How do I find out what rear differential the car has?

[LeffeBrune]

How do I find out what rear differential the car has without taking the rear axle out? The car does not have the little plate with the information installed.

The various calculations I made considering tire dimension, RPM, and speed all point to a 3.54 and not a 3.07. Is there a simple way to have certainty?

Thanks.

[Gfhug]

Hi, I had a similar problem and assumed my rpm were correct and when I compared these against a gps speed showed my car to have a 3.54 diff. Yet the tacho was over-reading by the 15% that is the difference between 3.07 and 3.54.
I had the tacho modified to RVC and calibrated and it now shows the correct rpm and confirmed 3.07 diff.

This may not help much but can be a warning against assuming rpm are correct. Maybe fit an inexpensive digital tacho to check the one on the car and do some more runs?

You can take the rear plate off the diff and look for the numbers on the gear, but that'll mean taking the IRS out.

Geoff

[MarekH]

Just put it in neutral, lift the rear wheels into the air and turn them round 3.5 times. If the propshaft has only turned once then it's a 3.54.

It's time to leave the country.

kind regards
Marek

[Gfhug]

Marek, that is so simple and obvious, why didn't I think of that (answers on postcards only )

A point to note for LeffeBrune is that if the rear wheels can rotate independently (ie hold one and you can still turn the other) then the diff is not a limited slip one. The limited slip one will see the other wheel turn as well.

[mgcjag]

Hi All..
To check whether you have a LS Diff or not, jack the rear wheels off the ground and spin one wheel, with a standard diff the wheel on the other side should rotate in the opposite direction whilst a Limited Slip Diff should rotate in the same direction.
You would need to select neutral,ensure handbrake is off and check for any sticking brakes hope this is helpfull... Steve

[LeffeBrune]

Thanks guys.

@Marek: Would it not be the other way round? For a 3.54 differential, you need 3.54 turns of the propshaft for one turn of the wheel. If I would get 3.54 turns of the wheel for one propshaft turn, I would be breaking the speed of sound at 400 RPM!

I currently don't have the car but I assume that one can see how often the propshaft turns when turning the wheel. If that is the case, then it would indeed be an easy method, provided one does see the difference between a 3.54 and a 3.07 turn.

@Geoff: I am sure that RPM are correct. The noise tells me so. And I did measure speed with a GPS.

Georg

[trondvo]

Put white marking fluid on the propshaft flange to diff and a position on the wheel, start rotating the wheel, unless your middlename is Hulk... then rotate the propshaft. Keep counting the propshaft rotation, guesstimate from the mark on propshaft when the wheel has done a full turn.

I checked with a 3.54 and a 3.07, the 3.07 is almost lined up but the 3.54 is on the other side of the white marking.

If in doubt make a paper strip that has the same length as OD on your prop, divide it in 10 with a pen, fit it with a tape (but you will not really need to do that).

[MarekH]

You are allowed to turn the wheels more than three times:- a 3.54:1 will be close to 10&1/2:3 whilst 3.07:1 will be close to 9&1/4:3

kind regards
Marek

[trondvo]

To find the ratio on the diff you turn the wheel or in this photo the output flange once while counting rotations on the input flange. The hose clamp is my marker but I already knew this diff.

This is a 3.54, after a full "wheel" rotation the tape on input flange has rotated 3.54 times around the starting point, a 3.07 will be close to the starting point and so on.

Start point.


A full rotation on the wheel, rotated pinion approx 3.5 times (ie 3.54)


I dont know how many variations you can get on crown wheel pinions for Series 1 cars but this is what give the ratio. Teeth on pinion vs teeth on crown wheel.