Another trans/diff oil post - 75w90 or 75w140 help?

[BmacIL]

My diff is at the shop getting 4.09's installed. Optimum Performance Mustang told me to use 75W140 because the 4.09's will produce more thermal load than the 3.73's - any opinions on that thought?

Seems logical. Higher rpm, more heat.

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[Mikepol2]

Just chatted with Ford Performance. They said even with the 4.09's and Torsen, they recommend staying with 75W85 and adding the friction modifier.

 

[GregO]

Just chatted with Ford Performance. They said even with the 4.09's and Torsen, they recommend staying with 75W85 and adding the friction modifier.

You may want to reach out to Torsen directly.
The 75W85/90 is logical. I’m not so sure about adding friction modifier to the Torsen.

 

[aleccesarenriquez]

Just to add my 2 cents,

When I swapped pumpkins from 3.73s to 3.31s, we added 75w140 as I push the car hard pretty often. Turned out to be a bad idea because the diff started to make a pretty loud "whirring" noise whenever I was turning. Switched back to 75w90 and it went away. If people aren't even recommending 75w140 for 4.09s, then it makes sense that 3.31s would definitely not like it.

 

[Mikepol2]

I'm no expert but based on what I've read...friction modifiers improve surface lubricity, and Torsens operate completely mechanically through gear action, so it would seem reasonable that the modifier may not be necessary but in the worse case couldn't hurt. Ford Performance is probably just covering their butts looking out for the warranty though.

 

[BmacIL]

I'm no expert but based on what I've read...friction modifiers improve surface lubricity, and Torsens operate completely mechanically through gear action, so it would seem reasonable that the modifier may not be necessary but in the worse case couldn't hurt. Ford Performance is probably just covering their butts looking out for the warranty though.

The friction modifier reduces friction and reduces the torque bias ratio (TBR) capability of the torsen. The torsen uses the internal friction to split the torque, and the wheel with the most grip will generally get about 2.8 times more torque than the other. Without the modifier it's closer to 3:1. It's not a big change but it is some. The upside of the modifier is significant reduced groaning/grinding noises in slow corners on the street. Thicker oil will also reduce the TBR further still, though will help with temperature management.

In summary: if the car regularly sees street duty, put the modifier in.

 

[Ghost50]

So...I put in 75-140 this past weekend. Should I swap back to the 75-90? It’s the BG fluid with the LSA.

MT82 with 3:73’s.

 

[GregO]

The upside of the modifier is significant reduced groaning/grinding noises in slow corners on the street.

I didn’t experience groaning/grinding noise when changing to Gear 300 75W90 non-LS fluid.
I did feel the increase in Torque Bias Ratio.
I’m not disputing that other fluids without modifiers may cause groaning/grinding.

 

[Nickel]

I used this BG kit about 350 miles ago, and it's been great. I used the 75w90 and the labels on the back said that the diff fluid is for high heat applications and doesn't require a friction modifier. Considering it's a Torsen, I can understand why.

https://www.opmustang.com/store/p7/...NG_-_Transmission_and_Rear_Axle_Fluid_.html#/

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