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Old 02-11-2013, 01:45 PM   #1
Scooby Guru
Member#: 261612
Join Date: Oct 2010
Location: Minneapolis, MN
2002 WRX - 750WHP
9.5s @ 150mph

Default 6 Speed Swap: 2011 STI 6MT with Spec B Long Gears into a 2011 WRX

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The car now has a new motor/turbo setup and makes around 600WHP.


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Added info on the R180 Rear Diff, STI Driveshaft, and STI Axles swap.


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I recently finished up a 6 speed swap in my 2011 WRX. The car has about 480WHP/500WTQ from a Blouch 20GXTR on E85 (27-28psi peak, 24-25psi readline). Last June it broke the stock third gear while I was on the road course. I replaced the gearset with a set of stronger gears from PAR. Here is some more info on that:


I used their newer "650HP" set of synchronized gears. I originally had a straight cut first and helical 2-4 (stock 5th), but then switched 2nd to be straight cut as well. I chose to do a built 5 speed over a 6 speed swap due to complexity (much easier) and because I liked the 5 speed ratios. Here is some more info on the car overall:


Recently I started toying with the idea of changing things up. Not because I was unhappy with what I had, but because I wanted something different. Mainly I wanted to try limited slip diff's and/or a DCCD as well as shorter/closer gear ratios. Looking into LSD's for the 5 speed, I eventually decided that with these goals the OEM 6 speed made the most sense.
At this point I started researching everything I would need, and also trying to choose the parts I wanted. There have been a few other 08+ WRX's that have done this, so they were of great help to me. This is what I found as the very least that would be needed to complete the swap on an 08+ WRX:
Originally Posted by d4rce
04/05 STI trans (complete with clutch release fork, slave cylinder, clutch line, etc)
any STI trans mount
any STI starter
any STI clutch/pressure plate/flywheel + 3 extra flywheel bolts
08+ STI shifter linkage/shift knob
08+ Impreza driveshaft, automatic
DCCD controller
Transmission fluid
There may be other little things, but those are the main ones. That's the minimum list. You can certainly do more. Late 04 (early 04 had axle stubs) and 05 STI tranny's are the easiest because they use a 3.9 rear diff. The downside (for some people) is that they have the short gearing. In general, this is usually the cheapest way to do things. 04-05 STI tranny's can be found used pretty easily, but with varying levels of wear and varying prices and you keep your stock rear diff and front and rear axles.

If you switch to a 3.54 FD rear diff, then you can use any 06+ STI tranny. The 06 is still the short gears, but the 07+ have the longer 2-4 gears. For a 3.54 rear diff you can use an 02-05 WRX r160 or an STI r180. The r180 is stronger but requires custom axles, Spec B axles, or STI axles. If you go with STI axles, then you need to change the hubs, brakes, wheels, etc. Same for the front. If you want the STI axles you have to out other components.

I decided that I wanted to keep things simple. Use my stock hubs, axles, brakes, etc. So it basically came down to 04-05 STI (cheapest/easiest) or 06+ STI and 02-05 WRX rear diff with 3.54 FD. I went out and basically was just letting prices dictate what I would get (but also considering gear ratios and mileage). I contacted many people. Mostly private sellers. I was close to buying two different 04 STI tranny's and then very close to getting an 06 STI tranny. But none of them were working out how I hoped (slow shipping, poor communication, didn't have all the parts, etc, etc).

Part of the decision I had to make was what ratios I wanted. I was thinking that it might be fun to try to short gears for a while. They're a very big departure from the long 5 speed gears. I made many plots comparing the different gearsets and ratios that I was considering. To me the USDM STI ratios are all somewhat of a compromise. 04-06 are very tight 1-4 and then a big gap to 5 and 6 (but 6 is actually shorter than the WRX 5th). This would be good for someone who needs close spacing, but that wasn't really what I wanted. Here is a plot of the WRX 5MT vs the 04-06 STI 6MT:

The 07+ makes 2-4 a bit longer, but 5 and 6 are the same. This is a little better but still not ideal. 4 and 5 are very close to each other. The ideal ratios, IMO, are the Spec B ratios. They are longer 2-4 like the 07+ STI, but they also have longer 5 and 6 so all the ratios are evenly spaced. I figured I could upgrade to the Spec B ratios after I tried the normal STI ratios for a while. They seemed to be what I really wanted at some point. Here are some more gear ratio comparison plots:

I eventually found a 2011 STI tranny available with low miles and in great condition. It was obviously more expensive than what I had been looking at, but I thought it would be worth it to get something so much newer. At this point I had the option of installing a 1:1 viscous center diff (50/50), or keeping the normal 1.1:1 DCCD. The 1:1 center diff would have allowed me to use my stock 3.9 rear diff and I wouldn't need a DCCD controller. But I chose to keep the DCCD because that's one of the things I wanted.

Also, I needed to decide if I wanted to have the SpecB long gears installed. Since this is an 07+ tranny with longer 2nd-4th already, I would only need 5th and 6th replaced. If I were getting an older tranny I would have just held off and gotten the long gears later, but since it already had the longer 2-4 it made sense to do it now. I had RalliSpec open it up and install the long ratio gears. They also replaced the 3rd gear synchros, upgraded 4th-6th to the stronger carbon synchros, replaced the 3rd/4th sleeve&hub, and replaced the 3rd/4th shift fork.

Last edited by xluben; 04-29-2013 at 10:22 AM.
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