What is the pro and con of a 25% diff vs a 40% diff?

[eriknetherlands]

I'm thinking about adding parts in my garage; gathering stuff for a 5spd conversion, and an LSD contributes nicely to that.

1. My car has a carbed 3.0 engine; I think 3:45 will fit nicely with my engine (stock) and driving (just a bit spirited I'd say; I'm not pulling black tire stripes on the tarmac). Missing something here?

2. When considering LSD's, one option is 25 vs 40% ; what is the argument going for either of them?

3. I've seen that there is a difference in 8 vs 10 bolts on the carrier. I think that only touches upon serviceability / spare parts availability, right?
Aspects like driving, installation and/or being an ideal combination to a 5spd are not a consideration, right?

Anyone have any knowledge/hints/experiences to share?

(I have read through the FAQ section on gearboxes and differentials, but these 2 aspects weren't covered)

 

[Klassic]

Road use, 25%, track 66%. Going to 40% for the road will cause more tyre wear and a less alive car on turn in IMO.

 

[Breiti]

Exact what klassik wrote.
Road use better less.
Especial when you got a wet road, you can't handle your tail anymore if the ls% is high.

Breiti

 

[JFENG]

less alive car on turn

Which means more under-steer when on throttle coming out of a curve.

 

[JFENG]

Especial when you got a wet road

This is dependent on the type and design of the LSD. A classic clutch + torque plate type LSD (like in our E9’s), can be made to have a different % of limited-slip for when the engine drives the wheels (positive torque) and when the wheels are putting load back into the power/drivetrain (negative torque). This is done via the 2 different ramp angles on the torque plate.

On a slippery road, more LSD means more chance to have both rear tires break/lose grip. This can be reduced by reducing the % of anti-slip (via higher ramp angles). For me, the value of LSD is mainly for acceleration, and I would setup for very little “limited slip” under negative torque conditions because it maximizes stability & grip when slowing down into a corner or when braking on an asymmetric mu surface. This minimized the chance of losing all rear tire traction and skidding/spinning.

I agree that for the street 25% is fine on the positive torque side (40% is ok for skilled drivers who can compensate for the greater on-throttle understeer), but I disagree with the stock ramp angles for negative torque conditions.

 

[gwittman]

The 40% will require more attention when accelerating while turning with wet pavement. Most of us don't drive in the rain, so that is normally not a problem. However, this can also raise its ugly head on slippery surfaces like some concrete.

Another thing that you can get with a 40% LSD is when accelerating around corners, especially sharp corners, is the inside tire tries to break loose if you are on the accelerator pedal a little too much. This is usually not a problem but does create a little more tire wear as mentioned before.. My tires usually get old before they wear out so that is not a problem for me.

I have never experienced understeer on turn in, but I can see where it might be an issue on really quick sharp turns. Our Coupes are a little nose heavy for really quick sharp turns anyway.

 

[Arde]

Tire marks on the pavement? LSD does not change that.
A dogleg tranny can help, or a shorter differential gear ratio also.
I may change that on my E24, I want to cruise around 3500 to 3800 RPM.
Alternately you can use cheaper tires .

 

[eriknetherlands]

Thanks for the valuable input .

Just bought a 3:45 25% LSD based on your inputs!
Erik.

 

[eriknetherlands]

Just recieved; quick check to confirm I didn't buy a cat in a bag: it's an S25!

 

[Breiti]

I just figure out that mine is also an LSD...
I just wonder, who bought a automatic with a LSD.

BREITI

 

[autokunst]

Just recieved; quick check to confirm I didn't buy a cat in a bag: it's an S25!

That is the same rear unit that I plan to run (3.45 @ 25%). Nice find!