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
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
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.
