Re-built 5MT vs. 6MT wap

[czei]

In 4 years of rallycrossing I've been through 3 phase 1 transmissions, and am tired of replacing them. The latest transmission only has a few races on it and is already starting to go, so I want to invest in something that won't require yet another weekend spent under the car in a year.

One option is to rebuild a 5MT. The local transmission guy said phase 2 transmission are far more robust, and he prefers the hydraulic clutches, so I could theoretically could upgrade the car for those. The base rebuild would be with a RA gearset, roughly $2,200 in parts plus an unknown amount of labour. I'd probably want to go with a 20kfg center diff at the same time, which would push it to roughly $3,200 plus labor, maybe $3,700 to $4,000.

For roughly that price though I could get the parts to do a 6MT swap and after-market DCCD. The 6MT wouldn't be new gears, but theoretically could take the rigors of rallycross just fine.

Any advice on which to choose? This is a dedicated rallyx car so I don't car how it handles on the street.

[Legacy777]

In the long run I think the 6spd is going to be more robust. It has its own little things that need to be addressed and does add weight to the car over the 5spd. The ratios on the 6spd are rather short, so I think that's something you need to look into a little more and may be the thing that tips you one way or another.

Have you driven a 6spd? If not, see if you can find one to drive that is similar to what you would swap in so you get an idea of the gearing. If you find that the gearing isn't right or you'd be shifting right at where the sweet spot is on the 5spd then maybe going with the built 5spd is a better option. You just need to look into a little more.

[czei]

That's a good point: I've ridden in a 2.5RS with PPG gears at a rallyx and the ratios were perfect. The RA set is close, and so I assume it would also work. The longer 1st gear and shorter 2nd are perfect for optimizing the amount of power from a smaller engine.

The STi's do very well at rallyx, so I assumed the gear ratios would be fine, but an STi has twice the power available, and may or may not be suitable when mated with a 2.2L or 2.5L NA. Its difficult to simply compare the gear ratios since I drive a 4.11 final drive, and the STi is 3.9.

[Legacy777]

Yeah, you need to look at the overall ratios. You could do a little graph and see how they overlap.

[czei]

I found an online site with a calculator. With 15" tires and assuming the car is 6,000 rpm for each shift here's the speed at a the shift points:

Gear, Stock 5MT, PPG 5MT, RA 5MT, 6MT
1st: 18, 21, 21, 19
2nd: 33, 35, 32, 29
3rd: 48, 50, 42, 39
4th: 67, 68, 57, 51

The MPH span of each gear is another way to think about it:

Gear, Stock 5MT, PPG 5MT, RA 5MT, 6MT
1st: 18, 21, 21, 19
2nd: 15, 14, 11, 10
3rd: 15, 15, 10, 10
4th: 19, 18, 15, 12

From my 4 years of rallyx on the stock phase 1 4.11 transmission, 1st is too short, suitable only for launch and digging out of a handbrake-tight u-turn. 2nd, however, is way too tall, and it takes a LONG time to get up to speed. The PPG and RA gear sets give a taller 1st, so it could be used in a the typical tight turns where I would have had to bog down in low second. I saw that at VIR, where an opponent was able to effectively use 1st in all the smaller turns, which turned into speed increases of roughly 1 second each turn.

Looking at the speed range of 2nd, the clear winner is the RA gear set, keeping the range of 2nd small, while keeping a taller 1st.

The 6MT is only 1mph taller in 1st, which is only slightly better, and thus would not be as useful. It, like the RA, also keeps the tight gear ratios in 2nd and 3rd. A 6MT version would have a DCCD, but that would be competing with a simple 20kfg center diff in the 5MT builds. I have no experience driving a car with a DCCD, but presumably it would be comparable to a dumb center diff in the dirt/gravel. I'm tempted to see that any after-market DCCD advantage wouldn't be as useful as a usable 1st gear.

[Legacy777]

Good research and info.

A DCCD center diff without a controller is like an open diff, so not overly useful. However the torque bias is 45/55 to the rear so in "open" mode the added RWD may be beneficial.

[jamal]

Torque split depends on which year it's out of. Some are 35:65 (04-05 usdm?), newer ones are 41:59. I think the 5-speed dccd boxes might be 45:55. I think it's all listed in the rallispec trans chart:

http://forums.nasioc.com/forums/showthr ... ?t=2173258

DCCD basically works by adding lockup when there is more throttle and less steering, and dialing it back with more steering and less throttle. So you get the good traction of a locked diff when you are accelerating mostly straight, but less of the car wanting to push straight forward on corner entry.

[Legacy777]

I keep getting the split numbers mixed up. I've got an 04-05, so 35/65.

[czei]

Torque split depends on which year it's out of. Some are 35:65 (04-05 usdm?), newer ones are 41:59. I think the 5-speed dccd boxes might be 45:55. I think it's all listed in the rallispec trans chart:

http://forums.nasioc.com/forums/showthr ... ?t=2173258

DCCD basically works by adding lockup when there is more throttle and less steering, and dialing it back with more steering and less throttle. So you get the good traction of a locked diff when you are accelerating mostly straight, but less of the car wanting to push straight forward on corner entry.

That makes sense, but how is that different from a 20kfg viscous center diff?

[James614]

The viscous center differential doesn't vary it's locking action at all. It will always aggressively apply the same amount of lock when the front and rear wheels turn different speeds, regardless of whether you're flying straight through a mud hole at 80mph, or trying to parallel park your car. The DCCD is intelligent and locks or opens depending on when it's needed, making it versatile for daily driving as well as all different kinds of motorsport. For example, in autocross you don't want the same type of differentials you'd want is stage rally.

I personally would go for the option that gets me DCCD over a 20kg viscous. But that's just a suggestion from a guy who's done a lot of bench analysis with no hard experience on either. And as mentioned before, there's the other factors like the gear ratios, weight increase, and whether you want to go down the rabbit hole of a swap and installing the DCCD controller properly.

[czei]

Because of the ideal gear ratios for rallyx I'm going with the phase 2 transmission with the RA gears. Since the center diff can be swapped out later I'll leave that later. Ideally one would want to test the transmission gear swap first without throwing the center diff into the mix.

[jamal]

Doesn't the 6mt have the shortest 2nd gear and the closest gap between 2nd and 3rd? seems like that would be helpful for rallyx, along with dccd.

Plus with the 6-speed you get a good front lsd, and if you scrouge up the full drivetrain a nice rear lsd as well.

[czei]

Everything's a tradeoff. The 6MT's 2nd is the shortest, but also has a 1st that practically unusably short. That's a problem on an engine with only 160HP since 1st can lay down much more torque in 1st than 2nd to power out of corners. Going with a 6MT means giving up 1st for the most part. 2nd may work with a 6MT, and its certainly better than the stock 2nd, but there aren't any 6MT transmissions with RA engines around for me to test drive.

The front diff and DCCD could be helpful-- I'd have to drive an STi to find out. There's only one that shows up to races regularly and I've never seen him let anyone else drive it, so that would be difficult.

Another issue is installation hassle. I could have a rebuilt 5MT installed in a weekend and not lose any racing time, while the 6MT is a major upgrade and could take a month or two to sort out, meaning lost racing time.

Resale value is undoubtedly better for a 6MT-- the market for a rally gear'd 5MT is fairly limited. A 6MT swap would be easy to part out.

[czei]

Just ran the numbers on a 5MT DCCD. Its 2mph faster in first, just 1mph slower than the RA I wanted, and the rest of the gears are just as close together.

Gear, Stock 5MT, PPG 5MT, RA 5MT, 6MT, 4.44 5MT w/DCCD
1st: 18, 21, 21, 19, 20
2nd: 33, 35, 32, 29, 29
3rd: 48, 50, 42, 39, 39
4th: 67, 68, 57, 51, 52

The MPH span of each gear is another way to think about it:

Gear, Stock 5MT, PPG 5MT, RA 5MT, 6MT, 4.44 5MT w/DCCD
1st: 18, 21, 21, 19, 20
2nd: 15, 14, 11, 10, 9
3rd: 15, 15, 10, 10, 10
4th: 19, 18, 15, 12, 13


Its a TY752VB6EA; does anyone have a idea if a 5MT DCCD is any easier to install than a 6MT? The one I found includes the rear diff, but not any of the axels or driveshaft that may or may not require modification for a '93 Legacy. Will need to do some research!

[91legacyawdturbo]

Have you looked into the moores performance blast plates?? Might be an option since a lot of the 5 speeds seem the break because of the torque deflection of the actual cases. I'm on my third turbo legacy transmission.

[czei]

Have you looked into the moores performance blast plates?? Might be an option since a lot of the 5 speeds seem the break because of the torque deflection of the actual cases. I'm on my third turbo legacy transmission.

Never heard of 'em, but thanks, I'll check those out. Unfortunately the darn things cost more than the used transmissions

[Mister Anderson]

Just ran the numbers on a 5MT DCCD. Its 2mph faster in first, just 1mph slower than the RA I wanted, and the rest of the gears are just as close together.

Gear, Stock 5MT, PPG 5MT, RA 5MT, 6MT, 4.44 5MT w/DCCD
1st: 18, 21, 21, 19, 20
2nd: 33, 35, 32, 29, 29
3rd: 48, 50, 42, 39, 39
4th: 67, 68, 57, 51, 52

The MPH span of each gear is another way to think about it:

Gear, Stock 5MT, PPG 5MT, RA 5MT, 6MT, 4.44 5MT w/DCCD
1st: 18, 21, 21, 19, 20
2nd: 15, 14, 11, 10, 9
3rd: 15, 15, 10, 10, 10
4th: 19, 18, 15, 12, 13


Its a TY752VB6EA; does anyone have a idea if a 5MT DCCD is any easier to install than a 6MT? The one I found includes the rear diff, but not any of the axels or driveshaft that may or may not require modification for a '93 Legacy. Will need to do some research!

With my RA 5 speed W/DCCD I had to use a manual legacy drive shaft, the Impreza DS I had was around 3 inches too short (I only had the back half of it). The R180 rear-end required me to build custom hybrid axles using the stock legacy axles/outter DOJ's, and STI inner DOJ's with special races. The input flange of the diff also had to be changed to the R160 version since the R180 flange has a larger bolt pattern diameter which makes it non-compatible with the legacy DS. Subaru also made 4.44 R160's if you don't want to convert axles, or do a hub/strut assembly swap.

my build thread has the axle/rear-end rebuild pics in it, about 2/3 of the way down. http://bbs.legacycentral.org/viewtopic.php?f=25&t=48826