Best Diff Ratio for a 325i Track Car

[Stan318is]

Would it be better for track use to put a 4.1 ratio diff into a 325 rather than the 3.76 ratio that is in it?
To my knowledge, the 4.1 would have better accaleration but a lower top end speed which would suit a track car more, i think.... You rarely break 100mph+ on a track so the final top end speed is just wasted....
Can anyone shed any light on this or has anyone tried this before?
Thanks...
Graham

[320Touring]

Would it be better for track use to put a 4.1 ratio diff into a 325 rather than the 3.76 ratio that is in it?
To my knowledge, the 4.1 would have better accaleration but a lower top end speed which would suit a track car more, i think.... You rarely break 100mph+ on a track so the final top end speed is just wasted....
Can anyone shed any light on this or has anyone tried this before?
Thanks...
Graham

hi mate!

with regards to the diff ratio's

on a 325i, fitting a 3.91 LSD for track use is probably the best bet

the 4.10 is most likely to be a small case diff, and may not be as willing to suffer track abuse..

the 3.91 LSD is what was fitted to facelift 325i sports, and coupled with a sport gearbox (close ratios in 2nd and 3rd) should give you a decent clip of speed around the track

Budget on Ԛ£250ish for the 3.91 LSD and Ԛ£150ish for the "sport" gearbox.

hope this helps mate!

[Stan318is]

Thanks 320Touring, Helps a fair bit....
Have a small case 4.1 open diff which i was thinking on welding. Would the power from the 325 just eat the diff and mangle it?
Only just after buying a new diff last week for my road 318is so wont be able to afford the 3.91 quite yet..
Does the sport gear box make a big difference?
Thanks again,
Graham

[320Touring]

Thanks 320Touring, Helps a fair bit....
Have a small case 4.1 open diff which i was thinking on welding. Would the power from the 325 just eat the diff and mangle it?
Only just after buying a new diff last week for my road 318is so wont be able to afford the 3.91 quite yet..
Does the sport gear box make a big difference?
Thanks again,
Graham

if you weld the 4.10 i'd say it may be "fragile"
as for using it on the public road

an LSD would let you drive it to and from the track.

you could get a 3.91 open diff off of a 325i so you had the accel but not all of the handling-i'd expect Ԛ£50-100 to cover that.

as for the sport gearbox, it basically removes the big "step" from 2nd to 3rd that you get with a normal box. this means you can stay in the powerband for the 2.5 engine.

another cheaper option would be to get a zone chip so that the car revs to 7k, thus putting your revs up when you change into third. send a PM to ant for info..

[Stan318is]

This car is never going to touch a public road over here in Ireland, costs over a grand just to tax it and good luck to getting insured on a 2.5 when 24....
Are the sport gearboxes found easily second hand? From what you are saying, they sound like the right job for the track...
Not stupid enough to try and use a welded diff on the road, i'd want to put winches on the car to pull me out of all the ditches i'd go off into....
Thanks a lot for the help bud....
Graham

[Taffy]

Would it be better for track use to put a 4.1 ratio diff into a 325 rather than the 3.76 ratio that is in it?
To my knowledge, the 4.1 would have better accaleration but a lower top end speed which would suit a track car more, i think.... You rarely break 100mph+ on a track so the final top end speed is just wasted....
Can anyone shed any light on this or has anyone tried this before?
Thanks...
Graham

that'll only be any good somewhere like Croix or a go kart track! 100mph on track is quite low. We ran a 3.91 for most tracks which used to top out at 122 and a 3.64 for Thruxton and Snetterton which would hit around 135 and that is spot on,

[karlp]

When I was building my car I was told by many people, that prepare cars including Nigel Moseley and Lester Owen, that the top speed is ideally around 140 mph.

You work out how high the revs are going to be then select rear diff.

However it is not quite as simple as all that as get it wrong and the car just will not accelerate sufficiently quickly.

Usually the max revs are higher (around 8000 or 8500).

So in a nut shell as Taffy has already stated the 3.91 diff is the best allrounder to get. A 4.1 is only useful for about 2 circuits in the UK (Brands and one other that I cannot remember).

Without being rude if you are only going about 100 mph you are not revving it hard enough.

A 4.1 diff would cost you more than the 3.91.

Karl

[Chaos]

even on llandow which is short (where i hold my trackdays) most cars are getting close to 100.

on the average circuit u will be doing a fair bit more than a ton at the highest speed point