This wasn’t a modification I was planning on doing any time soon. However, a deal popped up and I had to jump on it. For this install, we will be swapping to an STi 6MT as cheaply as possible. That means stock rear R160 diff and no DCCD controller. We are also keeping our relatively new 5MT clutch and flywheel to keep costs down.
|Transmission||2006-2007 STi 6MT (1.1 Transfer Gear)|
+Axle Stubs Kit (link)
|Driveshaft||2002-2007 WRX Auto Transmission or;|
2004-2007 STi (Requires R180 Flange [link])
|DCCD Wiring Harness||iWire (link)|
To remove the old 5MT, we need to remove the TMIC, starter, downpipe, mid-pipe, driveshaft, drain transmission fluid, release clutch fork, pop front axles out and disconnect the shifter linkage. I also had to loosen the VF37 turbo on the up-pipe as the flange hit the bellhousing.
I recommend placing a jack or jack stand under the engine to keep it from tilting forward on you once you pull the trans away as the engine mounts are at the back of the engine. I would also recommend using a transmission jack as it gives you more control over the transmission; especially when you need to put the very heavy 6MT in.
With the 5MT out, we can compare the 5MT and 6MT.
The 6MT is clearly bigger, longer, and unfortunately, heavier than the 5MT (200lbs vs 120lbs). They are both pull style so we can reuse all of the Bugeye clutch parts.
Getting the 6MT in was a chore. First, you need to get it on the trans jack while underneath the car. I had my car as high as I could get it on jack stands and the 6MT barely fit under the car without the trans jack. Once, I had it under the car I lifted one end of the transmission and placed scrap wood underneath each end until it was high enough to transfer the trans over to the jack.
However, once the 6MT is on the jack, it’s just a matter of raising it up to mate with the engine. The hardest part of this process is getting the trans onto the lower studs before the trans hits the tunnel.
Once the trans is close, I spin the engine over to line up the splines on the clutch and the input shaft. This is also a good time to finish installing the clutch fork on the throwout bearing.
I picked up a new clutch fork boot. I recommend putting this on ASAP. I have dropped a bolt into that opening before and it sucks to have to pull everything apart again just to find it.
For wiring, I went with iWire’s DCCD harness. I don’t have a DCCD controller just yet so we will just use the neutral/reverse harness. If you need to save some money I think you can swap over the 5MT harness and cut the DCCD wires.
For axles, if your 5MT has stubs, then you can pick up an axle stub kit from Flatirons. I should have gone this route but I tried to save a few bucks on eBay. The vendor only shipped one stub and couldn’t locate the third stub listed on their page. I probably should have returned the stub for a full refund but I thought I would be able to find another used one.
A friend had some front axles for sale so I ended up buying his male end axles.
The next item is the rear diff flange. The R180 flange is slightly larger than the R160 so we need to swap them out. If we had a WRX auto driveshaft we could skip this step. The R180 flange came off pretty easy but the R160 flange put up a fight. I had to use a puller to remove it. If you don’t have an R180 flange, Flatirons sells those as well.
Swapping the flanges at this point is pretty straightforward. Gutentight or approximately 135 ft-lbs. I think it is the same torque as the axle nut.
Then we can throw in the STI driveshaft. I repainted them a few months ago.
At this point, it is just basic reassembly. Before putting the TMIC back on I would recommend filling up the 6MT with fluid.
I chose Motul Gear 300 over Extra-S. It was cheaper and I’ve heard good reviews about Motul.
After a couple of test drives, I had a popping noise on hard turns. It was the pinch bolt on my ball joint. I gave it an 1/8th of a turn and the noise went away.
I have only driven this setup for a couple of days, however, I can say that this gearing is definitely better suited for the EJ207 vs the EJ257. The shorter first and second gears were much needed to make up for the lack of low-end torque on the EJ207. And the slightly overdriven 5th is better for that semi-highway style driving in hilly East Tennessee.
One thing I miss is my tall boi shifter. I kept reaching for it right next to my steering wheel and I ended up grabbing air. Maybe I will get used to it, maybe I will build another tall boi shifter.
It’s been a couple of weeks since the install, and overall, I’m a little conflicted over this modification. On the one hand, the gearing is a good match for the EJ207. It very much feels like a different car. I don’t have to rev out first gear to over 5,000 RPMs anymore to avoid the grand canyon gap between first and second gears. The front tires also have a ton of grip now with the helical front diff. The car just claws through corners like my old STi did. And that is kind of the problem. I didn’t really want to build a clone of an STi anymore, and yet here I am, nearly 80% there. The only things I am missing are the rear diff (which I have) and the rear knuckles and Brembos. The fun, nimble feel of the Bugeye is gone. And I think on some level, I expected this and it’s why I avoided the 6MT swap for so long.
Performance-wise, all the stats are up. The Bugeye grips well, even in cold temps with summer tires. However, steering response, something I’ve worked very hard at, is down. It doesn’t cut as willfully with a turn of the steering wheel like it used to. Initial turn-in feels muddy, even though there is more steering feedback. This is probably a side effect of the front diff wanting to straighten the car out, along with the 265 tires. I can probably dial in some EPAS assistance to counter this effect. I wonder if my scrub radius is playing a role as well.
Basically, the car will turn, but you need to commit to it, overwise it feels like it will understeer, which is a bummer. Adding DCCD will make this even worse. I used to drive my STi with the DCCD in open mode because it sharpened the steering response. Ironically, a carbon fiber driveshaft helped with this issue. Unfortunately, carbon driveshafts have jumped in price along with everything else. I think reducing the rotational weight of the wheels and rotors will help as well.
At any rate, I ordered a front subframe brace to help stiffen up the front end. We’ll see if that helps with turn-in.
I used to dream of the day that I would swap a 6 speed into my wagon.
Now that I’m older and wiser, I’m not sure if I’ll ever want to make that jump. Personally I think the 5 speed fits my driving style well and it does most of what I need it to do fairly well. I’ll keep it around for as long as I can, and maybe rebuild it with some extras if the time ever comes. 🤷🏻♂️
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I hear ya.
I really liked the EJ255/5MT combo in my old 2009 WRX. I’m not chasing big power but I couldn’t find 5MT ratios that would work well with the EJ207. I really wanted a short first gear and a close ratio second. The 6MT was really the only option.
Did you consider/look into the STi RA 5 speed gear set? Flatirons and Rallispec have the ratios posted and they seem to be much shorter than the USDM ratios.
If I ever need to pull my 5 speed apart I’ve considered going with the STi/LGT 5 speed gears for the improved strength and slight change in 1-4 ratios.
Props to you for doing the 6 speed swap though, probably the last transmission you’ll ever need. Lol
My 5MT has RA 3rd and 4th gears. I put them in after 3rd gear broke. The stock first gear is the shortest of the 5MTs. And for whatever reason, Subaru paired it with a really long second gear.
I’m pretty sure the 08-14 WRX has the same LGT 5MT gears. The GR WRX is more or less a LGT, especially up front.