diff ratios and combos

**YLD-16L** · 30-01-2007, 04:33 PM

Originally Posted by strad

on that topic then, how the hell do I find out what ratio I have??

Without removal..

Jack up rear of car, support with stands, put in neutral, mark the yoke on the diff and diff, mark tyre and guard. Count number of revolutions of the tailshaft for one wheel revolution...............or something like that.

**stradlater** · 30-01-2007, 04:41 PM

AH, ok

so,

1 (Wheel revolution) : 4.1 (Yoke revolution)

Makes sense to me. Thanks man.

**oldcorollas** · 30-01-2007, 04:49 PM

Originally Posted by stradlater

AH, ok

so,

1 (Wheel revolution) : 4.1 (Yoke revolution)

Makes sense to me. Thanks man.

actually

if you have both wheels turning same mount (both jacked up) it will be 4.1:1.

if you have only one wheel up (assumign the LSD allows to to spin the wheel) then the pinion will turn half the amount (since the stationary wheel causes the other one to turn twice as fast)

either way, best bet is to turn the wheel 9 or 10 times, so then it is easier to tell the difference between 3.7 (33.3/37), 4.1 (36.9/41), and 4.3 (38.7/43) (=turns of pinion assuming both wheeels are turning, halve it if only 1 is turning, or spin single wheel 18-20 times

)

or did i get it backwards?

**Norbie** · 30-01-2007, 04:55 PM

Originally Posted by bigmat

just found it in the forums. type diffs in search. but still couldnt find stuff for 3.9 for rn25 housing.

You can use any G series ring and pinion set in an RN25 housing, so for taller ratios check Supra/Soarer etc.

**stradlater** · 30-01-2007, 04:59 PM

Well mine is supposed to be lsd, but it tends to only spin one wheel when up in the air.

does that mean I should halve or double?

**oldcorollas** · 30-01-2007, 05:06 PM

iirc (and maybe i don't) then the pinion will spin slower and the wheel faster, when only one is spinning
so when turning the wheel...
umm... it will either be half or double

if the pinion turns twice for 1 wheel rev, then halve it. if the pinion turns 8 times, double it

(if you have pinion stopped, the wheels will spin the same amount in opposite directions, so if one wheel is stopped, the other should double? )

i think it will turn approx twice for each wheel rev, so when you count the number of pinion turns, then divide by the wheel turns..... you should double it to get ratio.

ie, wheel turns 10 times, pinion turns 20.5 turns, ratio is 4.1

**bigmat** · 30-01-2007, 05:08 PM

i just got a price for 3.9 second hand gears with lsd $1400. problem being i told this guy it was for an rn25 diff housing and he said well i only know them as hiux diffs. these are people that have been in it for 50 years. i need recomendations for sa work shops.

**stradlater** · 30-01-2007, 06:20 PM

Shit! 3.9 gears with a g series lsd for $1400!?

I got mine, practically new, from ARB for $300. LSD + gears, it was just a full center. Only problem is I don't know the ratio.

**bigmat** · 30-01-2007, 06:49 PM

who is ARB and there number. ill give them a call.

**hypo23** · 30-01-2007, 07:12 PM

arb are like an after market 4wd company

**stradlater** · 30-01-2007, 08:32 PM

They are an after market 4wd place.

I can't give you a number, because the only useful one I have is in vic.

Do a search, even google or god forbid, yellowpages. they will have a website.

They sell locker kits and air locker diffs for hilux's and landcruisers, so they have the old centers lying around. (old NEW centers, the replaced ones) and they may be of use to you.

**brett_celicacoupe** · 30-01-2007, 09:13 PM

better solution is to find a MX83 grande. 3.9:1 ratio, 4 pinion clutch type LSD. they usually sell for about 300$

regarding the ratio check thing,

i think of it like this. the sum of the output torque must equal the total output torque of the engine, forgetting torque multiplication of gears. for arguments sake, we'll say 100nm total torque.

its kind of like the same old thing your high school teacher would tell you. what goes in comes out. the sum of the 100nm in equals the sum of the torque out (50 per wheel).

if one axle is clamped, the other ouputs the total amount and spins at twice the the speed.

so to check your gear ratio;
clamp one wheel, rotate the other 20 times (same effect as spinning both 10 times together).

the number of turns the tailshaft does, divided by 10 is your ratio.

it should be 37, 39, 41, or 43 (common ratios).

2 man job

**bigmat** · 30-01-2007, 10:37 PM

sorry brett but does this fit into an rn25 housing as it is all ready modified?

**SillyCarS** · 30-01-2007, 11:29 PM

im also in amongst an rg turbo build, ive gone for a 4.1:1, 4 pinnion lsd in a g series diff, mainly i want it to be a strong street performer if it were for pure drag id go lower, as mentioned above turbo's love load

**chris davey** · 31-01-2007, 01:04 PM

I am one person who can definitely prove the turbo cars do not do well with short ratios. I have a 4.625:1 diff in my car and have tried for a long time to sort the auto to get it to make good boost in 1st gear. I can make fully boost in 3rd gear at 4100-4200rpm but in first it is made at about 5000rpm.

I am now biting the bullet and looking to get a 3.7:1 diff. Even for street use I would go for the tallest one possible. 4.1's are good for v8's with lots of low down torque. Turbo cars need the load on the engine to make boost and therefore torque.