04/05 Tranny Synchro Tech Rant

Upshifting from 4-5 gear is grinding every time at WOT, 3/4 throttle, and other times after warm/longer trips. No other gears grind!! 2004 STI with about 58,000 miles. Brand new clutch and tranny/diff fluid (Amsoil). so more then likely a synchro issue. Did some some soul searching and found people see its eased with regular gear oil, some others changed synchros with stock synchros to fix problem but some to fail again. Others have replaced the synchros with 06/07's and the issue to never return. Parts havent seemed to be too expensive. But labor probably will be. With all mechanical parts, wear happens and since premature wear for 04/05 synchros is seeming common and i havent found a thread for synchros here (maybe my eyes deceive me), why not open a thread?? Synchros are a biotch! More info and input would be appreciated. People have different beliefs...just look around on NASIOC, WRXTuners, and ISWTI.

The synchros need a little friction to work properly and some people feel using synthetic gear oil reduces friction enough to cause the synchro to seem like its failing. I personally dont believe that going from dino to synthetic causes failure. But that it unmasks a problem that is happening/starting to happen with the synchro....premature or normal wear depending on driving style and/or mileage.

Replacing the synchro is a costly fix but solves the problem (usually). If you didnt know (im still not 100% sure either so no quotes!!) but the 04/05 USDM tranny has the 5th/6th gears at a higher ratio for fuel economy reasons. They also have standard synchros but with the higher gear ratio, the synchros work harder making them fail sooner. The JDM had closer ratio gears so the synchros didnt work as hard. In 06/07, USDM STI went to a harder material (Carbon) for the 4-5-6 gear synchros which fixed the problem. If you have synchro problems in those gears...id find the JDM gears paired with Carbon synchros or just do the carbon synchros. ***Note: From my understanding, these are all Genuine Subaru replacement parts I am referring to, not aftermarket*** I am still searching for prices/availability and ive found right now is:

flatironstuning.com for the JDM gears and carbon synchros are $518
http://www.mzmperformance.com/gear-sets.html for JDM gears w/ synchros $425
 
With work and this my DD....I doubt it. I tried tackling the clutch change myself and was kinda left behind with help getting the tranny back it. Tranny is too heavy and bulky to do it alone so I will prolly have to send her off :( I did talk to some people about the JDM gears and yea, I can't do those. They said 80mph cruising is around 4500 rpm....I will pass, rofl. They limit you to about 125mph max speed.
 

HolyCrapItsFast

Drinks beer!
Hmmm... I have searched all over and I can't find any information regarding that question. I would imagine they would be the same. The 07 has different gearing but I think the synchro hub is the same size. I will compare the 04 service manual with other service manuals when I get home tonight to see if there are any differences.
 
Hmmm... I have searched all over and I can't find any information regarding that question. I would imagine they would be the same. The 07 has different gearing but I think the synchro hub is the same size. I will compare the 04 service manual with other service manuals when I get home tonight to see if there are any differences.

Yup. I've searched here too. Called my local shop and flatironstuning.com out of Colorado with no luck yet. My local shop may know a suby tranny expert in north Carolina. Hopefully this is true and I can get the info needed. Will post up results when I get them.
 
The 04 to 06 trannies should all mate up. I believe the 05-06s had different material syncros. The 07s had different gears so im not sure if the syncros would match. My 04 has 100k on her and 4th to 5th, shes grinded since 60k. I just got used to it and changed the fluid every 3 track days. Its a common issue in the 04s. If its that bad, have them replaced but like I said, I believe the 05-06 share the same syncros and they will work on the 04's considering they all had the same gearing/trannies. The 07s are def different.

I will have to let it go for now. Save up and pull the tranny sometime soon. It's super annoying and I'm very particular on stuff like that.
 
I agree. Another route is getting a new trans. In the end youll proabally end up spending just as much on another trans as u would a rebuild.

That is another route. I dont have anywhere to store an extra tranny tho, lol. Would be nice to have a spare. Especially if I go with the closer ratio JDM 5/6 gears. I'd def get into racing more.
 

bugeye_fever

New member
I always figured someday if I needed to upgrade gears I'd go for the high speed gears in the 6mt:
http://www.rallispec.com/prod_trans.htm
Down toward the bottom of the page right under the close ratio 5-6 kit. I actually have a close ratio 6mt in the car now, and its alot of fun in autoX, but crusing at 35mph i'm in 5th... I'd love to have some tall gears for the highway milage!
 
I always figured someday if I needed to upgrade gears I'd go for the high speed gears in the 6mt:
http://www.rallispec.com/prod_trans.htm
Down toward the bottom of the page right under the close ratio 5-6 kit. I actually have a close ratio 6mt in the car now, and its alot of fun in autoX, but crusing at 35mph i'm in 5th... I'd love to have some tall gears for the highway milage!

You have this kit ??:

RST-F145kit High Speed 2nd-6th Ratio Conversion W/ REM ISF $1319
(Ratios -- STD / 2.235 / 1.521 / 1.137 / 0.891 / 0.707)

This was what I was talking about :

RST-1145kit JDM 6MT Close Ratio 5/6 Conversion $456
(Ratios -- 1.062 / 0.842)
 
Talked to the tech group from rallispec.com. And got good info. Also found a good gear ratio calculator. Looks like the JDM 5/6 gears have a 361 rpm increase at 80 mph with 225/45/17 tires....a little extra torque from lower gears might be worth that difference since I can't really do high speeds in Ohio anyway...make driving back roads bit more fun. Any inputs??
 
I will save up. The billet shift forks and the stret quick shifter are looking mighty fine too. If I have to be in there, might as well save and start my setup.
 

bugeye_fever

New member
You have this kit ??:

RST-F145kit High Speed 2nd-6th Ratio Conversion W/ REM ISF $1319
(Ratios -- STD / 2.235 / 1.521 / 1.137 / 0.891 / 0.707)

This was what I was talking about :

RST-1145kit JDM 6MT Close Ratio 5/6 Conversion $456
(Ratios -- 1.062 / 0.842)

Sorry for the delay! I do not have that kit, I have a JDM V7 non DCCD 6MT. I haven't found alot of info on it, but I do believe it has a final drive of 4.44, and I think I once found a reference to it as "gravel spec" But I'm pretty hazy on that, it was a couple years ago. Here's what I do know: 3600 rpm= 77 mph in 6th on a 225/45 starspec. 25" dia per tire rack. I just can't imagine 6 gears closer than that, but its late and I could be confussing "close" with "short" ratio's.

For what its worth, if your gonna build something I like rek's idea.
 
Sorry for the delay! I do not have that kit, I have a JDM V7 non DCCD 6MT. I haven't found alot of info on it, but I do believe it has a final drive of 4.44, and I think I once found a reference to it as "gravel spec" But I'm pretty hazy on that, it was a couple years ago. Here's what I do know: 3600 rpm= 77 mph in 6th on a 225/45 starspec. 25" dia per tire rack. I just can't imagine 6 gears closer than that, but its late and I could be confussing "close" with "short" ratio's.

For what its worth, if your gonna build something I like rek's idea.

Yea, rallispec has the GroupN ratio 6mt for 1700. I didn't want to get too much into this as I don't plan to race. I do like the 5/6 combo for the money and a little more fun on the back roads. The gearing isn't bad as it sits. As for the billet shift forks, I prefer bend then shatter. Im not adding any more power right now so I'll wait until she blows later to do a full rebuild. I'm just super picky with the synchro issue. I can't let it go for long, it annoys the piss outta me.
 

STimedic

New member
I was eyeing the RalliSpec longer gears for a minute and if I Had the money, would do a slightly longer gearset for better MPG on the freeway. I'd have to get the car to cooperate first.
 
I was eyeing the RalliSpec longer gears for a minute and if I Had the money, would do a slightly longer gearset for better MPG on the freeway. I'd have to get the car to cooperate first.

Not looking for better mpg. Lol a couple hundred higher rpm for a lil bit more fun is worth it. Still looking for a mark 2 or 3 VW golf TDI beater to drive to work and back, etc. :):)
 

icudruln

New member
dont know if this will help but 08 sti have same gearing as 04 just cant remember my source

??? I woud've thought they left the same gear ratios they changed to in MY07 in the 08+ model years, which makes me question your source.
 

icudruln

New member
In regards to replacing synchros in the STi 6 speed, when ordering the part(s), order the synchro set for a MY06 STi, as it has Carbon-reinforced synchros on 4th, 5th and 6th gears. These can be found through a few online vendors that typically carry tranny parts, or obviously the dealer.

I personally run Motul tranny fluid for the 6 Speed (and Rear Diff), and have preferred it over the stock Subaru fluid in the roughly 10,000 miles I've used it for; we'll see how the Motul lasts. In talking to a few shops, many have had success with cars that had a grinding issue (possible synchro failing), and switched to run 1/2 Redline Shockproof fluid and 1/2 Motul fluid for the STi 6 speed transmission.
 
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