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Built 5MT vs STi 6MT


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Built 5MT vs STi 6MT  

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  1. 1. Built 5MT vs STi 6MT

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    • STi 6MT
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So, I've built every aspect of my car other than the drivetrain. Limited my recent dyno tune to 372 wheel torque so my trans would last a bit longer. From what I know, our stock 5MT is limited to roughly 350 wheel torque. I'm only at about 50k miles and on my old Stage 2+ setup I had to replace my rear diff, trans, and viscous coupling (twice) all under warranty. :eek:

 

I've researched the STi setup, which looks killer, but I've also enjoyed the gearing of the 5MT, especially for road racing. Anybody have experience with PPG gears or any other options? I've searched the forum and I can't seem to find much on here about building the 5MT. Mind you, the car is mainly track only with occasional street/canyon use.

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Noise is not an issue... I've already changed out all my bushings and mounts, so it's quite noisy already. This is NOT my daily driver. I also plan on pushing past 400/400 and I want the trans to be bullet proof.
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I've owned, at one point or another, several different PPG gearsets. A straight cut 1-4 synchro set (while RWD), a straight-cut 1/2 helical 3/4 PPG set, and finally a straight cut dogbox.

 

Gear ratios are fantastic in the 5MT, and get even better with the PPG set. There are other companies out there, as well. PAR has made some interesting gearsets lately, and I'd probably suggest that route at this point.

 

Noise is only an issue on straight-cut sets. If you go helical, it is hardly any louder than stock.

 

What kind of things do you want to know? Synchro sets drive like stock transmissions. No real downsides, other than cost. You'll realistically spend about the same swapping a 6MT as you will building a 5MT.

[URL="http://legacygt.com/forums/showthread.php/proper-flip-key-interesti-159894.html"]Flip Key Development Thread[/URL] "Genius may have its limitations, but stupidity is not thus handicapped." - E. Hubbard
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Any time I've raced an STi, whether on a straight or in the turns, I've noticed my gearing advantage as they seem to be constantly rowing trough the gears. I just want to make sure I go the right route considering the cost. I plan on getting what I can from this turbo, which should be about 450/450.
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I would rather have the 6MT for the durability and the lighter clutch. I am a little jaded though because I lost 5th gear for no good reason. And IIRC you don't change out 5th gear with any of the built sets.
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The rule of thumb is shorter gears should be faster.

 

No it's not. Anyone who has even a basic understanding of performance knows that isn't true.

[URL="http://legacygt.com/forums/showthread.php/proper-flip-key-interesti-159894.html"]Flip Key Development Thread[/URL] "Genius may have its limitations, but stupidity is not thus handicapped." - E. Hubbard
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I would rather have the 6MT for the durability and the lighter clutch. I am a little jaded though because I lost 5th gear for no good reason. And IIRC you don't change out 5th gear with any of the built sets.

 

The 6-speed clutch is heavier than the 5MT clutch.

 

Unless you mean pedal feel, which has nothing to do with the transmission and everything to do with the clutch.

[URL="http://legacygt.com/forums/showthread.php/proper-flip-key-interesti-159894.html"]Flip Key Development Thread[/URL] "Genius may have its limitations, but stupidity is not thus handicapped." - E. Hubbard
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Simplicity: 5 speed. Pull it, get it built, break it in, beat on it. The 6MT is a lot of mixing and matching of parts. Been done plenty of times but still more complicated, IMO.

 

Cost: As stated, about the same.

 

Durability: Prolly close. The 6spd is known to be a damn tough trans.

 

Unique factor: 5 speed all the way. Big power cars seem to use the 6 speed a lot and it would be killer to see a built 5MT make dumb numbers.

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I want durability and I'm assuming a built 5MT would hold up better than an STi 6MT.

 

It depends. A "bases covered" 5MT is fairly stout. The only issue is synchro failure, and those odds are slim. SOME cars have dropped 5th gear, but that is a random event.

 

The 6MT is plagued with transfer gear/bearing issues, so it's not without fault.

[URL="http://legacygt.com/forums/showthread.php/proper-flip-key-interesti-159894.html"]Flip Key Development Thread[/URL] "Genius may have its limitations, but stupidity is not thus handicapped." - E. Hubbard
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It depends. A "bases covered" 5MT is fairly stout. The only issue is synchro failure, and those odds are slim. SOME cars have dropped 5th gear, but that is a random event.

 

The 6MT is plagued with transfer gear/bearing issues, so it's not without fault.

 

Really? I was thinking of upgrading to a 6MT, then using the 1-4 gears out of the 5MT, making 5th gear a progression of the 1-4 gears, and 6th as an overdrive. Nuts?

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Really? I was thinking of upgrading to a 6MT, then using the 1-4 gears out of the 5MT, making 5th gear a progression of the 1-4 gears, and 6th as an overdrive. Nuts?

 

5MT gears are not interchangeable with 6MT gears.

[URL="http://legacygt.com/forums/showthread.php/proper-flip-key-interesti-159894.html"]Flip Key Development Thread[/URL] "Genius may have its limitations, but stupidity is not thus handicapped." - E. Hubbard
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well, here are a few things that I have considered-

weight- advantage 5mt

the 6MT is quite a bit heavier (i have lifted both, 6mt is way heavier) some people say 60lbs, others 80lbs more. I have a hunch there are variations between the weight due to different configurations (dccd, helical front, gear set, year to year casting differences)

the R180 is a bit heavier, but that is optional

the additional weight is about in the middle of the car, so it would probably not throw balance off too much in a street car

gearing-

gear chart

there are 3 final drive choices for the 5mt, 4.44:1, 4.11:1, 3.90:1

there is only 1 choice for the 6mt from what I can tell, 3.90:1, i would love to get a 3.45:1 final drive ratio, then i should be able to get to almost 200mph :D

there are quite a few gearsets available for the 5mt, but from what i can tell, there are no high power 5th gears, only 1-4th, but I am probably wrong about this

there are 3 gearsets for the 6mt, japanese sti short (1-4 short, 5-6 same as us sti), us sti has middle of the road 1-4th, and shortish 5-6, LGT 6mt has 1-4 same as us sti and a longer 5-6, but this is all off the top of my head, there is a whole chart for you to pour over.

from what i can tell, for a street driven 300-400hp 3200-3300lb car, the sti gearset with a 3.45:1FD will yield perfect gears for everything fun, the front diff in the 08+STI's is a 3.45, with a 1.1:1 center diff, combining with the 3.90 rear end, so i just need to find a 3.45:1 rear differential for either a R160 or R180, hopefully I don't have to get one made.

in drag racing, you want to shift as little as possible, after optimizing for the launch and keeping everything in the power band (including the launch, nothing worse than bogging)

for street, um, proving, longer gears seem to be a bit better, usually, proof is carried out in 1st to second, or second to third (0-60mph-stoplight, or 40-95mph-freeway)

for daily driving, the 6mt with us sti gears, 6th puts you at just about the same rpm as the 05LGT w/ 4.11:1 gears, and just a bit lower with the 5-6th from the LGT gears (from reading NSFW's thread, his build is inspirational)

after driving a 08STI, stage 1, i found that the 1-3 shift, or 2-4th shift was totally usable, and rowing the gears at WOT was fun, but became tiresome, and i missed the longer gears in my 5mt quickly.

cost- flip a coin

seems like building a 5mt starts off at about 4K$, and probably more to do it right, including cold treating things, good fluids, new fasteners, seals, tranquil TOB kit etc. granted, this gives you basically a brand new transmission with all new parts inside, so that is an advantage, but there are a number of engineering disadvantages to the 5mt, the main one being the shaft centers being too close, and the shafts being thin, allowing flex in the shafts because of the gears pushing apart at higher power levels (good article somewhere), want to say the shaft centers are 25mm in the 5mt, and 35mm in the 6mt, but that is just my old man memory.

the 6mt build is about what you are willing to sacrifice, bare transmissions sell from 2700-4400, going from well used, to new from stealership, then you need a bunch of parts, and bunch of small parts, you are probably going to pay about 5K$+, doing most of the work yourself

traction/differential options-

rear diffs are totally interchangeable between 6mt and 5mt, you just need to make sure your ratios match up, there are claims that the R180 plays with more power, but there are cases of folks breaking both, the really nice differentials are more available for the R180, but there are good options for the R160 also, and the R160 is lighter, but this is in the back of the car, where most subarus can stand to gain some weight,

the center diffs are a different story, for the 5mt, it seems that there is only the viscous center, or the open center, whereas the 6mt you get the early us sti center, 35 front, 65 rear, 1:1 center, then the 06-07 sti, and the 08+ change up a bunch, all the details are on the list, there was also a guy from europe shopping around a quaife 50/50 center diff group buy direct from quaife, also the 50/50 open diff from the specB's (also available for a number of japanese transmissions)

there are only a few front differential choices for either transmission, many 6mt's have helical fronts from the factory, no additional cost, where i am not sure if any 5mt's have a differential from the factory, adding 1K$+ to attain equal footing with a 6mt

but that is just my opinion

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This really gripes me, spending like $4000 to drop in a new motor to make great and healthy power and now my 90k mile grinding tranny is holding me back. just keep throwing more money at it and hope it stays together long enough to be considered "well that was worth it" which is never usually the feeling you get when something goes pop.
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