wcm ultralite rear diff fluid options (subaru forester rear)

[JohnK]

These data are accurate, WestTexasS2K confirmed them a while ago.

https://dl.dropboxusercontent.com/u/49212285/AsstdStuff/MPH-RPM02.html

 

If you want to change values for things like your tire's fitted circumference (and you calculate from ROLLING radius, which is not equal to nominal radius) and this works in your spreadsheet try

https://dl.dropboxusercontent.com/u/49212285/AsstdStuff/MPH-RPM02.xls

or

https://dl.dropboxusercontent.com/u/49212285/AsstdStuff/MPH-RPM02.ods

 

Tire Rack, for some of their tires, gives tire revolutions per mile - as an example of the error involved between nominal radius and rolling-radius based circumference, the difference between the published diameter of my Yokohama S.drive 245/45R17 (25.6") and the "revolutions per mile (812) comes out to about 3%. Yeah, I'm being a nerd here, but little things can add up and as the good Mr. Smith continually tells us, "Races are won by tenths."

Edited November 16, 2013 by JohnK
additions

[WestTexasS2K]

The F22 Trans does have a slighty taller 5th and 6th gear. To get a good calculation you need to measure the actual circumfrence of your tire and then calculate the diameter. Actual sizes vary between brands. I have seen as much as 1" difference between brands. That make a huge difference when calculating gearing.

[rss]

so, i evidently have a 3.5455 differential. 77mph by gps at ~3500 rpm. calculating by tire size i should be going 79mph with a 3.5455 at 3500 rpm. my car was billed as a 4.44 at sale and i didn't doubt it because the math works out pretty close if a secondary reduction gear is not considered. and i just found out about the secondary reduction gear. no wonder i have been confused. i mean, shifts just don't come up lightning fast in my car like they are described with a 4.44.

anyway, i think it's just as well, as i'm happy with my current ratio. a 4.11 might be nice but i'm pretty sure for my purposes a 4.44 would be too short.

[rnr]

Jim and I just measured the one in my car and its a 3.9. Seeing that I'm already revving high on street and track I'm not sure how people live with a 4.44 unless they have larger diameter wheels. The 4.11 would be as high as I would go personally.

[powderbrake]

Jim and I just measured the one in my car and its a 3.9. Seeing that I'm already revving high on street and track I'm not sure how people live with a 4.44 unless they have larger diameter wheels. The 4.11 would be as high as I would go personally.

 

I don't have any problem with the revs on the street. On the highway, 4000 rpm at 70 mph is a bit "buzzy", and I would not like to take a 3000 mile trip on the interstates, but a little 200 mile blat is lots of fun. I am running the 245/45/R17 RT615K Azenis on the rear.

 

Besides, this is a fun toy, and it's loud and windy, and fun, and fast, and fun, and ferocious, so it doesn't have to be "normal", and it has 5000 rpm left over at 70 mph.

 

This concept of the S2000 not having enough torque is not applicable to a 1400 pound car with a 4:44 diff. It cruises around town at 30 mph in 6th gear, and does not shudder or hesitate when you press on the go pedal.

It simply drives like a regular car, however one that you can drop down a couple gears, floor it and scare the stuffing out of a passenger.

 

On the track it seems to work well. Practice makes perfect, and keeps you out of the rev limiter. I am sure that different ratios work better on different tracks, but I don't track it any more, I just use it a a fun toy.

[rnr]

I don't have any problem with the revs on the street. On the highway, 4000 rpm at 70 mph is a bit "buzzy", and I would not like to take a 3000 mile trip on the interstates, but a little 200 mile blat is lots of fun. I am running the 245/45/R17 RT615K Azenis on the rear.

 

I'm at 4k RPM at 70 as well which is interesting - I'll see if I can go out later today and remeasure just to be sure. My Elise is at 4k rpm at 75 as well so I dont have a problem with staying higher in the rev range on the street.

 

My issue with going higher would actually be around using it on track - I'm already in 5th gear down the front straight and dont really want to have even more shifting. It would IMHO also be harder to optimize on track as well as you'd have to keep deciding which gear to take the corner as the vtech band wont cover long turns in the same gear. But I guess being data-intensive about that is what racing is all about.

Edited November 17, 2013 by rnr

[jimrankin]

I had figured that RNR was running the 4:11 rear gear because of the difference in our RPM's at freeway speed and his superior acceleration when we are side by side at speed. He may also have the earlier 6 speed so in 5th and 6th he does have additional overall reduction. I'm running the 3:55 rear to his 3:90 and if the transmission is also adding a little bit of reduction that would also drop his final drive ratio (in 5the and 6th only).

RNR also brought up the fact that going to 4:11 or 4:44 might not be the best thing for some of the tracks we run due to "running out of RPM" on some corners or corner complexes. Might leave you either feathering the throttle to stay out of the rev limiter, short shifting between complex corners or just having to need to shift too many times compared to the higher ratios. Looks like the only way to tell is to use two different ratios and a data logger at the track to see. Unfortunately it's a bit of a chore to pull he differential, almost something you would need to do at a two day event with the change at the end of the first day.

Any of the track drivers on the forum have experience with the 4:11/4:44 gears, especially if you had used a higher gear before and changed?

Darn...now he has me wondering what to do, what to do..... Guess I'll go butcher some more wood and think about it.

[jimrankin]

RNR's spare differential is a 4:11 but it is the non locker with spider gears and no clutch pack or I'd "borrow it" to see if worked on track. Might just go ahead and buy a 4:11 LSD, can always go back to 3:55 or get a 3:90 that

seems to work well for RNR.

[WestTexasS2K]

RNR how did you measure to get 3:90? Im not aware of the 3:90 being an option on the Subi R160 diff. Only ratios I have seen is 3:54, 3:70 (rare) 4:11 and 4:44. The STI r180 is available in 3:54, 3:90 and 4:44 (Japan only) some have made it state side.

I prefer the 4:44 for street and track personally. Running 275/40/17 rpm are 3900-4000 at 70. Having driven 10k miles over a 2 week period I can honestly say that the engine noise is beyond annoying after a few hours of freeway driving. If I was going to do some touring I would throw a 3:54 in for the tour and then swap back to the 4:44 for run around town and track events. If you are running a 15 slick for track use then 4:11 would likely be the best option.

 

If you have power adder like 7evin then 3:90 is pretty good. Its almost impossible to shift 2nd - 4th without hitting the rev limiter because it revs so fast. The 4:44 would be near impossible to shift fast enough.

[rnr]

RNR how did you measure to get 3:90? Im not aware of the 3:90 being an option on the Subi R160 diff. Only ratios I have seen is 3:54, 3:70 (rare) 4:11 and 4:44. The STI r180 is available in 3:54, 3:90 and 4:44 (Japan only) some have made it state side.

 

JimRankin counted input shaft bolts while I turned the rear wheels. One rear wheel rotation was 15 bolts which at 4 bolts on the shaft equates to 3.75-4 so we guessed it was 3.9. It is possible that it could be a 3.7 I guess.

 

I just drove the car and verified that 70 mph (GPS) is 4100 rpm and 50 mph is just over 2800 rpm. I assume my gearbox is an AP1 as the original engine was an AP1.

Edited November 17, 2013 by rnr

[jimrankin]

The Subaru shop manual I have on disk shows the 3:90 as one of the "available option" ratios for American import cars. It was only used in a few models, and then as the option. I can spin the disk up to see which ones if anyone wants to know.

We were just over 15 bolts so just over the 3:75 reduction, probably just enough to get close to the other .15 to make the 3:90 ratio.

[Mondo]

This Subaru site, among others, says STI after 04 were 3.90

 

http://forums.nasioc.com/forums/showthread.php?t=1112781

[m wirth]

Hope this doesn't digress too much. I have, supposedly out of a Forester, a 3.90 viscous LSD, which is just fine and seems to be a good ratio for the tracks I drive. Two questions: do all the r160 diffs use the same output drive shaft splines? and has anyone installed a Quaife, Torsen or Cusco LSD center section in an r160. I plan on get another open 3.90 and install one of the above.

thanks

mike

[jimrankin]

Going out tomorrow, weather permitting, and do an RPM to GPS 70MPH check to see just what I'm turning. When we checked RNR's car we determined it was just over 15 bolts (four for one full revolution of the drive shaft) so we were just over 3.75 to one, probably a 3:90 as that is a stock ratio.

Holly Crap Batboy, what do I have ??? Just counted this several times and I'm at 11.2 bolts for a full axle revolution. No slop in the differential and it's at full lock up as both wheels are turning in the same direction at the same speed. Unless I'm doing the math wrong this is a reduction of closer to 2:80. Nothing in any info I could find has a "final drive" ratio lower than 3:54.

If Subaru is counting the reduction in the overdrive 6th gear this would make my rear gear about in the 3.50ish range. Or would that multiply it and make it a 2:26? I'm confused, but that's not unusual

Maybe I'll jack it up again and try spinning the other wheel but I can't see that making a difference.

.

[JohnK]

Arithmetic: the spreadsheets I posted have the arithmetic I used in the cells, so if you download the *.xls file you can use it as a reference. A while back Loren saw the figures I posted for this thread and said they were accurate, given slight variations for the 17" tires different people run. The Primary Reduction comes from a third shaft in the trans. The Mainshaft plugs into the clutch, like god intended, and the Mainshaft drives the Countershaft. The final output is not the Countershaft but a third shaft that's driven by the Countershaft, and that's where that additional reduction comes in.

A while back I accurately measured the different ratios on a Ford Type 9 (Superformance S1) by attaching a degree wheel to an axle and then turning the input to the diff exactly one full turn, if I remember correctly, and then reading the degrees off the axle. Lock the axle you're not reading from to prevent its movement.

Reading a manual I got from WRXINFO website (earlier) a 2006 Impreza WRX with automatic trans has an LSD (viscous) and a 39/10 (3.9000) ratio.

[jimrankin]

Sun finally came out and I just got a chance to run the car at speed. 70MPH is just getting to 3000RPM in 6th gear. I was hard to see the GPS on my phone and the tach at the same time so may be off 50 RPM one way or the other, but close.

I'm running NT01's 275-40-ZR17 rears so a fairly good sized tire but not enough to make the really large difference in RPM @70MPH that others have posted here.

[rss]

jim, it sounds like you have a 3.55, eh?

[jimrankin]

I will venture to guess that with 3K RPM @ 70MPH it has to be the tallest gear available, and that seems to be a 3:55. RNR is over 1,000 RPM higher at 70 so that should put him higher than 3:90 because it's basically 25% more RPM so it should be a 25% difference in final drive. Part of that has to do with the overdrive built into top gear so it's not just as easy as 3.55 X 1.25 = 4.438. Powderbrake said he is at about 4,000RPM at 70MPH so that would put RNR into the same bracket.

Since my last high school math class is really pushing 50 years ago it seems I've forgotten just about every formula I ever learned. Time to bring up johnk's spread sheet I guess.

[JohnK]

I updated my diff ratios spreadsheet

https://dl.dropboxusercontent.com/u/49212285/AsstdStuff/MPH-RPM05.xls

and the webpage

https://dl.dropboxusercontent.com/u/49212285/AsstdStuff/MPH-RPM06a.html

data

 

I have a sheet from a Subaru Parts desk giving ALL the pinion-crown combinations delivered on the different Subaru models (back when I thought buying a gear set was the way to change my ratio.)

These are:

Ratio crown/pinion teeth.

4.444 = 40/9

4.111 = 37/9

3.900 = 39/10*

3.700 = 37/10*

3.545 = 39/11

*for a 2006WRX AT is 3.900, MT is a 3.700, both are (viscous) LSD according to the 2006 manual at WRKINFO.com.

 

N.B. the following:

- the RPM-MPH XLS file has human-readable names for all the paramteters in the sheet so you can see how I'm doing the calculations. That also means it's easier to make changes, like for different wheel circumferences.

Note that Tire Rack gives (sometimes) REVOLUTIONS PER MILE for the different sizes it sells. This value is a lot more accurate since the circumference calculated from the wheel/tire diameter don't take into account the distortion due to load.

-Primary reduction is different for different year ranges:

1:1 for S2000 engines 2000 - 2003

1:203 for S2000 engines 2004-2005

 

Now, the gear set data I have has codes for the different models which would be useful if anyone can decipher the codes, which I assume is based on encoded model abbreviations known to some others, but not me. Here're a few of the lines. If this can be figured out, it'd be very useful for people trying to find a diff with the correct ration for their needs. Another question, is this diff unique to Subaru or is it fitted on any other models?

 

Applicable model G/R

(TS+OBK).MT 3.900

RS.AT 4.444

RS.MT + (WRX+TS+OBK).AT 4.111

MT.205 3.545

S.AT.251 4.444

S.MT.251 +W.AT +AT.205 4.111

W.MT.251 3.900

5MT.205 3.545

6MT 3.900

RS.5MT +(TS+OBK+WRX).AT 4.111

Edited November 27, 2013 by JohnK