Pros/Cons of 75W140 vs 75W90...?

[joecar]

Originally Posted By: d00df00d
Originally Posted By: Ben99GT
Originally Posted By: joecar
Originally Posted By: ARCOgraphite
You loose 6-8% HP in the rearend where the power transfer makes a 90 deg turn - why make it worse?

You say I will lose 24-32 HP (that sounds over-estimated)...? How do you come up with 6-8%...?

The difference isn't anywhere near that much at temp...

http://www.redlineoil.com/content/files/tech/Gear Oil Tech Info.pdf

[Posts trimmed for concision]

ARCOgraphite didn't say the gear oil will cause that much power loss. He said the differential inherently causes that much, so there's no reason to make it worse by going with a thicker oil.

I don't know if the numbers are accurate, but that was the point.
Ok, understood, ring/pinion loses that much.

 

[joecar]

Originally Posted By: d00df00d
How old is that recommendation? The 75w-90 scale was split a few years ago (2003 IIRC) and the thicker half became 75w-110. There aren't many 75w-110 gear oils on the market but I know Amsoil makes one, and their gear oils are darn good stuff.

If you want to go with 75w-90, I would highly recommend Motul Gear 300. That's used straight out of the bottle by full-on race cars. Can't get much better than that.

If you really want to go thicker, you'll want the best stuff you can get so that your temps stay low (thick oil doesn't cool as efficiently). Castrol SAF-XJ would be my recommendation. You can get it from BMW or Nissan dealerships since it is factory and service fill for all the BMW M cars and the Nissan GT-R.

Or, you could just stick with the GM fluid. It's probably fine, to be honest. But that's up to you.
Service manual is dated 2004.

I have a choice between Mobil 1 75W/90 or 75W/140.

 

[joecar]

So, can we come up with a summary of pros and cons...?

 

[d00df00d]

How much power does your car make bone stock? How much is the rear end supposed to take?

 

[joecar]

Originally Posted By: d00df00d
How much power does your car make bone stock? How much is the rear end supposed to take?
Oh, something like 330-340bhp and 350ftlb at the flywheel... I think it may be ok upto 400 since it has auto trans.

 

[SteveSRT8]

I seriously doubt the choice of viscosity will have any notable effect on the lifespan.

As always, it's TRACTION that breaks them. Run street tires and you're generally good to go.

I'd use the thinnest that met factory spec, choose any quality synth.

 

[d00df00d]

Originally Posted By: joecar
Originally Posted By: d00df00d
How much power does your car make bone stock? How much is the rear end supposed to take?
Oh, something like 330-340bhp and 350ftlb at the flywheel... I think it may be ok upto 400 since it has auto trans.

It sounds like you're not making a huge amount more power than stock. You should be fine with the stock viscosity.

 

[cven]

GM 10 bolt axle with Torsen...

joecar... I have the same diff in my 2000 Z-28. It's not the strongest unit to start with. Many people have had problems in F-bodies with this diff. A 6 speed just makes it worse, thats what I have and my brother did as well and we've both had problems even with the stock fluid. If you shift hard with sticky tires no oil in the world is gonna help ya. Those that have an auto trans seem to have better luck. FWIW I'm useing schaeffer's #267 80-90 at the moment.

 

[[RT] ProjUltraZ]

also have the same, i've run Redline heavy and light shockproof, RL 75-90 (without LSD additive), but now i'm running DuraBlend 80W-90. it's all good but i'm not at 400hp. you can also get an alum diff cover with the bolts that stabilize the internals, that can really help stop it moving out and breaking. and yes traction is what will break it

 

[joecar]

Yes, I have heard both sides... some break (common factor seems to be M6 and drag radials), some don't (common factor seems to be A4).

I will soon be slightly over 400 with heads/cam install... maybe a little more with long tube headers.

I am on regular street radials.

I am still trying to understand the *technical* pros/cons of 75W90 vs 75W140.

 

[[RT] ProjUltraZ]

the first two replies had it right, 140 won't stop it from breaking, just maybe less wear on the friction parts but a small power loss

 

[[RT] ProjUltraZ]

look at Durablend, it's a beauty http://www.valvoline.com/pdf/Durablend_Gear_Oil.pdf

 

[joecar]

Ok, thanks.