Questions about EJ25/EJ22 Hybrid High Compression engine

Zac86 · 04-15-2009, 09:55 PM

looking at doing this build myself in the near future. Can use any EJ25 bottom end? is there a better one? Also about the EJ22 heads? what year is the best? I also heard you can use a EJ25 ECU to raise the revlimit another 500RPM? This engine will be going into my 98 legacy which currently has a EJ22. Will using a phase 2 EJ25 ECU remove my need to have a MAF sensor?

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PhilSine · 04-15-2009, 10:06 PM

Compression could be around 10.6:1 depending on which pistons you use. If you used a thin enough Cometic head gasket you could get as high as 11.9:1 or so.

I don't know enough about head gasket particulars to offer any pro vs con advice with respect to thickness.

I think the biggest problem you'd run into is the fact that the EJ22 SOHC heads may not flow enough air to really be able to accomodate the increase in compression in NA form. I have no data on this so it is something you'd have to explore further.

For all those wondering, I have a compression ratio calculator spreadsheet made up by Matt Jones (of NASIOC) but I am uncertain of just how accurate they are. I have run the math to check his calculations on various set-ups based upon information I found published and my numbers were often very close to his.

Also...

You can raise the rev limit in your 98 EJ22 ECU by soldering in a larger crystal that controls the rev limit. It's very easy to do and cheap too.

Zac86 · 04-15-2009, 10:10 PM

Quote:

Originally Posted by PhilSine

Compression could be around 10.6:1 depending on which pistons you use. If you used a thin enough Cometic head gasket you could get as high as 11.9:1 or so.

I don't know enough about head gasket particulars to offer any pro vs con advice with respect to thickness.

I think the biggest problem you'd run into is the fact that the EJ22 SOHC heads may not flow enough air to really be able to accomodate the increase in compression in NA form. I have no data on this so it is something you'd have to explore further.

For all those wondering, I have a compression ratio calculator spreadsheet made up by Matt Jones (of NASIOC) but I am uncertain of just how accurate they are. I have run the math to check his calculations on various set-ups based upon information I found published and my numbers were often very close to his.

I'm aware of the heads not being able to flow properly so I'm ready to learn how to PnP properly so the heads flow well all the way up to 7000RPM.

PhilSine · 04-15-2009, 10:13 PM

I edited for the rev limit raising so please re-read the bottom of my first post.

04-15-2009, 09:55 PM	#1
Zac86 Scooby Newbie Member#: 136919 Join Date: Jan 2007 Chapter/Region: W. Canada Location: North Pole Vehicle: 2005 Legacy GT Wagon ABP	Questions about EJ25/EJ22 Hybrid High Compression engine looking at doing this build myself in the near future. Can use any EJ25 bottom end? is there a better one? Also about the EJ22 heads? what year is the best? I also heard you can use a EJ25 ECU to raise the revlimit another 500RPM? This engine will be going into my 98 legacy which currently has a EJ22. Will using a phase 2 EJ25 ECU remove my need to have a MAF sensor? * Registered users of the site do not see these ads.

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04-15-2009, 10:06 PM	#2
PhilSine Scooby Specialist Member#: 130489 Join Date: Oct 2006 Chapter/Region: MWSOC Location: Superior, WI Vehicle: 02 WRX Wagon (WRB) 98 OBS-T, 91 Legacy SS	Compression could be around 10.6:1 depending on which pistons you use. If you used a thin enough Cometic head gasket you could get as high as 11.9:1 or so. I don't know enough about head gasket particulars to offer any pro vs con advice with respect to thickness. I think the biggest problem you'd run into is the fact that the EJ22 SOHC heads may not flow enough air to really be able to accomodate the increase in compression in NA form. I have no data on this so it is something you'd have to explore further. For all those wondering, I have a compression ratio calculator spreadsheet made up by Matt Jones (of NASIOC) but I am uncertain of just how accurate they are. I have run the math to check his calculations on various set-ups based upon information I found published and my numbers were often very close to his. Also... You can raise the rev limit in your 98 EJ22 ECU by soldering in a larger crystal that controls the rev limit. It's very easy to do and cheap too.

04-15-2009, 10:13 PM	#4
PhilSine Scooby Specialist Member#: 130489 Join Date: Oct 2006 Chapter/Region: MWSOC Location: Superior, WI Vehicle: 02 WRX Wagon (WRB) 98 OBS-T, 91 Legacy SS	I edited for the rev limit raising so please re-read the bottom of my first post.