Acceleration & Top End

[no1_jazz]

As stated in the subject guys. I am after the best possible bottom end and top end speeds.

Which LSD is the best for our cars?

Which LSD will able me to gain these differences?

All help is much appreciated

Many thanks
Jazz

[colin86325]

If you improve acceleration with your diff ratio, you will take away from your top end speed.  And vice versa.

[Wise Old Dog]

You can't have both bottom end and top end performance. You have to decide what a happy medium is. Stock is a 4.10, if the gears are too short for you, then you have several choices. 3.91  3.73  3.64
I went with the 3.91 and it knocked about 300 rpm's off at highway speed. Still plenty fast for me, but I don't track it.
Or if you want better power on the bottom, you can go with the 4.27 and IIRC they have one more gear that's shorter than that.

[nomad]

I thought it was the opposite?

[vonkamp]

Looks right to me...

WOD. do you get any better gas mileage with the 3.91?

[Wise Old Dog]

Hard to tell, did a lot of other stuff at the same time. But it does good at 26-27 around town. Used to get 24-25.

[nomad]

So the ratio is:
For every 4.1 revs of the driveshaft the wheel will spin once?
So now it makes sense that for every 3.73 revs of the shaft the wheels spin once = higher top end speed and lower revs at any given speed over stock.

I was switching numbers in my head. I would like to get a 4.27 lsd or higher for autocross. Good launch out of a corner.

[RouteZeroDesign]

Yes for every 4.1 turns of the pinion (which is connected to the output shaft of the gearbox) the rear axle will rotate once.

There is a sweet spot in the middle of low gearing and high gearing, where you have good acceleration, dont need to change gear too often, have decent top end (even though top end is relatively irrelevant) and allows good fuel economy.

My solution to this is to fit a slightly higher ratio diff and a six speed gearbox, where the 6th gear is an overdrive, allowing me good motorway fuel economy.

[colin86325]

What 6-speed tranny did you fit?

[tony92ic]

The 4.27 would be good for a tight, slow autocross, but if you run courses that tend to have longer straights or fast sweepers it seems that you are going to run out of 2nd gear and have to shift into and out of 3rd which will give up any gains you made coming out of the corner.

We tend to run longer courses so I am already on the rev limiter in 2nd in a few spots on our typical course.  The 4.27 would slow me down i think. If you are running courses on smaller parking lots then the 4.27 would make sense.

[RouteZeroDesign]

Tony
I guess thats why most racing teams have shelves full of differentials, so that they can swap in a diff to match any circuit.
Ideally, you want the car to reach the rev limiter in 6th gear, at the end of the fastest straight in the circuit.
This is why its so hard to pick a diff for road use, as there are an infinite (kinda number of different corners and straights to choose from.

Colin
Im in the process of fitting the 6 speed gearbox from the e36 m3 (my 318is is an e36)
The gearing is the exact same as my 5 speed, except that it has an additional gear as an overdrive.

[colin86325]

Thanks for clearing that up.

Cheers,
Colin

Tony
I guess thats why most racing teams have shelves full of differentials, so that they can swap in a diff to match any circuit.
Ideally, you want the car to reach the rev limiter in 6th gear, at the end of the fastest straight in the circuit.
This is why its so hard to pick a diff for road use, as there are an infinite (kinda number of different corners and straights to choose from.

Colin
Im in the process of fitting the 6 speed gearbox from the e36 m3 (my 318is is an e36)
The gearing is the exact same as my 5 speed, except that it has an additional gear as an overdrive.

[xwill112x]

4:45's are wicked street gears.

[no1_jazz]

Appreciate all the help guys i think ill go with the 3.91 many thanks

Jazz