Low Compression Build EJ25D

[Chrisg03]

So a friend of mine has a bunch of spare parts laying around that he is willing to basically donateto me since he has no use for them any more. One of those items being an EJ25D and he has a set of Sti pistons i was wondering if it was possible to install those piston in the EJ25D to achieve a low compression Turboable engine that wouldnt see anything above 18psi..
If not what can be done? that and thicker HG? help me out here
EM is not a problem at the time

[Legacy777]

Have you done the calculations on what the compression ratio would be with the resulting heads & pistons? I'd start there to see where you end up. The other comment I'll make is that in the long run you end up spending a lot more money and time putting stuff together that wasn't exactly made to go together vs. buying stuff that was made to go together.

I think what you're asking about doing is possible, but I don't think you're going to save a lot of money in the long run, and also may not be happy with the final product.

[Chrisg03]

No i havent im not sure how to thats why i was asking.. oem sti pistons and ej25d DOHC NA heads

[86BRATMAN]

With an sti head gasket you'll be looking at 9:1, but still stuck with the 48mm rod journals unless it's a 99 ej25d

[Chrisg03]

I do have the crank and rods off an 02 LGT but do 3mm really make that much of a difference?

[86BRATMAN]

It can, I would highly recomend rebuilding the bottom end. You never truely know what has happened to an engine in its lifetime, so new bearings, rings, seals, bolts could be the difference in 18 psi reliable and fun or 18 psi spun bearing.

[Chrisg03]

Yes a rebuild is a definite yes i just want to know what it takes to make it have a comp ratio of 9 or lower preferably 8.5 as that is what robtune is recommended for .... it seems i basically have access to everything i need to do it for free (or so i think) with the exception of bearings and gaskets which isnt a problem at all. iknow building an open deck 2.5 isnt popular but its not like i compete in anything or even race i just like being able to pull on the freeway or when i have to.

[86BRATMAN]

If you can find some 96 ej25d heads it will put you in the cr range you want to be in for the rob tune. Sti pistons with those heads will land you at 8.3-8.5:1

[Chrisg03]

okay awesome thank you very much!

[macipusy]

Doesn't the STi have a 8.5:1 CR from the factory. It also has 52cc combustion chambers vs 46.6cc on the 25D heads. IIRC, that volume is correct for the 25D head combustion chamber.

Let's assume the STi has 8.5:1 CR stock. If you just swap the 25D heads (46.6cc) and keep all else equal, you jump to 9:1 CR. Thicker HG will bring it down a tad.

The online CR calculators are a great tool to get an idea of what magnitude of changes to the system considerably affect the CR.

[86BRATMAN]

96 ej25d heads are 53cc or 57cc depending on where you read.

[Chrisg03]

Good info thanks guys
Making my projects easier one step at a time !

[macipusy]

The 98 DOHC heads are the non HLA 25D heads. I know 100% these have 46.6cc combustion chambers. I have volume tested many heads of this type.

I thought 96 HLA ej25D heads were around 50cc. I believe 57cc is not accurate. I don't think there is a DOHC head with a larger combusiton chamber volume then the ej257. I would strongly recommend volume testing this style of 25D heads before doing any CR calculations.

[Chrisg03]

i see okay thanks for your input assuming they are only 50cc that would mean i need a new wet of pistons right?

[86BRATMAN]

Early Legacy ej20g have 57cc chambers as well. They're identified by 2 bolt holes in around the spark plug hole. That is clearly documented here, and other sources.

I have 2 sets of 96 heads, and there is clearly more than 3cc difference between them and 97-99 head.

I don't have any real equipment to cc heads, but I can make a stand and make use of one of my 1cc insulin needles to get an approximate cc.

[86BRATMAN]

With my half assed method I was about to get 56cc's on the 96 head. Definitely not suggesting that is the end all be all measurement. But I will contact some local shops to see if they can give an exact reading with proper equipment without charging me an arm or something.

[Chrisg03]

Nice 96 heads would be much cheaper than different pistons or turbo specific heads

[86BRATMAN]

One small detail makes the 96's not a great candidate for an ej20/22, the combination chamber width is slightly larger than standard sized ej2 pistons. That said my friend is running them in a 97 GT with an ej22e bottom end and a td04 with no detectable detonation, which was my initial concern.