Information is subject to change as more information comes.
I have a ’05 WRX and I have NOW upgraded my turbo to a decent TD06H-20G-8cm^2, FMIC, EWG with ported heads, 272* cams, 9.0:1CR pistons, Cosworth rods/bearings and running 110oct. What I’m afraid of is breaking my transmission now. The car was tuned to 22psi up top and we push 364whp and 331wtq. I don’t know what gears I want to hold the power. Some were saying go PPG’s and others were saying ’07 STi 6spd.
WELL…. The MOST important question is, what are you going to be doing with your car? Gearing is determined by the car’s use. A person who drag races a car has much different gearing compared to a person who does circuit/lapping days. The stock gearing on the car is much different from BOTH of the guys who does the 1320 and track races.
A drag racer much rather prefer rolling through 1-2-3-4 on a typical 5spd tranny with an average 280-450whp. Gear ratios are much longer and further spread out. Less shifting means more time for acceleration. First gear for get up and go, second gear for gaining ground when there’s not enough wind drag yet, third gear to pull through with the power made, and forth gear to show “wassup” about TOP END.
A track racer much rather prefers rolling through 2-3-4-5 on a typical 5spd tranny with an average 280-450whp. Gear ratios are much shorter and closer to each other. Staying in the power band even after a shift means power on the tap 99% of the time so you can zoom out of a corner faster. Second gear is usually for slower turns, third gear to get you to the next corner, forth gear for that high speed turn, and fifth gear to pull through the back 1000ft straight. An autoX guy will like a nice short 2nd gear and 3rd gear, more like the STi 6spd 2nd gear and 3rd gear.
Now the Daily driver much prefers being able to go through all gears on a 5spd with an average 280-450whp. Gear ratios are much wider in the first 3 gears and shorter as they go up or get shorter as they go up to 4th and 5th gear would be a little longer. Less shifting in the lower speeds means more concentration on the ROAD, closer ratios from 3-4 means staying in the power band and NOT using a lot of gas, and a nice low ratio 5th gear for that NICE hiway cruising engine speed.
Now you should have an idea what you want now. Remember that these are the KEY POINTS in choosing the gear ratios that meets your needs. Don’t expect the Daily driver gearset to keep you IN THE POWERBAND all the time and don’t expect it to give you MAXIMUM acceleration getting up and going from a stand still on the side of the hiway.
This is where it gets technical. There are final drive numbers on each tranny can vary on year. Also are your engines’ powerband and window of RPM operation. The gear ratios will determine how much gap in engine revs there are between shifts and some speed. The final drive ratio will change the overall SPEED of each gear and how much overall speed once in top gear. Sometimes a tranny will have a short 4.444 final drive ratio, but its speed carried through the gears given the engine’s rev limit is really close to the same as one with a 3.900 final drive. The ’95 WRX STi Ver.II 4.111 tranny(3.454 2.062 1.448 1.088 0.825) and ’99 WRX STi Ver.VI 4.444 tranny(3.166 1.882 1.296 0.972 0.738) would be a good comparison; STi VerII 60mph@2880rpms and STi VerVI 60mph@2785rpms both in 5th gear.
Typical Subaru’s with an average 280-450whp will rev up to 6500-7500rpms. Most gear ratios out there will work fine with the factory final drive ratio given the engine’s rev limits. One would put STi RA 5spd ratios(3.083 2.062 1.545 1.151 0.825) in a ’06 WRX tranny(3.700FD). It’ll give you decent acceleration and still a forgiving 5th gear hiway gear running 60mph@2590rpm and 80mph@3455rpms. Putting the same STi RA 5spd ratios in a ’05 WRX(3.900FD) would mean a bit quicker running through the gears, however a slightly higher engine speed; 2730@60mph and 3640@80mph both in 5th gear.
Those who are running the PPG gears have all out acceleration performance through the straight with an average 280-450whp with up to 7500rpms rev limit. Let us look at these ratios; 3.08 / 1.87 / 1.32 / 0.95 / .738(OEM) with a stock final drive of 3.900 on ’02-’05 USDM WRXs. Running through first gear you’ll get to approx 41.3mph, second gear starting at about 4200rpms carrying speed to 68.1mph, then third gear starting at 4900rpms and carrying speed to 96.4, forth gear starting at 5000rpms and carrying speed to 134mph, then fifth gear starting at 5400rpms and carrying it up to 170mph if given enough straight shifting at about 7000rpms. You can carry enough speed through the quarter mile in 4th gear giving you plenty of time to accelerate shifting only 3 times for the drag racers.
Now when you put in aftermarket gears, you have a variety of ratios available. Knowing what YOU are going to do with the car will allow you to make the right choice in selecting the right ratios for you. You can also consult with a shop that specializes in rebuilding transmissions and they can help you narrow your gear sets down to the one that suits your application.
Now what about the 6spd? Oh when you go with the 6spd you are jumping into a whole different ball game. You have LSDs, DCCD, rear biased, and short gearing. The 6spd was made for track driving and was NEVER intended for 1/4mi use, however it will put a decent time. They come with and without DCCD depending if you get JDM stuff or USDM stuff. In Japan they have non-DCCD 6spds for the purpose of owners’ preference. DCCD 6spds will always be rear-biased except very few models and the newer models; 35:65, 41:59, & 45:55. The strength is also very incredible. You will be able to run the tranny hard and don’t have to worry about breaking gears with the average 280-450whp.
Gearing is changed a little bit for certain countries. For example the JDM 6spd is much shorter than what’s offered here when looking at ’07 models. For the ’04 STi that first came here in the USA, we had the same gearing 1-4 as the JDM 6spd, but with LONGER 5th and 6th gears. This was changed out for the ’07 models using longer gears that are the same as the Legacy Spec B 2-4th gears while retaining the same 1st, 5th, and 6th gear ratios. The USDM Legacy Spec B 6spd ratios are like the USDM ’07 STI, but with much longer 5th and 6th gear evening the spacing out.
For the guys who run and average of 280whp-450whp, the 6spd will get things done well for most applications. The gearing is short enough so you don’t have to worry about long gears when you run at the track. They are also long enough to enjoy rolling through the 1320. The longer ratio ’07 STI tranny is will suit those who do the 1320. The ’04-‘05 STi tranny will suit those who autoX and do circuit. The JDM STi 6spd will suit those who are doing PURE time attack and road course events. If the car has HIGH HP, the Legacy Spec B ratios will suit them better for its longer ratios.
Now selecting the ratios does go hand in hand with the amount of power as well as window of power with engine speed. For example would be if I wanted a QUICK & FAST street car running 540whp revving to 7500rpms on a built 2.5L, I would definitely do a 6spd and have Legacy Spec B 2-6 ratios installed. The car would have enough power to push through the longer ratio gears. The longer ratios also help because of the lower engine rev compared to the JDM models with the 8000rpm redline. If I was to have JDM STi 6spd I would run out of gearing so fast that I wouldn’t have anytime to use the power for acceleration. There’s just too much shifting with shorter ratios where you can have longer ratios to gain more ground accelerating. If it was a pure short/medium track or rally machine where top speed isn’t a factor but more so acceleration, the JDM ratios would fit right in. Anytime out of a corner power would be on TAP.