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Old 04-22-2007, 10:44 PM   #1
Davenow
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Default 06-07 Forester XT 5mt power handling limits? And a couple Qs about the recent 5MTs

So its been a long time since I thought about subaru transmission power capabilities, and I am a bit out of the loop.
I know that subaru has done some upgrading on the 5MT since my 02 wrx.
I also know that Forester XTs are geared lower, and that lets them take a bit more power reliably.
I bought an STI shortly after that, and stopped paying much attention to the 5MT.

So combining the improvments and the final drive gearing difference, what is about the limit on an FXT for a reliable gearbox?

I ran 309whp 278wtq in my 02 WRX on the stock gearbox with an aftermarket clutch for 3 years without breaking the box. So driver error shouldnt be a problem.

My goals for this thing are relatively modest, Im looking for say 310whp 350wtq or so, that seems reasonable on the FXT as they seem to make a lot more tq than hp. I may up that into the 350whp range once I learn the limits on this thing and get bored with the power.

I know the gearbox should handle the HP just fine, but with the much higher TQ of the 2.5, and the fact that I want a big buffer zone as far as limits goes, I want to get some more info.
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Last edited by Davenow; 04-22-2007 at 10:58 PM.
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Old 04-23-2007, 01:47 AM   #2
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I'll have to double check, but I'm pretty sure your box is identical to the newer WRX boxes with a 4.44 final drive.

A nice, cheap option for upgrading (being proactive) is to use the 1-2 gears from a Legacy GT gearbox. The gaps in ratios are closer and the tooth/root is thicker than WRX gears.

Either way, you should probably be fine. The 1-2 gear upgrade isn't just for strength, it's also for keeping your gearing in the power range. If you do a search on Legacy GT gears, you'll find all the data we've compiled over the last couple of months.
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Old 04-23-2007, 09:50 AM   #3
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Like you've more or less said, don't drive crazy and it'll go for a while. I have 20k miles on my stocker. At about 8k miles I went to a FP Green and then steadily added power. I'm running 358whp on the MZM dyno and the transmission is still holding. This includes moderate shifting and launches at the dragstrip and the TX mile. About 3k miles ago I put in an ACT street clutch and it's still holding up to that. I'm in the process of getting a PPG'd transmission built right now. I'm sure that my stocker will last a while, but I'd rather spend the $$ for peace of mind since this is my daily driver.
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Old 04-23-2007, 07:11 PM   #4
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Cool. 358 on MZMs dyno is probably more than I even really am shooting for (to those that dont know, that dyno is INSANE low. 195whp from a stock STI)
I am shooting for 320-330whp on an "average" reading dyno.

I know I will get bored with the 16G in a year or less, but Ill just have it converted to an 18G (or sell it for an 18G) or MAYBE sell it for a 20G
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Old 04-23-2007, 10:39 PM   #5
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I've found that it's the mid-smaller turbo setups with fast spool and high-torque that often breaks transmissions.

You're probably at more risk with the 16-20g vs. a rotated setup. That said, you should be fine regardless as long as you don't go too crazy on the clutch.
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Old 04-24-2007, 08:59 AM   #6
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The FXT tranny is terribly tough due to the 4.444 final drive. You drive like a grandma anyways, so you will be fine. I'd start to worry at 400, but anything up to that should be fine barring a rogue breakage due to a defect or unfortunate wheel hop incident. Davenow = n00b
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Old 04-25-2007, 11:34 PM   #7
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Unabomber=suck it


I may actually be picking up a TD05-18G now... I found one locally for $400. Its tack welded for external wg, but looks easily converted back.
My end goal is a TD06 20G, but with the rest of the stuff I want to do to it first, that is a good year off.
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Old 04-26-2007, 12:10 PM   #8
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later '03+ WRXs got the widen 1-3 gears. This applied to all the other turbo trannies; FXT & Baja XT, not sure of the LGT(4.111) or Outback XT(4.444FD).

The Forester, Baja and WRX runs the same ratios, just the different FD; FXT Baja XT w/ 4.444 and WRX w/ 3.900. I'm pretty sure the Outback XT tranny has that splined part in the tranny for the speed gear & speed sensor.
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Old 04-26-2007, 12:33 PM   #9
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The cases all have the spot for the speedo sensor, but it's plugged and missing the sensor gear in the cars that use the abs system to sense speed... and aperantly you have to take the case halves apart to get the speedo sensor gear in there. The LGT gears should be pretty much identical to earlier STi non-RA 5mts.

The FXT gears should be identical to later USDM WRX gearing, but the 4.44 would help, and it really doesn't make it as high-winding as you would think (my 4.44s leave me at 3500rpm @ 75mph on the same 5th gear ratio and stock wheels and tires size). What I will say, however, is that you don't realize just how big of an rpm gap there is in the wrx/fxt ratios in 1st-2nd until you drive a better matched set of ratios (LGT/STi 5mt/PPG et.). After driving my STi box around a bit, I'm completely conviced that the common second gear failure has A LOT to do with how much shock load is put into 2nd gear with the wrx ratios when the clutch is engaged more harshly (quick/hard shift). With the us wrx, you get a clunck out of the rear end under normal driving conditions if you let go of the stock clutch even slightly to quickly, now it's literally difficult to get that shock shifting into second even if you try. I'm thinking that the fxt 4.44FD would help a bit, but there's nothing like getting the 1-5 ratios nicely matched in the first place. I'd really recommend an older STi box, LGT internals in a wrx case, or something along those lines... The improved driveability of the car really is worth it.

I guess as far as the FXT gearbox's limit... well... I'd guess that it's about ~12-ish% "stronger" than an '03/04+ us wrx gearbox (however strong that is) because of the 12.2% difference in final drive gearing. Engineers/gear gurus: feel free to tweak this figure if you have better info.

Last edited by jhargis; 04-26-2007 at 02:03 PM.
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Old 04-29-2007, 12:04 PM   #10
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I would think that the percentage of difference in the final drive would not be locked to the percentage of the increase in stength. Wouldnt it ramp up? Like is its 12% lower geared, it may be 15% stronger? Or something like that.
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Old 04-29-2007, 02:20 PM   #11
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You'r probably right, I'm just giving a ball-park... could be somewhere between say 10%-20% figuring in actual forces acting on the gear teeth, tires sizes, etc. there are a lot of variables to take into consideration, and I'm no physicist. But even a 10% increase would be a good noticable improvement, so it's probably worth it. I love the 4.44s... Sorta high freeway rpm, but it's great piece of mind and makes the high winding 2.0 a lot happier around town.
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Old 05-02-2007, 10:15 AM   #12
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My FXT tranny lasted right around 30k miles (22k with a VF30 at 19psi peak) before 2nd gear ate itself.

http://www.subaruforester.org/vbulle...ad.php?t=14214
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Old 05-02-2007, 11:30 PM   #13
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That's pretty much what I would expect. I'm pretty sure the fxt trans is just a wrx box with 4.44 gearing, and most stength gains through the final drive change are negated by increased tire diameter on the bigger forester.

I think you don't hear about so many broken fxt gearboxes because there are

A- Fewer FXTs (at least it sure seems that way around here)
B- You don't see too many modified FXTs while it's hard to find an unmodified WRX
C- Driver demographics. FXTs tend to be driven by older, more experienced, and less padel happy drivers than you would often see behind the wheel of a WRX.

Go to a one of those super-kiddie-ricer street races and you'll see some OMGBOV WRXs driven by spoiled children garunteed, but I doubt you'll see too many FXTs.
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