Final Drive Ratio - Front Diff Ring Gear Differences

[95REGF150]

So came to find that a built trans I bought is a 97-99 ratio and my transfer case is a 90-96 ratio.

Since transfer cases are so hard to come by these days it looks as though it may be easier just to swap out the ring gear in the transmission. However, I have a couple questions regarding that. When looking up I find people referencing the 58 vs 57 teeth on the ring gear.

From what I have read all the 1G cars & the 95-96 2G cars had 58 teeth on the ring gear. Then for 97-99 they went to 57 teeth for some reason. I also found some references to some of the 1G cars having a different helix angle on the ring and pinion set but no specifics on that.

So the main question is can you use a 1G 58 tooth ring gear in the 2G trans? Does anyone know which years had the different helix angle? Was this a 90' only difference similar to the 22 spline on the transfer case? Is there any way to visually tell the helix angle on the gear by looking at it?

Also there is a seller on Ebay selling some front differentials from 1G's. In his ad he says confirm the notches machined into the OD of the ring gear are the same as on yours. Can anyone confirm that these notches mean anything? I have found some pictures of 2G ring gears with only 1 notch.

2 notches:

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3 notches:

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1 notch:

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Looking at the above photos the helix angle looks greater on the 1G rings but it's hard to tell

 

[FIBA M - EC17PSE]

my 2G auto has 2 notch but they wide notch. one on the edge and one near the middle

 

[bastarddsm]

So came to find that a built trans I bought is a 97-99 ratio and my transfer case is a 90-96 ratio.

I'll trade you a matched pair 90-96 ring and pinion for your 97-99 set. Cover the shipping is all I ask.

 

[95REGF150]

@bastarddsm thanks I'll take you up on that. Be sending you a PM.

I know you do trans rebuilds do you have any insight on the ratio difference? Why do you think they did it? When they profile shifted the ring gear to get 57 teeth at the same contact diameter they did end up with wider tooth thickness. So was this the purpose just to pick up some strength?

To that end have you seen more failures of the 58 tooth ring compared to the 57 tooth ring?

 

[95REGF150]

The further I look into this the more interesting all this is for me. Looking up DSM gear ratios everywhere seems to have the same chart (VFAQ, RRE, etc.) which is shown below:

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Now it only shows a single category for 2G MT cars with the 57T ring gear. Then it shows the 1G MT cars as a 58T ring. Then the two transfer cases are 1.074 & 1.090 respectively to make up for the difference so that both cars use the same 3.545 rear diff which has all been understood before.

However, the weird thing is everyone only ever lists 1 final drive ratio for both the 1G & 2G cars 4.929:1. But as the center differential primary reduction never changed it is 1.275:1 on both cars which makes the final drive ratios between 90-96 & 97-99 different:

1G 90-96 final drive (1.275 X 3.866) or (1.266 X 1.090 X 3.545)= 4.929:1
2G 97-99 final drive (1.275 X 3.800) or (1.275 X 1.071 X 3.545) = 4.845:1

This isn't a huge difference but that would make the later cars slightly taller geared. For instance at 65mph in 5th with a 25" tall tire that would make RPM's:

1G 90-96 final drive - 2870 RPM
2G 97-99 final drive - 2820 RPM

It's only a 50 RPM difference so I see why this would go unnoticed. I just never hear anyone make any mention of the final drive ratio difference when talking about the early vs late cars.

Am I missing something here? Can anyone confirm what I'm getting at? @twicks69

 

[twicks69]

It is a typo in the VFAQ.

95-96 have 58-tooth front diff ring gear and 1.090 ratio (24/22 tooth) transfer case
97-99 have 57-tooth front diff ring gear and 1.074 ratio (29/27 tooth) transfer case

The pinion shaft in both 95-96 vs 97-99 is the same, as is the rest of the gear ratios.

The Evo 1 and Evo 2 use the 1.090 ratio transfer case, and the Evo 3 use the 1.074 ratio transfer case but use a different front diff final drive to match with the different rear diff final drive.

As for what happens when you mix up front diff to transfer case final drive ratios.... well, it might seem inconsequential to some, but you would destroy your center differential quickly as well as nuking your viscous coupler. The car would also not roll easily and would be binding up causing it to put significant load on the engine under any rolling condition and will cause issues like frying a head gasket, nuking a turbo, overheating, and then the additional stress on all of the drivetrain.

For reasoning as to why Mitsubishi chose to change the final drives, I have no clue. The 97-99 transfer cases are not any stronger, and the front diff ring gears are not either.

 

[95REGF150]

It is a typo in the VFAQ.

95-96 have 58-tooth front diff ring gear and 1.090 ratio (24/22 tooth) transfer case
97-99 have 57-tooth front diff ring gear and 1.074 ratio (29/27 tooth) transfer case

The pinion shaft in both 95-96 vs 97-99 is the same, as is the rest of the gear ratios.

The Evo 1 and Evo 2 use the 1.090 ratio transfer case, and the Evo 3 use the 1.074 ratio transfer case but use a different front diff final drive to match with the different rear diff final drive.

As for what happens when you mix up front diff to transfer case final drive ratios.... well, it might seem inconsequential to some, but you would destroy your center differential quickly as well as nuking your viscous coupler. The car would also not roll easily and would be binding up causing it to put significant load on the engine under any rolling condition and will cause issues like frying a head gasket, nuking a turbo, overheating, and then the additional stress on all of the drivetrain.

For reasoning as to why Mitsubishi chose to change the final drives, I have no clue. The 97-99 transfer cases are not any stronger, and the front diff ring gears are not either.

Thanks Tim I really appreciate the info. Gonna move ahead with changing my tranny over to the early final drive.

I wonder how different can the ratio be front to back without tearing up the center diff? In the early ratio you have 58/15=3.866 combined with 1.090x3.545=3.864 so that's still a difference of about 0.002:1 front to back. Then on the late cars you have 57/15=3.800 combined with 1.074x3.545=3.807 which is even worse a difference of about 0.007:1 front to back. So the original ratio is actually slightly better matched.

I totally get mixing and matching would be bad. Using a late case on a early trans is a 0.059:1 difference and then a early case on a late trans is even worse at 0.064:1 difference front to back.

 

[bastarddsm]

I think I remember reading that there was a spec somewhere of a couple revs a mile difference was tolerable. I think it was actually for 3s cars, as those guys like running staggered wheels, but they have a similar center diff setup. If you actually run the tooth counts, the front and rear are not exactly the same ratio. EDIT: I see that you realized that. For more accuracy, run the actual tooth counts as the ratios they publish are rounded off.

 

[DSMPT]

@95REGF150 Just FYI, if you have a hard time to find a manual or auto 2gb 1.074 t-case, check out some JDM cars, too. In Japan, Mitsubishi changed the t-case gears to 1.074 from 1995 models. So basically post '95 JDM AWD cars that came with a W5M3- or a W4A3- have the 1.074 t-case, such as EVO 3, RVR, Libero etc. Also post '95 Expo and Summit have the 1.074 t-case I think. Those t-case's gears are the same as 2gb DSM's gears and you can mount it on a DSM. Some cases design are slightly different like the EVO3 t-case has a better design for cooling than the EVO 1-2's but other than that they are basically the same.

 

[95REGF150]

@95REGF150 Just FYI, if you have a hard time to find a manual or auto 2gb 1.074 t-case, check out some JDM cars, too. In Japan, Mitsubishi changed the t-case gears to 1.074 from 1995 models. So basically post '95 JDM AWD cars that came with a W5M3- or a W4A3- have the 1.074 t-case, such as EVO 3, RVR, Libero etc. Also post '95 Expo and Summit have the 1.074 t-case I think. Those t-case's gears are the same as 2gb DSM's gears and you can mount it on a DSM. Some cases design are slightly different like the EVO3 t-case has a better design for cooling than the EVO 1-2's but other than that they are basically the same.

Thanks for the reply. Actually I see a few auto T-Cases for the W4A3- for sale but the mount is different so I wasn't looking at those. Can you use the 1.074 gears from an auto T-case and put them into a manual 1.090 T-Case?

Edit: After Some research the only difference between the Manual and Auto transfer case appears to be the center housing. And Boostin is even making those billet now. So the answer is you can build an auto T-Case into a Manual T-Case. Maybe I will head that direction instead of splitting my trans apart.

Manual:

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Auto:

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[DSMPT]

Thanks for the reply. Actually I see a few auto T-Cases for the W4A3- for sale but the mount is different so I wasn't looking at those. Can you use the 1.074 gears from an auto T-case and put them into a manual 1.090 T-Case?

Edit: After Some research the only difference between the Manual and Auto transfer case appears to be the center housing. And Boostin is even making those billet now. So the answer is you can build an auto T-Case into a Manual T-Case. Maybe I will head that direction instead of splitting my trans apart.

Manual:

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Auto:

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Of course the cases are different between manual and auto. I meant that if your car is manual, then look for a manual t-case. And yes, the t-case gears are the same between manual 1.090 and auto 1.090, manual 1.074 and auto 1.074, they are interchangeable but doing it by yourself wouldn't be a good idea unless you know how to rebuild properly. Maybe you can ask Tim @twicks69 to swap the gears and rebuild by sending two -t-cases manual 1.090 and auto 1.074 and swap, if the budget would allow you. Maybe you should also look for in some early evo communities in facebook or Yahoo auction in Japan. In case if you find one, just make sure with the seller that the one is surely a 1.074 t-case, because many people actually don't know what they are selling...

 

[95REGF150]

Of course the cases are different between manual and auto. I meant that if your car is manual, then look for a manual t-case. And yes, the t-case gears are the same between manual 1.090 and auto 1.090, manual 1.074 and auto 1.074, they are interchangeable but doing it by yourself wouldn't be a good idea unless you know how to rebuild properly. Maybe you can ask Tim @twicks69 to swap the gears and rebuild by sending two -t-cases manual 1.090 and auto 1.074 and swap, if the budget would allow you. Maybe you should also look for in some early evo communities in facebook or Yahoo auction in Japan. In case if you find one, just make sure with the seller that the one is surely a 1.074 t-case, because many people actually don't know what they are selling...

Ya I totally understand what you were getting at now. I just had never looked that close at the auto T-Case before I didn't realize that it really is exactly the same T-case with a different mid housing. Posted those pictures for anyone else who stumbles on this and didn't know that either.

I'm no stranger to setting the lash on a differential and I wouldn't think this little 90° gearbox would be any different. I think I may pick up an auto T-Case and give it a go just swapping out the mid housing and resetting the lash. Seems easier than splitting the case on the tranny and swapping the ring and pinion for the front diff.

 

[twicks69]

You guys have some wrong info.

The 5 speed and auto tcases have a few variables.
*Input sleeve length is different. Auto input sleeves are about 3mm shorter and can be used in a 5 speed tcase housing, but not the other way around unless you grind down a 5 speed input sleeve to fit in an auto tcase. It has nothing to do with the tcase housing as a fitment concern, it actually is at the splines on the transmission output shaft. An auto tcase with a manual input sleeve installed requires grinding of the input sleeve snout to fit the auto trans output shaft. If not properly clearanced you will be dealing with a fitment problem as well as a leakage problem potentially.

*The auto transfer case mid housing is 5mm shorter than tha 5 speed mid housing and also has different mounting locations. They are not interchangeable.

*The auto transfer case front housing is 5mm offset forward and has different mounting locations. They are not interchangeable.

*The 90 full aluminum front housing tcases are also unique as they use a different dimension side cover than the 91-99 stuff so you cannot use a billet side cover or billet tcase brace on it or a cast iron 91-99 side cover.
*All of the 91-99 cast iron front housings are interchangeable if it is all manual, or if it is all auto. They are not interchangeable between auto to manual.
*There are no additional reinforcements or cooling stuff on the Evo 3 stuff. All evo 1-3, RVR, libero, 3000gt NA AWD transfer case housings are identical. Only the gear ratio changed from Evo 1/2 of 1.090 to Evo 3 1.074 in 1995.
*Tcase gear sets are the same manual and auto from 1989-1996 USDM and are 1.090.
*Tcase gear sets are the same manual and auto from 1997-1999, JDM 1995+ Evo 3, late year RVR, late year Libero, etc and are 1.074.

 

[DSMPT]

You guys have some wrong info.

The 5 speed and auto tcases have a few variables.
*Input sleeve length is different. Auto input sleeves are about 3mm shorter and can be used in a 5 speed tcase housing, but not the other way around unless you grind down a 5 speed input sleeve to fit in an auto tcase. It has nothing to do with the tcase housing as a fitment concern, it actually is at the splines on the transmission output shaft. An auto tcase with a manual input sleeve installed requires grinding of the input sleeve snout to fit the auto trans output shaft. If not properly clearanced you will be dealing with a fitment problem as well as a leakage problem potentially.

*The auto transfer case mid housing is 5mm shorter than tha 5 speed mid housing and also has different mounting locations. They are not interchangeable.

*The auto transfer case front housing is 5mm offset forward and has different mounting locations. They are not interchangeable.

*The 90 full aluminum front housing tcases are also unique as they use a different dimension side cover than the 91-99 stuff so you cannot use a billet side cover or billet tcase brace on it or a cast iron 91-99 side cover.
*All of the 91-99 cast iron front housings are interchangeable if it is all manual, or if it is all auto. They are not interchangeable between auto to manual.
*There are no additional reinforcements or cooling stuff on the Evo 3 stuff. All evo 1-3, RVR, libero, 3000gt NA AWD transfer case housings are identical. Only the gear ratio changed from Evo 1/2 of 1.090 to Evo 3 1.074 in 1995.
*Tcase gear sets are the same manual and auto from 1989-1996 USDM and are 1.090.
*Tcase gear sets are the same manual and auto from 1997-1999, JDM 1995+ Evo 3, late year RVR, late year Libero, etc and are 1.074.

Tim, sorry if I made you confused. I was talking about only the gear set about the 1.090 MD756514 and the MD756623 1.074 that are used in many cars and those are interchangeable as you mentioned. And maybe I didn't explain myself very well that what I meant is the difference that EVO 3 has a bigger transfer air guide for better cooling than EVO 2.

 

[twicks69]

What air guide are you referring to?? To the breather bulb?? Show me the casting variation in the midhousing and tailhousing please as I have not recalled any major variation other than the early mid housing with the stupid fins vs the late mid housing with the two mounting bolt holes and the kick out, which is normal stuff (90.6-91.5 vs 91.6-99).

 

[DSMPT]

What I am talking about is an air guide panel for cooling the trans and t-case that goes to under the center member. I think I still have a booklet that came from Mitsubishi that explains the differences between EVO 2 and EVO 3. Let me look for it and take a pic.

 

[95REGF150]

You guys have some wrong info.

The 5 speed and auto tcases have a few variables.
*Input sleeve length is different. Auto input sleeves are about 3mm shorter and can be used in a 5 speed tcase housing, but not the other way around unless you grind down a 5 speed input sleeve to fit in an auto tcase. It has nothing to do with the tcase housing as a fitment concern, it actually is at the splines on the transmission output shaft. An auto tcase with a manual input sleeve installed requires grinding of the input sleeve snout to fit the auto trans output shaft. If not properly clearanced you will be dealing with a fitment problem as well as a leakage problem potentially.

*The auto transfer case mid housing is 5mm shorter than tha 5 speed mid housing and also has different mounting locations. They are not interchangeable.

*The auto transfer case front housing is 5mm offset forward and has different mounting locations. They are not interchangeable.

*The 90 full aluminum front housing tcases are also unique as they use a different dimension side cover than the 91-99 stuff so you cannot use a billet side cover or billet tcase brace on it or a cast iron 91-99 side cover.
*All of the 91-99 cast iron front housings are interchangeable if it is all manual, or if it is all auto. They are not interchangeable between auto to manual.
*There are no additional reinforcements or cooling stuff on the Evo 3 stuff. All evo 1-3, RVR, libero, 3000gt NA AWD transfer case housings are identical. Only the gear ratio changed from Evo 1/2 of 1.090 to Evo 3 1.074 in 1995.
*Tcase gear sets are the same manual and auto from 1989-1996 USDM and are 1.090.
*Tcase gear sets are the same manual and auto from 1997-1999, JDM 1995+ Evo 3, late year RVR, late year Libero, etc and are 1.074.

Tim thanks I really appreciate you clearing all this up for me. I had been looking through some old threads on here talking about transfer case compatibility and they incorrectly identified the mid housing mounts as the only differences between the auto and manual.

In their defense the 5mm offsets and sleeve lengths between the 2 is not a difference you would see with your eyes.

Well I'm back to changing the tranny to the early 58 tooth ring to work with the transfer case I have. Or buy a whole 1.074 auto T-Case just to rob the gearset which doesn't seem that practical

 

[DSMPT]

Tim, this is the one I was talking about.

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[twicks69]

I thought you were specifically talking about a physical difference in the housings of the transfer case.

 

[DSMPT]

I thought you were specifically talking about a physical difference in the housings of the transfer case.

Made a confusion, sorry about that.

 

[95REGF150]

So I got to thinking more about this after @bastarddsm mentioned staggered wheels on the 3S cars and wondered how much of a difference in tire size it would take to account for mismatched transmission and transfer case.

Even though the ratios are not perfect from the factory they are very close. Below is a breakdown of the early ratio:

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You can see at 3000 RPM in 5th there is only a 0.02 MPH difference in speed that the center diff has to account for. So it's almost a perfect match as with both wheels held the same there is around 1 RPM difference side to side on the center differential. That's almost nothing.

The later ratio is actually quite a bit worse:

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Still we are talking only a 0.15 MPH difference that the center is making up for. If you held both wheels the same at this speed its still only like 7 RPM difference at the center differential. That's really not that much if you think about how slow 7 RPM is. But its still 7 times what the original ratio is.

But now lets say you have a late trans and a early T-Case as I do. That would make it:

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We're talking a 1.21 MPH difference. With both wheels held the same as they would be on the highway that would be almost 60 RPM difference front to back at the center differential. Obviously that's too much as others have stated it would kill the center diff.

Now for the interesting part. Can we fix this with tire diameter and get back to 7 RPM center difference or below? All of the above was done assuming the same tire front and back and 25" was used for sake of argument. It wouldn't matter what size tire you used the differences at the center would be the same if you used the same size tires front and rear.

So on stock 16's what if we used 215/55/R16 on the back? The actual calculated number for this size is 25.31". Then run the OEM and slightly smaller 205/55/16 on the front which is 24.88" See the below:

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It's actually better matched than the OEM late ratio at only 0.04 MPH speed difference. That equals only 2 RPM difference at the center on the highway.

I know tires can vary in actual size from manufacturer to manufacturer but its the difference in diameter between the 2 being around 0.5" that makes it work. So I would think if you bought both tires in each size from the same manufacturer the difference would be correct.

I still think I'll go ahead with changing the front ring to go with the early ratio but it is nice to know it can be done with tire sizes in a pinch. It also really only works for a late trans with an early TCase. With an early trans and late TCase the larger tires would have to be on the front which would look dumb.

What all this really shows too is that if you have a welded center diff on a late ratio drivetrain it's stressing on the components just going in a straight line let alone when turning.

 

[bastarddsm]

Seems about right. I wish I could remember where I found the spec for max difference. I was working on a way to utilize the stronger getrag gearset from a 3s and keep the dsm like ratios, without using a 3/s tcase. I think it was like jdm 6 speed front ring and pinion, late dsm tcase and an auto rear end, then like 1/2" mismatch on tires.