Transmission Internals Compatibility Question

I've been day dreaming about doing a manual swap. I've got a 5EAT in my OB 3.0R. It's FDR is 3.272 and 5th gear is 0.834. This gives me a total Engine RPM to Wheel RPM ratio (called EWR from now on) of 2.728848:1.

I've been thinking it would be cool to do a manual swap (I don't mind doing wiring, nor buying a wrecked manual legacy/ob to pull parts from). It's probably more realistic to just get an OBXT/LGT manual to start with, but I love my EZ30.

However, I've noticed that most cheap 6 speeds I can find have a 6th gear of 0.756 and a final drive of 3.9. This gives me a EWR of 2.9484, this is about 8% higher than my current EWR. I want lower revs at highway speeds, not more.

Then I started doing some research about the transmissions in the US Spec Bs and JDM Forester STIs. Their 6th gear is 0.707. That is good. Then I found that the 06-07 WRX non-STI manual has a FDR of 3.7. The EWR for this hypothetical build is 2.6159, which is 4.1% lower than my current EWR.

I know it would be a pain and expensive collecting all the parts from all different vehicles, but would it be possible to grab the 3.7:1 ring and pinion from the 5 speed WRX and stick it in a 6 speed transmission? I don't know what is swappable between 6 speeds and 5 speeds, if anything. This is what my question is.

I know a 4.1% lower EWR won't do much. At 70 MPH, RPMs will go from ~2333 to ~2237. 4.1% lower speed means 4.1% more torque to make the same power for cruising. I'm hoping this will push the engine further into the low BSFC range and save me some fuel. Even if it doesn't, it would still be cool to have a Frankenstein's monster transmission.

I too owned a 3.0R in GRP and loved the EZ30. It was 5EAT, and like you, I searched for months on how to manual swap. Ultimately, I came to the conclusion that it would have been more feasible to purchase a 6MT from an EDM 3.0R and send it over here. I never played around with gear ratios, or Frankenstein-ing a 5MT. That just sounds pricey, then again so is shipping over a transmission. It's an idea though, hassle-free in comparison to creating your own transmission. You'll probably need the ECU* from a Legacy 3.0R too.

EDIT:- ECO > ECU

Ya, I considered that. I'm not sure how reputable eBay sellers are, but I found a few EZ30s paired with 6 speeds. Unfortunately it still would've had a slightly higher EWR. If I do this, I want to do it someway that isn't a trade off, especially if it SEEMS like it doesn't have to be.

It's also nice to know that I'm in good company with those who like the EZ30. Thanks for that.

If I do this, I want to do it someway that isn't a trade off, especially if it SEEMS like it doesn't have to be.

Some things to consider. If in theory, you could match up the 5 speed rear diff/front diff with a 6 speed Spec B. or Forester STI transmission, you'd still have a weak front/rear differential setup over the much stronger R180 assembly. If you're not adding power to your H6, that's probably acceptable, but the other problem is you could very easily go the wrong direction, where you make the gears too long and now your vehicle is considerably slower and passing in high gear becomes near impossible. Drivability could really be effected with gears that are too long and the car could be much less enjoyable to drive spiritedly.

you could contact PPG have a custom 6th gear made in whatever size you wanted. A really overdriven 6th would be almost useless except for long cruise mode.

The jdm forester sti 6speed 's 6th gear technically would give you 207mph at 8000rpm if you had the engine to drive it. Enough to finish in the top 10 at the silver states open road race. Not sure where else you'd ever use it.

Some things to consider. If in theory, you could match up the 5 speed rear diff/front diff with a 6 speed Spec B. or Forester STI transmission, you'd still have a weak front/rear differential setup over the much stronger R180 assembly. If you're not adding power to your H6, that's probably acceptable, but the other problem is you could very easily go the wrong direction, where you make the gears too long and now your vehicle is considerably slower and passing in high gear becomes near impossible. Drivability could really be effected with gears that are too long and the car could be much less enjoyable to drive spiritedly.

I've never owned a manual car before (driven them, but not extensively). I kinda wanted to ask this too. The EWR for first gear on my 5EAT is 11.58288, the EWR for the Spec B is 14.1804 (with a 3.9 FDR). The Spec B obviously has more leverage in 1st gear. Is this because it's a manual, because it's a sporty transmission, or both? Do you think having 6th gear as a cruising only gear would be an issue? To me, it makes sense that 6th gear is just for cruising, but I don't have lots of manual transmission experience.

you could contact PPG have a custom 6th gear made in whatever size you wanted. A really overdriven 6th would be almost useless except for long cruise mode.

 

The jdm forester sti 6speed 's 6th gear technically would give you 207mph at 8000rpm if you had the engine to drive it. Enough to finish in the top 10 at the silver states open road race. Not sure where else you'd ever use it.

Ooo, that sounds marvelously expensive. As for the 2010 Outback 6MT, I never considered it, nor did I know it existed. I'll do some research on it and if it shares parts with other vehicles that can be switched around.

I contacted MRT Performance, they said they haven't tried something like that. Although, they suggested I just put some bigger tires on it. I gotta laugh at myself, I never thought of that. I guess I should state my intentions with my car while I'm at it. I come from Toyota. Getting 21.6 MPG with 93 has certainly been an adjustment from 35 MPG with 87. I know I should expect this really, but 21.6? Really? I live 30 minutes from work and I like to take long road trips. Don't get me wrong, I want to make my car fast, but 6th gear to me is not the go fast gear. 6th gear, to me, should be the get-good-gas-mileage-and-drive-comfortably gear.

Okay, so I just downloaded the 2010 OB FSM. The 6MT for the 2.5L Turbo model has the EXACT same gearing as the 2015+ WRX non-STI. Even the FDR is the exact same. Looking at the assembly, it's almost identical to the 2015+ WRX. Only a few little changes here and there. I don't suppose anyone has done a 3.9 FDR swap to a 2010 LGT?

I guess if I really wanted to:

Don't get me wrong, I want to make my car fast, but 6th gear to me is not the go fast gear. 6th gear, to me, should be the get-good-gas-mileage-and-drive-comfortably gear.

Which is all well and good, but when you mess with final drive, you're changing the spacing of all the gears, not just 6th. Everything gets longer. Just some things you have to consider.

Are you okay downshifting to 5th when you need to pass someone? Are you okay with acceleration being slower to achieve a longer 6th gear? These are the things you'll likely encounter if you go for a super long 6th.

Something else to keep in mind; all 6 speeds aren't created equal. The 15+ WRX 6 speed and outback 6 speed transmissions aren't nearly as strong as an STI, Spec. B or Forester STI transmission.