3.73 or 3.91 Diff?

[LonZo]

1985 535 with ZF4hp22 automatic. I plan on building a custom turbo setup soon, shooting for 400 rwhp. Car currently has a LSD 3.25 rear diff. I was wondered from anyone here that might have experience or suggestions.

Should I go with the 3.73 rear Diff ?

Or the 3.91 ?

I was wondering if I chose the 3.91, I would regret it, and would then wish I had the 3.73 instead.

Thanks
Lonzo

[T_C_D]

Totally depends on your driving style. Start with a 3.73LSD. They are cheap and easy to find.
Todd

[Bill in MN]

My 3.73 is great for daily driving. I've no experience with the 3.91.

[bahnstormer]

the 3.91 was in my sisters 90 535ia
and we swapped it for a 4.27
much better oomph at the expense of some
top speed is in my opinion worth it...the auto's
need it esp in the e34

[canyoncarver]

used a 3.91 in my kid brothers NA 88 535 with the sport auto. engine revs are consistent with a 3.73 equipped 5 speed car.
offers much more response and fun factor than your current 3.25s.......

[LonZo]

Thanks for the response. Always great advice at this forum.

Cheers
Lonzo

[gol10dr1]

if you have 400 ish hp, that 3.25 would be great for highway driving and all around acceleration. if you plan on doing some drag racing then the 3.91 would come into play. i personally dont think you shoud change but i also have 420 ish hp and i have a 3.45 that i am pretty happy with so you might want to go for something taller. top end will be sacrificed but that means you can only go 130 mph instead of 135!

[LonZo]

[QUOTE="gol10dr1"]if you have 400 ish hp, that 3.25 would be great for highway driving and all around acceleration. if you plan on doing some drag racing then the 3.91 would come into play. i personally dont think you shoud change but i also have 420 ish hp and i have a 3.45 that i am pretty happy with so you might want to go for something taller. top end will be sacrificed but that means you can only go 130 mph instead of 135![/QUOTE]

Thanks, but do you also have an Automatic? If I had the 260 Manual Transmission, yes I would keep the 3.25 or go even with a 2.93.

Lonzo

[Tjn182]

I have a manual, I'm thinking about plopping on something a little shorter than my 3.25. I might hit up some junkyards when I get to Charlotte today.

[gol10dr1]

[QUOTE="LonZo"]Thanks, but do you also have an Automatic? If I had the 260 Manual Transmission, yes I would keep the 3.25 or go even with a 2.93.

Lonzo[/QUOTE]

nope, 5 speed.

[Bill in MN]

As far as I know, Todd and I are probably the only ones that have auto trannys in our turbo cars. My car didn't come alive until I put the 3.73 in it. You also may want to think about a 3k rpm torque converter. It too made a big difference in the overall performance.