Hia, planning my next stage of mods, and a forged build is required.

I've got a 2007 WRX with 58k km on the clock (36k miles), running a mild tune atm.

Induction, STi Topmount, 3' Decat, Remap. 320/360 according to the dyno.

Next I want to go 400+/400+ (i dont much care if its 401 or 420), and for that i need a forged build.

Locally, everyone suggest to go for oversized pistons, but that would mean extra machining costs and this that and the other. Is there any actual "benefit" of doing so? I'd suspect with "only" 36k on the clock, that the cylinders should be pretty neat, and any honing/machining isn't needed, and that I could go with STD size pistons.

Ive also got a question regarding the compression ratio: i can apparently get cosworth pistons with 8.2:1 cr or 9.2:1 cr, but I don't know which to get. Iirc, the stock STi cr is 8.2:1.
I would have though a higher cr is counterproductive to a turbo'd car?

Which brings me onto the next question, head gasket thickness. these seem the be available in .78mm, 1.1mm and 1.6mm. quick calc suggest going for the 1.6mm would allow for cr of 8.2:1, but again, I'm at a loss as to which to actually go for.

Similarly, what else would i definately need to purchase along with this:

Pistons
Conrods
Head Gaskets
Head Studs

Any help would be greatly apprecciated.

 

alright mate the machining work doesnt cost much if you wanted over size,as for thickness of hg's that needs to be worked out once all machining work has been carried out ie head skim/block if needed and piston size.
your shopping list will need to include main and big bearings,block seal kit and sealant for block,also inlet/exhaust gaskets and 4 cam seals and 1 front and rear crank seal and possibly rocker cover one's if your in any doubt as theses can leak for fun.

 

The standard cylinders walls are already very thin, machining them further will only weaken a weakpoint even more.

I would go with std piston sizes of 99.5mm

The head gasket needs to be decided once the pistons are in the block. Normally the pistons sit below the top of the block faces (circa 0.3mm below) and use a 0.78 gasket. If the new pistons sit higher than that then use the gasket to space to the same amount. I have just gone through this and my pistons were completely flush with the top of the block so I used 1.1mm gaskets.

As said, a full engine gasket set is needed and LOTS of time to clean everything and prepare prior to reassembly.

 

probably cheaper to sell the WRX and buy an STi and tune it to 400HP

 

You would think so, but cars are ridiculously expensive here. A new sti is around 70k, 07s still fetch around 40k, whereas my wrx is worth about 20k.

Thanks for the tips though, food for thought.

 

holy $hit didn't notice finland :-( unlucky chap

 

oh my god mate thats shocking prices

 

Am i reading this correct ?

70,000.00 FIM = 9,852.38 GBP
40,000.00 FIM = 5,629.43 GBP
20,000.00 FIM = 2,814.71 GBP
sounds bloody cheap to me

or is it

70,000.00 EUR = 58,546.25 GBP
40,000.00 EUR = 33,455.00 GBP
20,000.00 EUR = 16,727.50 GBP
as this sounds stupid prices

Rob

 

Euro mate, we havent had Finn Marks since 99.

 

Buy one over here and drive back, or do you drive on the wrong side of the road, lol.

Anders

 

We drive on the "wrong" side, also, if i were to buy one from the UK, id have to pay import tax and vat when i bring it to finland..

Been down that road once before, not doing it again

 

The problem is that the engine will destroy your gearbox, clutch and rear diff. giving half a chance.

Sorry ! I know someone running 360/390 on his.

dunx

P.S. An STI transmission transplant may be a better idea !