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Old 09-23-2010, 03:43 AM   #35
Scooby Guru
Member#: 4414
Join Date: Feb 2001
Location: Dublin, CA
2002 WRX Sedan
Midnight Black


1) You need AVCS. Period. AVCS rules under 4k, where you'll be a lot.
2) You can run higher compression than stock, and likely want to if running E85
3) You want a 79mm crank for your desired powerband (the crank in the EJ25x). THe 2.0L rod:stroke ratio is designed around high RPM. The 2.5L rod:stroke ratio is MUCH better with torque, even if paired with the smaller bore. I would not design an engine for your purpose around the 75mm crank on the 2.0L or 2.2L engine. The 79mm crank matches your desires much better.

I think the ideal options would be:
2.0L block + 2.5L crank + custom ~10:1 pistons to get the height right + JDM AVCS heads (~2125cc)
2.5L longblock split it and put in ~10:1 pistons (~2460cc)

Turbo size has already been mentioned. vf34, 16G, maybe 16GEvoIII max.

2.1L vs. 2.5L comparisons:

These are larger turbos, and the 2.1L in these do not have AVCS, which is worth 2-300 RPM in spool. These are nice comparisons because they are both same turbo on the same dyno. The Motion one isn't exact same config, but the older Gruppe-S one is (but with a MUCH larger turbo than you're going to want to use).

E85 road dynos of turbos the size you're looking at:

Tuning house Dyno charts don't work well for looking at spool area. They load VERY differently than the road. Some do alright, but most do not. Look at where the road dynos are making 200 torque, as these will be more accurate for looking at that area of the curve:
2.0L ~2900
2.5L ~2600
A 2.1L will be in-between. Higher compression will push this number lower. Also, tuners don't really like to start pulls at 1500 RPM or anything, so sometimes this can lead to inaccurate torque numbers at very low RPMs.

Another thing to keep in mind is that these are all AWD with probably 20+% driveline losses. You'll be more than 205 torque to the wheels at decently low RPM on any of these engines. You'll definitely be better off than your 205 torque engine, but I'd assume you'll be lower torque at VERY low RPMs. Don't know how often your south of 2k, or the exact shape of your old engine's torque curve, but there's no replacement for displacement at the extremely low end unless you want to talk a TD04 on a 2.5L. Those will hit 200+ torque immediately, but they are topped by 5k. You can't get around turbo cars having smaller torque-bands than N/A cars. However peak torque of 350 instead of 200 probably makes up for it.

TD04 on 2.0L and 2.5L for reference:

The 2.5L will likely be cheaper, as the easiest way to get all the parts for 2.1L will be a JDM swap, then split and replace pistons & crank. You would get twinscroll if you get a v8. The v8 twinscroll header is quite good.

JDM vf37 dynos (pump gas):

Imagine those with E85 and the torque curve shifted left a bit from the longer 79mm stroke. I really think that's your ideal (Subaru based) motor for the XP classing. 2.5L is a cheaper fallback but clearly superior for the flat weight racing.
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Last edited by Concillian; 09-23-2010 at 10:34 AM.
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