Best diff ratio for turbo cars

[cecotto_singh]

Just wondering if there is a particular ratio of diff that suits ours cars with turbo engines?

[UweM3]

I don't think there is "a" turbo diff ratio. All depends on how fast you want to go.
And what horsepower your engine is putting out and your max rpm. Taking into account that many turbo'ed engines make max bhp at lower rpm's than before I do not know any diff off the shelf with a suitable long ratio. Maybe Anders can chime in.

[Ted B]

One has to consider the overall gearing, which takes the transmission into account.

For U.S. cars, I elected to drop from the 4.1 to a 3.7 ratio. The lower numerical gearing (10%) loads the engine earlier and spools the turbo faster in the lower gears. Also, the much increased torque of the turbo engine works much better with somewhat taller gears, as the lower gears can be rendered useless due to traction issues. And finally, the taller gearing is better for engine longevity and fuel economy.

For the Euro cars, what is the factory ratio? 3.45? I would look at dropping down at least one step, maybe two. One issue with the Euro cars is the close ratio box is weaker than the U.S. box, so that can create an issue if there is enough torque on hand.

Also, I should mention that lower numerical ratios in the diff are physically stronger, which is a good thing.

[Az in Oz]

3.25 and 3.15 for evo Ted

[Ted B]

If that is the case, the 3.15 ratio would be preferable where the close ratio box is used. However, the stronger U.S. gearbox (and higher numerical diff) is a better option for turbo cars.

[OZM3]

I have a 4.10 diff. I find my first gear is all but useless now. I basically get iinto 2nd within about 10 or so metres from take off. I'll keep until I drop in my E34 M5 rear end which has a 3.91 diff.

[Ted B]

Which gearbox does your car have?

[OZM3]

US H Pattern.

[Ted B]

Just keep in mind that 4.10 to 3.91 is only about 5%, which is a minimal difference and may not be worth the effort. I located and rebuilt a diff from a 325 with a 3.7 ratio, which gives about an 11% difference, and I'm thinking that I could have gone even a step lower.

[OZM3]

Yes but keeping in with how I like to do things, try before I buy. So I may as well use it and if itsnt working then look lower. The E34 M5 rear end someways off at this stage.

[BingM3]

2.93-3.25 ratios

[Mortimer]

I have 2.93:1 with CR gearbox.
Gears 7500rpm, 215/45-17": 80-124-168-236-297km/h. Fifth gear 2000rpm = 80km/h.

[GGray]

I have a 3.73 in my 535 turbo and its still too short geared 1/2 are pretty much useless... 3/4/5 go like hell though.

I think a 3.46 would be better or even a 3.25 for higher HP cars.

In the E36M3 turbos highHP guys go from a 3.15/3.23(depending on year) to a 2.79 or a 2.97 they freaking haul balls with the longer gearing...

[OZM3]

What diff ratio's are available that bolt stright in?

[GGray]

2.79 eta cars
3.25 E28 535s
3.46 E24 635
3.73 E30 325
3.91 E28 M5/Auto E30
4.10 M3/Auto E30

I think thats it. These are US spec cars. Bolt in as in swap output and driveshaft flanges and diff bolts right in...