High Compression Build

samandw · Post by **samandw** » Thu Jan 22, 2009 10:36 pm

Hello Folks,

I'm new to the forum but not new to Subies or mechanics. I'm investigating the idea of building a custom 1990-1994 Legacy wagon and have decided I want to build a high-compression experimental engine. (I'm an engineer as well as a gearhead).

My question is this:
What is the highest compression ratio I can get out of an ej20, ej22, or ej25 before valve/piston interference becomes a problem? I'm aiming for stock or lower RPM's. I don't mind going the custom piston route if I have to . . .any gurus out there?

RJ93SS · Post by **RJ93SS** » Thu Jan 22, 2009 10:56 pm

yup, but not me. they'll be here

SLODRIVE · Post by **SLODRIVE** » Thu Jan 22, 2009 11:25 pm

If it's any help, these guys actually stock 11.2:1 pistons for EJ25's, and they also do 12.5:1 pistons...they might be worth looking up for further insight if you want more compression than that.

http://www.techworkseng.com/Products/Su ... istons.htm

samandw · Post by **samandw** » Thu Jan 22, 2009 11:29 pm

Thanks SLODRIVE!

Yeah 12.5:1 would be about the minimum. Ideally I'd like to get about 14:1 ala Toyota Prius (which should give you an idea of what I'm planning) . . .

SLODRIVE · Post by **SLODRIVE** » Thu Jan 22, 2009 11:32 pm

Dang, I was hoping to see a brutal alky motor.

JK...good luck and welcome to the site!

evolutionmovement · Post by **evolutionmovement** » Fri Jan 23, 2009 12:04 am

Atkinson cycle? You're going to need some custom cams as well. Are you going to try to mate it to a supercharger like in a Miller-cycle or mate it to an electric motor for low rpms? Variable valve timing would be nice for the low-end torque issues (where it could run like an Otto-cycle) and direct injection could help prevent detonation and the fueling under delayed intake closing. Could a stand alone handle the fueling change? I don't know how you'd do DI without available heads for the EJs anyway (come on, Subaru, get with the friggin' program—do DI so you can get some respectable mileage and I'll have a use for you again ... to get the heads off a wreck and retrofit them to an EJ22.)

Morgie got custom pistons from CP pistons for a reasonable cost a few years ago.

samandw · Post by **samandw** » Fri Jan 23, 2009 12:09 am

You nailed it. Aktinson cycle. Custom cams. I do plan a low-pressure remote-turbo setup as well though, so it should make stock or better power levels. I'm hoping I can make a stand-alone run it.

samandw · Post by **samandw** » Fri Jan 23, 2009 12:10 am

I don't think I'll need DI with what I have planned. . .

AWD_addict · Post by **AWD_addict** » Fri Jan 23, 2009 12:47 am

That's good, cause if you are shy of custom pistons think about custom heads for DI. Too much!

evolutionmovement · Post by **evolutionmovement** » Fri Jan 23, 2009 4:43 am

I was just extrapolating on the lines of using VVT to switch from Otto to new Atkinson, where DI would likely be necessary for the Otto cycle rpms. Would also allow better efficiency, but I can't see any affordable way of doing it.

samandw · Post by **samandw** » Fri Jan 23, 2009 5:44 pm

Yeah, I'm going to go the non-VVT route if/when I do this build. Still curious if anyone knows the max compression ratio those Subie engines can get too. . .

evolutionmovement · Post by **evolutionmovement** » Fri Jan 23, 2009 6:24 pm

So am I. I doubt anyone's gone above 12.5 at the very most as N/A builds are rare and that would require some serious fueling and money. I always wondered if the EJ22T could handle being a diesel, but the availability (more or less) of Subaru's diesel engine makes that a pointlessly expensive exercise.

samandw · Post by **samandw** » Wed Jan 28, 2009 8:16 pm

bump.

Surely someone knows valve-piston interference measurements and combustion chamber volumes. . .anybody?

legacycentral bbs

A forum for 89-94 BC-BF(BJ) Legacy Owners and Fans

High Compression Build

High Compression Build