5/6MT custom top gear ratio

Nemesis Destiny · #1 02-14-2006, 09:04 PM

Hi. I've been in the process of deciding what trans. to swap into my SVX while I wait for the 4EAT to die for the past two years now. I really want a 6-speed, but I want to be able to get some decent range / fuel economy in top gear on the highway.

So, my big question is: does anyone know if it's possible to swap in the .738 5th gear as the 6th ratio in a 6-speed box?

Alternately, does anyone know a shop that can custom fab a top gear for me? The taller the better. I'd love to be able to bog the thing out on the highway like a big V8; 2000rpm@75mph.

mbtoloczko · #2 02-15-2006, 08:47 AM

Swapping gears between the 5MT and 6MT is not possible. The parts are totally different.

Which 6MT are you getting? Here's a run down of the options:

- USDM WRX STi 6MT: 6th gear ratio is 0.756 which is pretty close to WRX 5MT 5th gear ratio of 0.738.
- JDM WRX STi 6MT: 6th gear ratio of 0.842.
- JDM Forester STi 6MT: 6th gear ratio of 0.707.

All of those tranny's have a 3.90 axle ratio.

Sounds to me like you'd want the JDM Forester STi 6MT, but since those cars are pretty rare, there's not much chance of finding one in Japan that's being parted out. If the USDM 6th gear ratio isn't low enough for you, you could buy a Forester 6MT gear set and have that installed in whatever 6MT you buy. Expect a pretty good gap between 5th and 6th gear if its installed into a JDM WRX STi 6MT.

I can get that Forester STi 6th gear set for you if you like.

PPG might be willing to make a custom 6th gear set, but it would be expensive. I wouldn't expect it to cost any less than $500, and it probably be closer to $750.

mbtoloczko · #3 02-15-2006, 09:22 AM

I can also track down a used Forester STi 6MT if you are interested in that route.

Nemesis Destiny · #4 02-15-2006, 09:24 AM

Hm, I have an extensive chart with all the gear ratios, but the Forester STi was not on it. The .707 ratio sounds about perfect. Do you have a listing of all the ratios in the Forester STi gearbox?

Could you look into getting it? Either the whole gearset (1-6), just 6th, or the whole box. I have no idea how much it would cost, but it sounds interesting. I can't commit to it *right now* but I will absolutely need to get a gearbox to install for the spring, so if you could quote me a price, that would be fantastic and much appreciated.

Does PPG have a website for that service that you are aware of? I looked all over their site and could only find glass, silicas, and paint info. I honestly thought having a custom gear made up would be more expensive than $500-750. I'd pay it, if it did what I wanted it to do; it would pay for itself in fuel savings eventually.

mbtoloczko · #5 02-15-2006, 09:59 AM

Personally, I'd go with the Forester STi 6th gear set over a custom 6th gear set. The Forester STi gear ratios are:

3.636, 2.235, 1.521, 1.137, 0.891, 0.707

I can definitely get you a price on a new 6th gear set and on a new 1-6 gear set. I'll do that today. Getting a price on an entire used tranny is not so easy though. My guy in Japan has to spend some time trying to track one down, and I don't want to ask him to do that unless you are serious about getting an entire box. I can say though that almost without a doubt, a used Forester STi 6MT gearbox (plus the other bits that go along with it) would cost at least USD$4000. Probably closer to USD$4500.

Nemesis Destiny · #6 02-15-2006, 10:26 AM

Thanks. I appreciate the help.

Yeah, I'll scrap the custom 6th gear idea; the .707 ratio looks like it will do nicely for highway cruise purposes. I will also scrap the idea of getting a whole JDM Forester STi gearbox, since it would be a little expensive.

This is assuming that the gears will swap right into a USDM gearbox.

Right now I've got two options for a person to do the actual work; supposedly there is a guy near me (about an hour away, in Guelph) that has swapped several SVXes over to 5-speed, and also TomsSvx has also offered to do the work for me down in NJ (It's 10x further away, but he has 6-speed experience).

The other thing is, if I'm going to do this, I want to do it right. I see a lot of people here with stiffened rear subframes, better springs, beefier swaybars, urethane bushings, etc. I could save a lot of money on labour if I have some of this stuff done while it's all apart...

Earthworm · #7 02-15-2006, 11:17 AM

You would need a 0.631 6th gear just to match the RPMS of a stock SVX 4EAT 4th gear.

Nemesis Destiny · #8 02-15-2006, 12:35 PM

Quote:

Originally Posted by Earthworm

You would need a 0.631 6th gear just to match the RPMS of a stock SVX 4EAT 4th gear.

Is that after factoring in the shorter 3.9 final drive?

Earthworm · #9 02-15-2006, 12:50 PM

Quote:

Originally Posted by Nemesis Destiny

Is that after factoring in the shorter 3.9 final drive?

yes:

My idea of the perfect tranny would be 3.90 diffs, 6 gears (first 5 the same ratio as the WRX 5MT plus a .631 6th gear).

Nemesis Destiny · #10 02-15-2006, 01:07 PM

Nice charts!

Originally, I was thinking along the same lines as you: 6 speed MT with 3.90 diffs, viscous coupling (no DCCD), WRX or 5MT STi RA ratios + a taller top gear. Personally, I would want to go a little more extreme for the 6th. I think the engine is flexible and torquey enough to get it done. Not too crazy or unrealistic; say a comfy interstate 70mph cruise at ~2000rpm. What kind of ratio would we be looking at? 0.555-ish?

Earthworm · #11 02-15-2006, 03:40 PM

Quote:

Originally Posted by Nemesis Destiny

say a comfy interstate 70mph cruise at ~2000rpm. What kind of ratio would we be looking at? 0.555-ish?

By my calculations 0.528.

Nemesis Destiny · #12 02-15-2006, 03:51 PM

I doubt I'll find one of *those* in a Subaru 6-speed box anytime soon

I will still try to find someone that can cut me a custom gear, but I won't hold my breath on that. I'll just keep my fingers crossed that Mychailo can hook me up with the Forester STi 6th gearset for a decent price... I should still be able to easily break 30mpg with the .707, and be able to defend my honour with the other 5 gears.