbft ) and the 350lbft by 4400 rpm (currently 320lbft) At this point I consider a 5 Spd WRX to be about the best it can be for the road.

None of the above makes financial sense to do unless you started with a much lower price car and just work your away along the route. Mine was £4250 in 2012
At the end of the day the less durable EJ205 engine block / its 5spd gearbox / Poor brakes / 17 inch wheel diameter are your WRX limiting factors. Changing them all costs more than a reasonable Sti

You should therefore look for a V7 V8 Sti which will have a 6 spd gear box / 18 inch dia wheels / Brembo Brakes / RW/FWD Torque transfer balance control / Better seats at £ 7- 8 K
Then you can add a few mods Re-Map and run pretty reliably 400 + Bhp on these cars without changing the engine internals, although most will in time and continue to improve the car.

Hope this helps.

I do agree, I guess I am not quite there money-wise at the moment. I have seen a few I like in the past month, but these are going for anything between 10-13k. The ones below are just examples, I have seen many more over the last few months in that price range. Unless they are high milage with rust bubbles appearing, then yes, can get cheaper i.e. 7-8k

For example:

https://www.ebay.co.uk/itm/SUBARU-IM...MAAOSwHXxfiyQH

https://www.ebay.co.uk/itm/Subaru-Im...4AAOSwV6VfiqxS

And another one, and this is 103k miles on the clock

https://www.ebay.co.uk/itm/2004-04-S...MAAOSw4PNfZMGs

And then this one:

https://www.ebay.co.uk/itm/Subaru-Im...sAAOSwj1hfYF7X

first one a type 25 , a favourite of mine , but on the negative they push quite a lot of power out of a 2.5 engine so can be an issue , the second one appears to be a jdm sti , it looks like strong money for it , see if you can find a bugeye wagon , maybe a jdm sti as they look good plus come in cheaper, again with a part forged engine from the factory 2001 i believe was the last of the wagons

Wait a couple of months for the **** to really hit the fan, and you'll pick up a cracking Sti for 8k.
I had a wrx 05 wagon, and then an 09 Sti spec C hatch, and although they are miles apart in performance, the wrx wagon would do 100 more miles to a tank, and still put loads of smiles on my face. Remember, your costs in terms of tyres, road tax, insurance etc will all be higher with an Sti.
Good luck

not strictly true...........

all MY01 & 02 STi's used forged factory pistons, at MY03> the JDM STi cars started to use the Hyperpeutic casting process on their 2.0 cars whereas the UK stayed with the factory forged version until MY05 - UK car stayed with X-drilled crank rather then the Nitrided version the MY03 JDM car got..........

at MY06> ALL STi's whether 2.0 or 2.5 used the Hyperpeutic process until the very rare MY19 US spec "S" car

Great info there Alyn :notworthy Some on here always seem to think they're right no matter what you tell them.

So has my 05 sti uk 20l widetrack got forged??

Just to reinforce Alyn's info
UK STI cars same pistons MY02 - MY05

https://cimg1.ibsrv.net/gimg/www.sco...03756fae92.png
JDM STI
MY01-02 same as UK

https://cimg8.ibsrv.net/gimg/www.sco...5643640d3c.png
MY03 onwards

https://cimg8.ibsrv.net/gimg/www.sco...ce51d7f018.png

yes as my post ALL UK 2.0 STi MY01-MY05 cars have the factory forged piston spec

Indeed.

There is nothing wrong with the hyper pistons at the power levels they were designed for, they have low expansion rates which allowed close piston to bore clearances which helps oil consumption and emissions.
The problem with the 2.5 ring land failures isn't because of the material used, they just cocked up the design and implementation.

The important comment is "at the power levels they were designed for" I suspect John knows a lot more to this..

Hypereutectic pistons are designed with low coefficient of thermal expansion to allow tighter piston to bore clearances, using a relative high silicon alloy content
in a process nominally cheaper than forged pistons are to produce. Anyone interested should read Wikipedia "Advantages" on Hypereutectic piston.

They may well have a low expansion rate, and associated emission advantages but I am not a fan of these close piston and ring clearance when we tune for power.

The higher level use of silicon makes the hypereutectic pistons harder but success depends heavily in the production process and the alloy cooling rate,
In the hypereutectic method the cool off rate becomes crucial to prevent phasing in the alloy. (a physical alloy structure)
potentially making them more brittle. A fine grain is more resistant to cracks than a rough one.

Its a significant issue as eutectic alloys do not cool in a linear fashion but during cooling actually raise in temperature minimally before finally cooling.
Its at this point that the fine grain structure can be lost if cooling is not controlled. ( I know that sound weird but it happens )

Why I don't like them with tight clearances in a tuned engine..

I managed to destroy a new EJ205 2.0 build using Sti hypereutectic pistons using Std piston to bore and Std ring clearances.

On a -5 Deg C night in Germany after 1-2 k miles on a 300bhp build I took to much of a liberty from a cold start and gave it full throttle and >140 mph after perhaps just 5 mins at low speed,
This resulted in all 4 hypereutectic pistons / rings expanding faster than the block does or can at -5deg C ambient temperature and such high speed.
My impatience resulted in big end failure, when I opened the engine up I discovered all four cylinder walls were polished with heat pretty much evenly around the whole length of stroke, but no damage to the piston crown. (no detonation)
naturally you would still clearly see the hone pattern in the bores at this mileage. Thus its never a good idea to give it welly before everything is properly warmed up and especially going fast in very cold air. Lesson learned the hard way.

However for our tuned higher outputs that most of us run ( that generate much more heat than originally designed ) you simply should not use factory clearances. ( if you follow the Wiki Article you can see these are designed to meet emission requirements)
There is a very simple way of course to mitigate this when you re-build simply to gap your 1st and 2nd compression rings with more growth tolerance which compensates for the tighter piston to wall clearance
that hypereutectic piston uses. I note that the RS25 uses mm clearance Top ring 0,20-0,35 Second ring 0,20-0,50 Oil 0,20-0,70 but that the wear limit is huge a 1.0mm gap

Id therefore be tempted on a new build of that engine to start with Top 0,40 Second 0,55 mm I would also suspect that the ring growth in its thickness may cause stress in its ring slot.
Race engines use thinner rings to alleviate binding , but don't care about seal life higher wear rate and have no relevance to emission law.

So would totally agree that the EJ 2.5 engine Piston land split would be less likely if they were used in the environment they were initially designed for.

But If I were rebuilding one with hypereutectic pistons for 350hp application Id be happy to give it a go on just using a little more gap on the rings. I'm not experienced enough to say if more piston to bore clearance would also work ,
or if more clearance in needed in the slot but someone else might want to jump in on that. Whatever above this power level on the EJ blocks and targeting tuning for 400 + Id want move to a different piston type. I think I'm right in saying most of us do this.

On my latest 2.0 build I have on Mahle pistons, these are of a different alloy that do expand more than hypereutectic do but start from design a larger clearance. (yes this makes them slap louder cold)
I also contacted Mahle US applications and described my 2.0 Turbo to be boosted 350 bhp at this level the build advice was you should open the ring gaps (using the table Mahle provide on line)
which normally increases 5 thou per bore inch on a modified engine like mine , on a boosted 450 bhp 2.0 that becomes 10 thou more.

As a foot note my MOT tester actually suspected that I have Forged pistons( yes he hears the piston slap) but notes the hydrocarbon result takes longer to stabilize than his Sti
Which makes perfect sense when you read https://en.wikipedia.org/wiki/Hypereutectic_piston