The performance limit of the WRX gearbox
Unfortunately, the standard five-speed WRX gearbox has a limit of 300 horsepower, and since the engine from the WRX is similar in many ways to the engine from the STI, the potential for increasing power is similar to that of the STI, mainly torque. In most cases, the stock WRX gearbox will fall apart at the intersection when you want to make yourself the biggest dude. The STI transmission is said to be made of granite.
Gearbox System and Differential
The standard gearbox from the WRX is differently geared compared to the gearbox from the STI. The WRX has longer gears and this makes the ride feel slower and more boring.
If we look at the differentials, the stock sti has the same "empty" differential up front as the WRX. This differential cannot be adjusted in any way. Some STI models had a torsen type differential between the front wheels for an extra charge, but this too cannot be adjusted. The STI Spec C had a limited slip differential, basically the best you can get. Of course, here the surge of slats can be set. The STI's rear differential has much more clearance than the WRX's. You can imagine the difference when the car slides in the snow in a controlled skid. With the STI, the car skids much more willingly.
When luck smiles upon you and the racing Gods wish you luck. They might bring you a transmission from a newer model STI 2005 - 2007, you will be blessed with the joy of a DCCD inter-axle differential. Thanks to this central differential, the options for adjusting the traction are moved several levels further, especially when we supplement this gearbox with a programmable traction control unit (Ing. Pavel Skácel).
Stregnth and Durability
All STI stuff is bigger, beefier and we can say a lot sturdier. For example, semi-axles are thicker, have larger joints and wider rods. The half axle is the part that suffers the most in Subaru after the engine. Pay attention to the year of manufacture of the axles, axles are not always identical. So what will i be installing in a six-speed gearbox in a WRX? In order for the conversion to be complete, it is advisable to replace absolutely everything that can be found on the STI chassis. Unfortunately, not all of us have a bottomless wallet, so I will mention here the all necessary parts to convert to a six.
These parts are not important to change:
1. Front axles
2. Front axle beam – it is not necassary to change. Unless we want to put Brembo brakes with 326mm dics on fronts.
3. Rear Brakes Brembo
4. Handbrake cables
These are the parts that are important and you must change:
1. Rear Axles
2. Rear differential
3. Rear axle beams
4. Six-speed gearbox STI, incl. Of all holders
5. Complete steering assembly
6. Drive shaft
8. Large clutch set (clutch, pusher, retainer, clutch bearings)
How much will it cost to convert a five-speed into a six-speed?
If we are talking about used parts, the price for a NON DCCD conversion is usually around 70 – 85,000(Czech Crowns). If the transmission will be DCCD and we want to add a traction control unit to the set from the start, it is necessary to add an additional amount of around 30,000,-(Czech Crowns).
What you should pay attention for?
When you're about to start remodeling, it's good to keep one thing in mind. If you are going to buy used parts, try to always buy the transmission and the rear differential from the same car. If you are going to buy parts one after the other, keep in mind the ratio of the gearbox to the rear differential. That is, if the ratio will be 1:1 or 1.1:1.