Hello guys,

Just wondering if Sti Version 3/4 pistons would fit in a Wrx Version 5 engine.

Also what factors to take into account in choosing performance rods?

Cheers

 

they would fit in terms of diameter, but I am sure all the standard pistons are stamped with a tolerance mark of a or b relating to the actual size.

I cant understand why you would need to do this. I assume you are putting in second hand pistons, which personally I wouldnt do. If you are buying new, why not just buy the proper ones?

Rods wise I would judge by envisaged strength, you can see how strong they look just by their design, also consider their weight, their strength to weight ratio can go through the roof just by removing a channel,

Consider billet rods as these will be very strong and also insist on arp bolts.

 

Adam's right, they will fit.

The main difference is in the skirt design, which makes the V5 pistons fractionally lighter. V5 pistons are also coated in molybdenum to supposedly make them more slippery in the bores. All late STi pistons are forged, but the reason is commonly misunderstood. A forged piston is substantially lighter than a cast one, meaning that the engine can rev to 8,000 without putting more load on the rods. It does not mean that they can take more boost.

There is a stamped mark on the top of block, just behind the alternator and this contains the engine build info. It looks something like:

2
4 B C
5
2 A B
5

The numbers are the main crank bearing sizes and the letters are the piston sizes. The letters relate to the piston size nearest to them. IE bottom left (A in this case) is piston number 1 and top right (C in this case) is piston number 4.

 

There are several things that need to be taken into account here.

1) V5 pistons are a different design to V4.

2) V5 runs a much thinner head gasket, but is still 8:1 compression.

3) Difference in piston deck height (?)

4) Difference in head volume.

Piston dia' tollerance is a minor issue, compared to the effects of the above points !!!

IMO, if you're going to the trouble of rebuilding an engine, with uprated rods (to run more boost ?) you don't compromise on the pistons.

Mark.

 

Surely a set of STI 5 forged pistons would be more appropriate for a version 5 with the phase 2 block and heads. The piston forms the bottom of the combustion chamber, and, iirc, the piston crown design was changed significantly in the transision between phase 1 and 2 engines.

 

Sh*t.....there's an echo in here

Mark.

 

Cheers guys,

Adam - the reason I am asking is because I sourced a set of V 3 pistons for 200 quid. You are right though, I should not put second hand pistons on second thought.

Pete- cheers for letting me know piston numbers

Mark - thanks mate, did not know that V 5 headgasket is thinner

 

Mark,

Just trying to help make your post sound more convincing.

Moray
bbs.22b.com

 

If the pistons are in good condition, and are, Im sure of very good quality, being original sti items, Then what is the point of buying new ones at perhaps 4 or 5 times the price?? That extra £500+ could be better spent on rods or something. Is a set of new pistons at near £1000 incl rings going to give more power??, make it last longer?? I doubt it. Are the second hand ones going to fall apart? If the pistons suit your head + gasket combination for the correct CR, then whats the problem, with saving some money?

 

"Im sure of very good quality, being original sti items"

Like the STI rods ?

None of these parts are going to give you "more power", in them selves, but using parts of an unknown quantity, for such a critical application, seems like a "false" economy to me.

They may, or may not be ok, but finding out, could be a very expensive experience.

That aside, the V4 pistons don't match the V5 engine.

Mark.

 

As I do have only limited experience with subaru engines, I ask... How many engines have you actually had with rod failure? There is another post somewhere recently, where people indicated that the rod bolts were the weakness, and other posts a while ago saying that the actual rods were just crap. I have only heard of about 2 engines with rod failure, I have heard of more std mundane cars with rod failure than subarus, does this mean that the other tens of thousands of that model's rods will break too? While it can make more sense to uprate with new components where possible, it does get bloody expensive. Even a simple rebuild, costs a lot without replacing every single bit of it.. why not replace the crank too, as some people seem to think they arent that strong either. better safe than sorry. False economy is also replacing parts that arent necesssary. While the engine is out, may as well buy a high perf. clutch, and rebuild the gearbox, with new straight cut gears too, as it may break in a few thousand miles if you dont, maybe a bigger better turbo too. Most people here im sure dont have the price of the car again, to spend on replacing otherwise good parts, with super strong bits. Are you saying that STI pistons are crap? STI pistons would be more than adequate for 98% of engines anyone will drive. Better ones will not make any more power, and as long as the engine is running correctly, should not break, ie melt etc. Detonation will destroy a brand new piston just as easily as a sceond hand one.
A lot of people in the tuning industry like to sell people things, so a lot would say that this need replaced at 100 bhp, and this at 101bhp...etc etc While it may not be totally relevant to subarus, I learnt a lot with my mini. Everyone claimed that over say 100bhp, you needed steel crank, rods, steel main bearing caps, ARP nuts bolts, washers..anything, Copper head gasket (why anyone would use such a crap gasket is beyond me ) Maybe I was lucky, but I have run my mini ( metro turbo engine ) with std crank, rods and pistons for over 60,000 very hard miles, running 1.5 bar boost, and would estimate 180bhp. I have melted pistons due to my fault, but otherwise the engine internals have gave no bother. The only thing to break regularly is the gearbox and diff, despite having straight cut gears. Again, another weakness they all claimed was the modern type CV joint driveshafts, again which have given me no problems at all.
There is a lot to be said for leaving things until they break, because it really can save you a lot of money, as more often than not, they wont break ( I only speak from my experience turbocharging various N/A engines ). If you really are seeking big power, then it probably is a wise idea to uprate, but there are very few I have heard of in the UK with big power. I dont quite understand why when subarus are so popular very few are making power. They seem to have no trouble getting horsepower in Australia, why are we so lagging behind?
Maybe ( probably )all of the above is waffle, but my thoughts anyway. I am always loathe to believe people who are selling things, when they tell me I have to buy this to make it reliable for anothr 5 bhp.
If enough people keep saying it then people will start to believe it. Doesnt a big dump valve make your car faster...WHOOOSH
And as you say, the v3 wont fit the v5 anyway due to different pin heights.


[Edited by ustolemyname??stevieturbo - 6/30/2002 3:24:16 AM]

 

Steve,

Firstly, this thread was about using V4 pistons, in a V5. That's an easy one, since they don't fit, at leats not correctly.

Then there is the issue of using secondhand parts, not for the purposes of a "cheap" rebuild, but with a view to being able to run much more boost. Well, not knowing the integrity of those parts, but knowing what they are intended for, it would be poor advice to say they will be fine.

I also know of rod failures, and put into context, they are very few, but, very few people are prepared to push their cars to the point where the rod are likely to break. "Most" of the people wanting to push the engines that far will uprate the internals first.

As for associated parts, well, it's reckoned that the crank is good for 400bhp, some run far more. You only need to change the clutch if it slips.

Gearbox's is an interesting one. I don't know what power my car was producing, but it had a big hybrid, Motec, and was max'ing out 550cc injectors. I was still on my original box, but others have had failed box's with far less power. Again, if you keep breaking box's, get an uprated one, if you don't, there's no problem.

The level of rebuild, should be based on what the owner wants to achieve, and the parts used, based on the experiences of people far more qualified than us.

If you want to be a guiney pig, with your own car, that's fine, but it's not something that should be recommended to others, when you can't guaranty the results.

Just because there aren't people posting that they have cars with big BHP, doesn't mean that they don't exist.

However, I do agree that there is a distinct lack of big power cars. IMO, it just shows the "real" level of ability of the so called "Subaru experts", in this country !!!.

Mark.

 

Steve - You got mail

 

Very true about the guinea pig bit. I do all my own work, so dont have to worry about paying people for labour which can be very expensive. I also do firmly believe if it doesnt break, dont fix it, so I have no problem trying something that others may be a little more cautious about as I am the one doing the repairs afterwards. Maybe some people here have responded to the Top 10 Street car shootout, run by Trackdaylus, and the 200mph Club. Myself, and a friend ( legacy turbo, mentioned elsewhere in my posts ) will be attending. While I will not be driving my subaru ( it just aint that quick, and still lying in bits ) I will be bringing my 79 Granada, with 4.6 rover V8 twin turbo. No idea what it will go like as I am still awaiting fuel pump and injectors, so it will be very tight getting it ready in time. Its a new engine, and I am newly fitting DTAFast Fuel injection to it also, after just blowing through a holley, with a welded dizzy before. May be primitive, but very effective.