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I have a rod knock on my 22t 20h head combo. My question is can i drop an ej257 short block in and run that? will there be an issue with the pistons hitting the heads? will it run safe?

I have ej20h heads from a 96 twin turbo legacy. I believe the current compression is 7.6:1

HI,
I found this POST of guy running the EJ257/EJ25D combo....should be similar to yours.

And this ONE...good info as I'd be cautious of the HG you use.
And look on Rs25.com.

If I ever have to replace mine, I'd like to do the same...need more low-end.

GL,
TD

Thanks TD.

You will have to run a 2.5L HG for sure. The thickness is up to you...depending on target CR. Just guessing, I think you'll have to run a little thicker HG then stock STi because of the smaller combustion chamber size....49cc on the 20H vs approx 52-54cc on the ej257 heads (I never personally CC'd these).

One thing to worry about is detonation while running pump gas. The smaller diameter combustion chambers (2.0L) could create hot spots of compressed gases near the edges where the larger pistons overlap the edge of the combustion chambers. I have seen quite a few engine failures due to this phenomenon on ej257/205 hybrid blocks (but only on 91 octane, on E85 there is greater resistance to knock/detonation).

To alleviate this, the combustion chambers can be machined to match the ej257 bore size. But is it worth the price/labor?...depends on end product quality goals.

Yeah That's what I'm reading up on now. Here we have 92 octane, So I don't know if that would matter or not. I guess I'll have to go to PIA and see what machine shop they use to get an idea. This http://www.rs25.com/forums/f128/t144200 ... heads.html states that it is possible. I haven't found any results yet on longevity. Must keep searching.

It is similar theoretically to all the ej257/205 hybrids, since the combustion chambers on the ej205 heads are the same volume and also designed for the 2.0L bore.

There is info out there on guys who had the combustion chambers cnc'd or machined. This might be a good option if the price is not too high, for safety insurance and peace of mind.

One important thing to observe is all the coolant and oil flow passages between the heads, HG and block. Make sure there is no obstructions due to hardware compatibility.

And there is the issue of some early 20g heads being different then later models (larger combustion chambers on the early design)...make sure you know which ones you have when doing the CR calculations for HG thickness.

Also found this

snowman4us wrote:I would disagree with you Jon. Ive opened up a couple of Ej20G's, and only 1/2 of them where closed deck. There is no consistancy with them.

And my good friend runs a EJ257 block with EJ20G heads, his CR is about 9.2:1, so his spoolup is realy fast. And on the dyno, he made 50HP more then i did at 3000RPM due to the higher CR and the extra .5L.

And i consider a stage2+ what i have. TBE, Intake, Bigger TMIC, MBC, PFC, im also running a TDo5H 16G, that came stock on the Sedans(my motor came out of a wagon, so i had a tdo4 stock)....so technicaly im at a stage3, but sense the turbo i have just came of a diff. body style (GC) i consider it stock.

As far as tuning goes, PFC is the best way to go. Its the cheapest, its a Plug and Play, and will do just as much as the high end stand alones will(almost)

http://www.rs25.com/forums/f128/t53808- ... nions.html