stevie1982

iTrader: (16)

guys i have been thinking of more power uprating internals but was just wondering if i was to go ahead on the standard 2.0 block why not change it for a 2.5 or simialr

is anyone running a 2.5 and can give me any pointers good and bad?

what they have gained or lossed

etc

any advice would be great

thanks

steve

MartynJ

Have driven many 2.5 conversions from 300-420bhp depending on turbo choice and found them all fantastic....You cant beat the torque you get from the 2.5 and it makes it an effortless drive , 1/4 inch of throttle response is great....

340BHP-WRX

iTrader: (6)

I was considering having a 2.5 build. Been told the weak point is the pistons,you need forged items to handle anything over 350ish bhp.

I was also told that the gearbox can't handle the torque and you would need either a 6 speed 'box or a ppg 'box.

suffolkdar

As any American tuning company, On standard internals the 2.5 block will easily handle 450 bhp, and a huge amount of torque, Which would IMO kill a standard 5 speed box,

340BHP-WRX

iTrader: (6)

API and Zen both say the 2.5 will only handle around 350 ish safely,maybe 400bhp but it won't be reliable.

I did speak to people who were running 400-450bhp on a standard 2.5 but who knows how long that would last ?

suffolkdar

Please note the word" AMERICAN" they have had the 2.5 lump for years, and Thousands of them run over 400bhp on standard internals, Uless lots of americans are liers!

stevie1982

iTrader: (16)

ok, i was thiniking of uprating the internals with forge bits on the standard 2.0 lump so assumed it would need to be done for the 2.5 anyway.

if this is a stupid question then sorry but between a 2.0 or 2.5 (on forge internals) if they both ran 450bhp which would be a better drive, would they both be as reliable as each other (so no real need for 2.5) would one out pull the other, faster point to point etc those kind of things

steve

Tim W

iTrader: (1)

The 2.5, 'the area under the graph' would be much greater

Basically a 2.5 will spool up sooner, produce it's peaks lower down the rev range and produce more torque right across the rev range.

burns

iTrader: (1)

Having had an 2.5 standard STI block which melted after 18000 miles running only 380 bhp.I can only say get a "Built" one you know it will be able to handle the power and mate it up to a sti 6 speeder.It makes for a great daily driver with plenty of torque on tap.
David at API is worth having a chat with regarding what sort of engine would suit your requirements.

APIDavid

iTrader: (4)

NOT a Chance, the pistons on the 2.5 standard are made of Hershey bars and will not take anything above 1.35 bar in my [ AND Litchfield's ] experience.

To get above 380 hp you WILL NEED steel rods and forged pistons to have any chance of engine safety.

David APi

johnfelstead

There is no need for steel rods at that level of power, the stock 2.5 rods are a nice piece of kit with good quality bolts, they are the same rods as used in the STi9. You also dont need forged pistons at that level of power.

What matters is the rpm levels you intend to use as much as anything, the 2.5 uses a longer stroke crank so the piston and rod velocities are higher than a 2.0, if you intend to stick with a 7k rev limit you should be OK on stock internals in the low to mid 400's based on what other people are achieveing, if you drop the revs to 6.5K it can handle more safely. It has to be mapped correctly of course.

If you want peace of mind then it's a relatively easy thing to swap the stock pistons for some drop in forged items, without the need to split the block. If you intend to rev it then you will need to improve the rods. It's rpm that kills these rods, not power sub 500BHP.

I recently rebuilt my STi5 engine and considered the 2.5 as an option because it is a good value way of going about it. I decided against it in the end because it would make insurance and selling the car more dificult, it would also need a diferent turbo to make it work as well as my 2.0 setup, plus i couldnt rev it which would have negated the benefits of the gearing on my car.

I ended up using an STi9 cross drilled and nitrided crank, STi9 rods and STi9 forged pistons, built into a 2.0 CDB with oil squirters, that makes a really strong bottom end for not a lot of money for sensible road power levels. You dont need to jump on the 2.5 band wagon to have a quality engine package. The currently quickest and strongest setups out there are 2.3 and 2.0 with the likes of Zen, AndyF and RCMS. Just dont lose sight of the fact it's a road car and you will want to sell it at some stage, going 2.5 narrows the market base IMHO.

APIDavid

iTrader: (4)

I beg to differ. On the Litchfield type 25's, we had a case of bent standard rods, which brought in steel rods for all subsequent engines.

I have another experience quite recently of STI 9 rods bending and I know for sure that a number of the rally guys have a problem with bent con rods.

While 'building' the engine why not make it future-upgrade-proof ?? There is a lot of labour involved in building Subaru engines and the thought of fitting a part that may not be up to it and needs changing in 6 months time means that a lot of money could be wasted.

David APi

johnfelstead

There are plenty of 2.5's out there running well over 380BHP/Lbft (some are running 500 in hillclimb aplication, such as John Stevenson, 2nd overall at TOTB this year on the handling course) on the stock late spec rods. It makes a lot of sense labour cost wise to not strip the short motor if you are not chasing big numbers, but uprate the pistons with the drop in wiseco's or similar. Instead of a days labour to strip and clean, then reasemble it's an hour to swap the pistons on a new short motor (plus it's a nice out of the factory assembled bottom end, a piston swap is a piece of cake and doesnt disturb 90% of the assembly).

The consensus on the earlier spec rods was anything over 370 was potentially an issue on the 2.0, but you have lots of those running over 400BHP. The latest spec rods are much better than these and dont seem to be an issue at mid 400's at the stock 2.5 rev limits. Go chasing higher rpms even at the mid 400's and you will be asking a lot of the rods though, the extra stroke is significant when you start to rev it.

If you are using the engine in a rally environment then that changes things, because you will be looking at running massive boost (2.5BAR) in the midrange, to get the performance out of a restricted engine, so headline power figures are low because they dont rev, but you cant use that as a basis of comparison when you are talking about people with road car aplications.

Of course it's worth future proofing the engine if you think there is a chance of you chasing bigger numbers later, but there are a lot of people out there who wont do that, and just want a relatively cheap 380-420BHP road spec engine with simple TD05 20G or similar bolted on, adding rods and the labour could put another £800 on the bill on top of the £2000 you just spent on pistons and base short motor, it soon adds up.

Lateral Performance

iTrader: (20)

John,

John Stevenson hasn't run the stock rods for some time, because he bent them, and now runs up rated rods.

Generally speaking, I suggest the stock rods are good for 400bhp/7000rpm, with a little head room, after that, I would suggest up rating them.

The stock 2.5lt pistons are just a liability for anything much over 350bhp, and I have heard that IM are seeing piston issues even on standard cars.

For the sake of changing the pistons, you'll have a 400-420bhp engine, so long as you keep to about 7k.

Do rods, and pistons, and circa 500/500 isn't uncommon with the right supporting mod's.


Mark.

johnfelstead

John bent his stock rods when he held a gear too long to go over a finish line without shifting, he took it past 8000rpm whilst running over 500BHP and expected it to fail because of that over rev. It's documented on 22B.com.

Gear Head

iTrader: (2)

ooooo, the plot thickens!

frayz

iTrader: (7)

Interesting read

So with a 7k rev limit the stock 2.5 with some drop in forged pistons will easily cope with 450/450...

Or greater?

Lateral Performance

iTrader: (20)

John,

Yes, I'm aware of John S's antics and there have been cases of rods bending at lower RPM too.

What we do know, is that there seems to be a very big range of levels where parts are failing. Some people are claiming very high figures on the standard parts, whilst others are seeing failures at relatively very low levels.

I need to err on the side of caution, and suggest limits with enough "head room", that I'm 99.9% certain there won't be any failures at those levels.


We know some people will push harder, and get away with it, but they are prepared to accept the consequences, whilst others are not.


Mark.

johnfelstead

Yes, i understand that Mark. You have to admit Davids statement on 380BHP being the absolute limit on the rods is extremely conservative. I can understand talking about your sugestion of low 400's though just to cover your **** as a suplier.

911

Possibly what David is actually doing too?

David builds engines to last and the customer expect that of course for the investment. Look at Burn's example.

JohnS is not in that catagory, and the win is everything, Rob Harriman is the same.

Where does the '2.2' fall in amongst all this?
Also, is it fair to say many 2.5 built engines are in RA/R gear ratio cars, so 8K is needed?

johnfelstead

380 is way lower than the latest rods will work at Graham, the old classic rods will handle that, how much margin do you want? Mark with his 420 on stock rods and dropin wiseco's is already building in plenty of safety.

I am not saying if you want an 8K reving 2.5 use stock rods, if you want that then you have to change the rods, but if sticking to the stock rev limits then they are up to it.

I wouldnt expect that many 2.5's to be in RA's because of the lower rev limit, you would need uprated rods to be able to get the most out of that gearing setup, thats one of the reasons why i didnt go the 2.5 route as costs would go through the roof by comparison for the small benefit you would see on the road. The 2.0 on RA gearing is a stunning setup and is easily overlooked these days with all the 2.5 talk.

2.2 is the same crank stroke as a 2.0, so you can use more revs with them on these rods, i'll be using 8K on these rods with my 2.0 because thats what they are designed for. (the valvetrain is also designed for that on my STi5 engine, dont go doing 8K on a UK spec top end).

If you are building a competition engine then of course it makes sense to go for uprated internals, but most people arnt, they want a cost efective fast road engine, there will be far more people wanting to run in the 380-420 bracket than any other on these 2.5's IMHO.

Jay m A

iTrader: (2)

One point regarding RA boxes and 2.5's - at the 400 of each level which arguably an OE 2.5 can take, the OE RA gearbox's aren't reliable at that torque. So an uprated gearset is need which usually has longer ratios.

But funnily enough these newage 2.5 STi's with their 6 speed boxes do have short gears - the UK boxes have almost the exact same ratios 1-5 as my RA box.

What is the rev limit on the OE 2.5?