Why MTL-P isn't for Diffs...

[edmundu]

I was wondering why MTL-P is NOT recommended for hypoid diffs, even though it is now dual rated GL4/5?

I was thinking of using it in a 5 spd man transaxle which contains a hypoid diff. Because it is in a cool climate, is the reason why I'm considering it.

 

[digitaldrifter91]

dont they have mt90 which is gl4 and 75w90 for the diff which is only rated for gl5 differential? they halso have a 75w90NS as well.

i hear that mt90 takes place of MTL and is a bit better...thats why im going mt90 in my 5 speed 300zx 86.

www.redlineoil.com

 

[edmundu]

I was referring to Specialty Formulations Inc. (Sorry for not being clear on that). And since I have a Subaru Legacy, it shares its front diff with the manual trans. But unlike other transaxles, Subaru's use a hypoid setup, which needs GL5 specs.

I just was posed a question of why can't MTL-P be used for a diff, when it is rated for GL5? Is it because it's too thin? If so, why is it ok to use in a hypoid equipped transaxle?

Ultimately, I would want to use this thinner fluid for cold winters.

 

[smthrj53]

The fluids recommended for manual transmissions have a different additive package from that used for hypoid drives. The synchronizers don't usually get along with the hypoid additives, and hypoids wear faster without that package.

I doubt if you would notice much difference initially, but reports are using the wrong one increases wear rates, reducing service life.

 

[edmundu]

So why would manufacturers like Subaru, allow front diffs to share the same fluid with the transmission? I can only choose to have nice shifting or protecting my diff?

If GL5 is the spec needed for hypoid diff protection, and a fluid meets it, shouldn't it be only temp dependant on which viscosity is chosen? Kind of like we choose different grades of oil for summer vs winter...

 

[MolaKule]

quote:

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I was wondering why MTL-P is NOT recommended for hypoid diffs, even though it is now dual rated GL4/5?

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Because MTL-P's viscosity is not high enough for use in hypoid drives. MTL-P has a 75W85 viscosity (11.3 cSt) such that it replaces Honda HG and Nissan/Infinity fluids and some Audi and BMW fluids for manual transmissions.

If you look at MTL-R's specs, which is designed for use in most MT's and transaxles requiring a 75W90, it does not carry that prohibition since it falls into the 75W90 viscosity range.

What the warning's are referring to are Final Drive hypoid type gearing in which the vehicle has separate differentials that are NOT integrated into the transaxle.

[ May 08, 2005, 07:31 PM: Message edited by: MolaKule ]

 

[Big Jim]

The diff in a transaxle is probably not hypoid.

Hypoid is when the pinion gear meets up with the ring gear near the bottom of the ring gear to permit a lower driveshaft which in turn provides a lower floorpan. This steeper angle where the gears meet puts mutch more pressure on the teeth, and thus the need for EP (extreme pressure additives).

 

[MolaKule]

Use the MTL-R in the Manual transaxle and 75W90 HDS-5 in the rear differential.

The HDS-5, and most diffy fluids, DO NOT have the proper friction modification for manual tansmission synchronizers.

 

[mechtech]

Are you sure that you don't have HELICAL final drive gears?

 

[MolaKule]

What year and model is the vehicle?

It does not make sense to design a hypoid drive into a transaxle. Hypoid drives were designed where the torque tube or drive pinion entered at an angle above the axis of the ring gear. These axles are space intensive and bulky. In a transaxle, the drive torque is inline with the CV joints and internal gearing.


Most transxles use helical gears with drive pinions in order to reduce the space required for mechanical differential action, just like Rear drive Lawn Mowers. In fact, the reason for the transaxle design was to allow a small structure that contained the transmission AND differential in one small package to reduce frontal area dynamic drag.

In the rear drive riding lawn mowers, the tranny is a pancake structure with an integral differential using helical gears and pinions. Otherwise, you would have this HUGE bump in the transaxle.

 

[edmundu]

quote:

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...What the warning's are referring to are Final Drive hypoid type gearing in which the vehicle has separate differentials that are NOT integrated into the transaxle.

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So being that my Subaru's front diff is within the same housing, I can use MTL-P for better cold performance?

And could I also use MTL-R instead of HDS-5 for my rear diff, also for reasons of better cold weather performance?

And one last question...

In the opposite weather, if it is regularly hot (steady 75-95), would using thicker fluid like HDS-5 be recommended over MTL-R for my transaxle?

 

[MolaKule]

quote:

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So being that my Subaru's front diff is within the same housing, I can use MTL-P for better cold performance?

And could I also use MTL-R instead of HDS-5 for my rear diff, also for reasons of better cold weather performance?

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Do not use MTL-R for the rear differential. MTL-R is optimized for MT's and transaxles, not separate final drive differentials. HDS-5 is optimized for use in final drive differentials.

While your transaxle may include a differential, are you sure it has a Hypoid design differential contained therein?

 

[edmundu]

Yeah, the FSM shows/describes it as a hypoid final drive for front & rear diffs.

So would I be able to run the HDS-5 in the transaxle? Would it only hinder cold shifting, by being thicker? Or would I give up any other advantages to MTL-R?

On another note, I really wish the manufacturers would be a little more specific on the oil specs they require. I mean, my manual only states that a GL5 grade with 75W-90 viscosity, for both the transaxle & rear diffrential. Which would lead one to believe that the same fluid can/should be used for both of them?

On top of that, they display a viscosity chart that actually lists 85W oil as being in the same temp range as the 75W-90? So that's why I was asking if the lighter MTL-P or MTL-R could be used in the transaxle and rear diff?

 

[MolaKule]

No changes in recommendation Ed.

The MTL-R has enough viscosity to handle it.

 

[edmundu]

The vehicle is a 2005 Subaru Legacy GT. But, Subaru actually uses the same design transaxle across all their cars! Only in their Auto transaxle applications do they separate the diff form the transmission sumps.

And after checking the FSM again, they are definitely listed as being hypoid final drive for both front and rear. Which makes sense as the engines are mounted longitudinally, so the power has to make a 90 degree turn.

So does the recommendation change any?

 

[tdi-rick]

quote:

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Hypoid is when the pinion gear meets up with the ring gear near the bottom of the ring gear to permit a lower driveshaft which in turn provides a lower floorpan.

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Getting

here, but the real reason hypoid differentials are used instead of spiral bevel designs is that for a given ratio, the pinion head is much larger in diameter, and the teeth engage a greater area, leading to a stronger pinion, and hence differential.
The ultimate extreme of this is a worm drive.
The downside of this is that with the sliding, wiping action of a hypoid diff, it is more inefficient at power transfer, converting this energy to heat.

The British are extremely fond of spiral bevel diffs, which, as an example, are used throughout Land Rovers range of vehicles.
The crown wheel/ring gear are almost 9" in diameter, yet Landy's are known for spitting teeth off the crown wheel and pinion. A popular modification here in Oz is the fitting of a Toyota 8" hypoid centre, which, even though it has a smaller ring diameter, its pinion head is 21mm larger for the same final drive ratio, leading to a much sronger differential.
One of the reasons Ford's famed 9" diff is the darling of the go faster set is its extreme hypoid design making it a strong design (as well as the pinion nose being supported by an extra bearing)

 

[Michael SR]

the subaru engine/tranny are longitudinal, not transverse. hence the hypoid.

-michael

 

[bulwnkl]

quote:

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Originally posted by Michael SR:
the subaru engine/tranny are longitudinal, not transverse. hence the hypoid.

-michael

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Correct. I asked this same question of Molakule a couple weeks ago via email. His answer was the same to me as it was here. My Subaru is a '91 XT6 AWD manual.

Actually, all the AWD (that means all Subies now) Subaru trannies have two differentials in them. Only one is hypoid, though. The "center" differential is not hypoid, only the front diff; it is a hypoid final drive.

Molakule, would you mind explaining how/why the MTL-R is not optimized for hypoid final drive use?

Also, please note that the prohibition IS on the MTL-R page regarding use in final drives.

FWIW, I run RedLine's 75W90 in my tranny because I cannot afford to kill my differential. I suspect very strongly that SF's MTL-R would shift better, but I am nervous about diff life given my lack of knowledge regarding the nature of the "optimizations" of MTL-R vs. HDS-5 and the limitations of each in a transmission vs. a final drive (remember, the Subarus are both.

[ May 12, 2005, 09:40 PM: Message edited by: bulwnkl ]

 

[bulwnkl]

Whoa! I messed up and can't edit. All Subaru AWD manual transmissions have 2 diffs. The autos only have the front hypoid unit. The center "diff" in their autos is not a diff at all; just an electro-hydraulically controlled clutch pack.

 

[MolaKule]

The viscosity of the MTL-R is sufficient for the small hypoid in the Subaru.

The HDS-5 is optimized in terms of viscosity and other adds for final-drive hypoid-type and other design differentials.