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Rear Diff Ratio


Guest NantLladron

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Guest NantLladron

Hi

 

My S7 is very long legged ( 85mph in 3rd quite easy ) , it has a Cortina rear axle and a sierra 5 speed type 9 gearbox , does anyone know how do i figure out what ratio the rear diff is and what vehicle should i source a lower ratio diff from ? im thinking P100 ?

 

Ta

 

Mark

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If there is no tag on the diff telling you the ratio, you can approximately guess by doing the following.

 

put a chalk mark on the tyre at the bottom. (This will be easier if the wheels are off the ground)

Then mark the top of the prop shaft on the front of the diff.

Turn the prop shaft and count the turns until the mark on the tyre has done one turn. (That's both wheels off the ground so both turn through one turn.

 

The ratio is then 3 and a bit turns of the prop to one turn of the wheels.

 

Sounds like you are looking for a HIGHER ratio than the one you have now, not lower.

 

(The crown wheel will also have this stamped on it, but that involves taking the back plate off with all the diff oil mess that goes with it.)

 

Simon.

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Working out the current ratio is fairly easy. Lift one rear wheel (chocking the car and supporting as necessary and safe). Now spin it by hand whilst watching how many times the prop-shaft turns. I'm not familiar with the Cortina standard ratios so can't suggest what to look for but if it were a Sierra with maybe a 3.62:1 diff I would be expecting 3.62 turns of the prop to equate to one turn of the wheels BUT since we only have one wheel off the ground and the other held stationary 3.62 turns of the prop will give 2 turns of the wheel.

 

Automatics tend to get very tall diffs. Has your car had a 4-speed box in the past? It's possible a previous owner fitted an auto diff to give an acceptable cruising engine speed with the 4 speed gearbox.

 

P100's use a Transit rear axle as far as I know so I don't think it would fit your Cortina unit (but I'm not 100% on that). You might find a Transit diff takes you from one extreme to another. If you do find a Transit diff will fit then generally Mini-bus variants of vans have the taller diffs so that might be something to consider.

 

From memory a Sierra with a type-9 and a 3.62 diff on original size wheels will be doing about 3krpm at 70mph. Wat does yours do.

 

I'm also assuming that your speedo is accurate and you really are doing 85 when it says you are?

 

Is your wheel/tyre combo around the same rolling radius as a Cortina/Sierra?

 

Iain

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Turn the prop shaft and count the turns until the mark on the tyre has done one turn. (That's both wheels off the ground so both turn through one turn.

 

Almost impossible to do it with both wheels off the ground unless it's a limited slip diff as one will always turn more than the other. Lift one wheel whilst keeping the other still and just remember that you are looking for 2 turns of the wheel rather than one.

 

Iain

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Hello Iain,

 

I suppose your way would be more accurate and would prevent the slip you talk about. It happens though that mine both turn equally when I did that. I am not disagreeing and find that your method would give a more accurate reading too.

 

Thanks for the tip.

 

Simon.

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Guest NantLladron

ive no idea if the origional donor was a auto ? will check the turns ( should have thought of that ) , my objective is to get the engine to rev more so your right higher ratio read diff ( i was thinking low range in my discovery )

 

I think in 5th at 70mph mine is 2krpm but i will check

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Guest NantLladron

I was guessing a bit as i dont use 5th in it cos its so long legged it could easily be 2500 ......let me check before i mislead everyone .....i have 17" wheels but low profile tyres so overall diameter is maybe a little bigger but not substantial

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85 in 3rd at 6000rpm with 185/60/14 and a 3.92 diff & 1.21 3rd gear (V6 box)

70 in 5th @ 2000??????

Should be 40 @ 2000

 

Google gear speed calculator, you can play with gearbox ratios, diff ratios and tyre/wheel combinations.

 

The standard Ford gearbox's are to wide, low 1st, 2nd, long 5th meaning that you compromise with diff ratios and tyre size to get good usable acceleration in 1st 2nd but then end up with impossible to reach top speed in 4th 5th.

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Guest NantLladron

....jacked the car up and the rear diff is a 3.6 ratio , with a 5 speed type 9 box and 17" wheels it looks from some other chats that a 3.9 would fit better , especially for track days

 

which fords had a 3.9 fitted? or are they special made?

 

ta , mark

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The Suerra generally had a 3.6 or 3.9, the way to tell is a tag on the diff or engraved on the crown wheel, you can get the code on the VIN plate, axle box letter code but only if the diff has not been changed.

What diff do you need, bolt on shafts or push in?

I have a 3.9 push fit and a 3.6 bolt on LSD in my spares

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