what is meant by 3.91 diff?

[noney82]

what does it mean? how much should i be paying for one of these

[Ziggy]

3.91's the ratio - one that'll give you fairly low gearing (may actually seem higher, depending on what you're coming from, of course). Common ratios are 4.10, 3.91, 3.73, 3.64. The higher the number, the lower the overall gearing, ie the higher then engine speed will be for a given road speed.

Higher numbers (lower gearing) will in most cases give better low end acceleration at the cost of higher revs on the motorway & increased fuel consumption.

& as for what it's worth, none of the open diffs are worth much (Ԛ£40 maybe?) but it's Ԛ£200 - Ԛ£250 for a LSD.

HTH

[SPADGE]

3.91 revolutions of the drive shafts for every 1 revolution of the prop

[noney82]

so for my 325 what diff i ratio i want?

[maxfield]

3.91
3.73
3.64

maybe 3.46

[noney82]

so the lower ratio i get the more juice she will use

[maxfield]

so the lower ratio i get the more juice she will use

3.91 will use more juice than 3.64 i.e
If in 5th with 3.91 its revving at 4k at 70 with 3.64 it will rev lower i.e 3.5k revs

Make sense?

[noney82]

iv been offered a 3.91 shall i pass on it then??

[maxfield]

LSD? limited slip differental

[noney82]

yeah

[maxfield]

I don't see why not you may have 3.91 open diff now im not too sure about them

[noney82]

i cant find any id on diff i got now so i dont know what mine is

[Gunni]

raise the back end, rotate the driveshaft and count the rotation of the tires,

[noney82]

raise the back end, rotate the driveshaft and count the rotation of the tires,

dont follow

[old_skool]

He means, chock the front wheels, jack the back end up with the car in neutral and the handbrake off so that one rear wheel is off the ground. Mark the propshaft with a strike of chalk or a piece of tape. Then rotate the rear wheel, counting the amount of wheel revolutions, or part revolutions until the prop has completed a full revolution. For example, if the wheel goes around nearly four times then it's a 3.91.

It may be a good idea to mark part of the wheel as well so you can clearly identify a full revolution)

Not exact science but if done carefully it will give you a good idea what you have.

[noney82]

ahhhhhhh i seee now lol silly me, how about knowing if its lsd

[old_skool]

Jack up rear end until both wheels are in the air. Turn one back wheel and if the other wheel spins in the opposite direction then it's an open diff. Cool?

[noney82]

ok i will look into that, so lsd's are all the rage thn

[old_skool]

so lsd's are all the rage thn

"Would Sir like one black line or two?"

[noney82]

lol its all new to me, lsd are good for grip aint they

[reggid]

3.91 revolutions of the drive shafts for every 1 revolution of the prop

its getting late here but doesn't it mean for every 3.91 rotations of the input the output (driveshaft) turns 1 rotation?

[maxfield]

I thought it was 1 turn of the input shaft. then it would be 3.91 turns of the output shaft???

[old_skool]

Other way round

[Brianmoooore]

If you're going to count it with an open diff, you need to jam one wheel still, and just count the other wheel.
With a 3.91 diff, twenty turns of the propshaft will turn one wheel 10.23 turns. (20/3.91 X 2).