Diff & Gearbox ratio's?

[Shovey]

Hi Guys

No problems here, but as my engine is just nearing total re-build completion I've learnt a s***t load.

Now I'm really keen to learn all about diff and gearbox ratios. (curious thats all!!)

Has anyone got the time to tell me what a 3.89 diff ratio means, what it does, why people change to other ratios and what effects they have to your cars performance.

I have a 1992 Narrow - It has a live axle and I understand is most likely to come from a MK11 ESCORT.  What ratio am I ilkely to have?

Same for the gear box ratio's - again do people target certain gearing ratio's and as a consequence chop and change individual gears. Whats the best recognised setup and why

Thanks

Al

[Hammy]

A 3.89 back axle means that for every 3.89 turns of the prop shaft the  half shaft will make one turn.  Generalising a bit in fourth gear ( in either a four of five speed box) then  the engine and prop shaft are connected "directly" at a 1:1 ratio. fifth gear would be .89 or whatever, An overdrive.

Gearing has the effect of multipling the torque from the engine (so a higher nurmically ratio'ed rear end will multiply the torque more -e.g a 3.89 will multiply torque more than a 3.54).

A lower ratio  (numerically higher  e.g 5:1) has two effects:

Initial acceleration is faster

The engine is turning faster for a given road speed.

The car is more flexible in each gear ( more torque at the wheels)

top speed may become rev limited

A higher ratio has the reverse effect:

top speed may become power limited (e.g Capri 2.8 is faster in 4th than fifth)

Rally cars tend to use lower ratio's for acceleration  and  have a lower top speed.

What you choose depends on your engine characteristic and how you like your performance and economy. ( e.g a low torque high reving bike Engined car  would want lower gearing  ( higher numeric ratio) than a V8 stump puller)

An internet search may give a more accurate/complete answer

A close ratio  gearbox, where the ratio step between each gear is small is handy for highly tuned engines that have a narrow rev band over which the produce the amjority of their power/torque. Thus when you change gear the revs only drop "slightly" and the engine stays in its power band. This gives better acceleration than with  a wider ratio box where the engine drops further out of its power band when you change gear.  Again obviously more important with a bike engine than a V8 which has a flatter torque characteristic.

I'd guess you've a 3.89, but could be 4.1ish or 3.54 - wind the half shaft for say 10 turns and count the revs of the prop shaft, then get your calculator out.

All sorts of posting about power and torque will now occur!! , but I hope my little knowledge helps.

[Shovey]

Hammy - thanks mate

Thats a really good description - not an easy one to describe!

Calculating diff ratio:

to rotate the half shaft, do I just rotate the rear wheels x 10 and then count the rotation of the prop?? or is there an easier way  

Out of interest what diff's are you guys running on and what would you suggest for my 2L Fiat TC.  Lots of Torque with an estimated bhp of 165 at the crank (yet to be confirmed post re-build completion)

Al

[adhawkins]

The diff ratio you need will depend on what your max RPM is, what ratios are in the gearbox and what your required top speed / acceleration characteristics are...

Andy

[Hammy]

Yuo need to turn the half shaft a number of times (with the other wheel on the ground)  to be able to eleminate as far as poss the errors in measuring the  fractions of a turn the prop will make.

Turn it say 10 times then work out your ratio = no of prop revs/no of half sahft revs.

The actual number of turns you make doesn't matter ,but the more you do the more accurate will be the result. do the whole thing  two or three times and compare results - they should be the same!

enjoy 1

've a 1700 X flow with 4sp g/box and 3.54:1 (RS2000) . rear end. I stand to be corrected but I think this is the Tallest rear ( lowerest numerical ratio)  end you can get in an Escort casing, I'd be gonig for that, but your 3.89 would do for now ( probably run out of revs at the top end)

[Shovey]

Until I get my car back from the garage I cant calculate my diff ratio. But i'm assuming 3.89

I've been doing some searching and since discovered that my 2.0L FIAT 131/132 gearbox has the following ratios:

1st = 3.612

2nd= 2.043

3rd= 1.363

4th= 1.00

5th=  0.83

further searches have revealed these are almost exactly the same as Fords MT75 box!!

further, further, further searches have also revealed that Westy owners don't rate the MT75 ratios's at all    

My understanding is the MT75's downfall is its ridiculously short 1st gear. (I also understand its not as interchangeable as the Type 9 box?)

I haven't got onto the Fiat forums to see if the 1st gear is commonly swapped yet - but I will soon.

I do know when driving mine the 1st gear certainly is very very short and b4 long i'm in 2nd (its not the smoothest box in the world so time is lost in a fumbling gear change sometimes!!!!)

If the 1st gear in the FIAT box could be swapped for a longer gear - would you reccomend it ? and what else do I need to be aware of as it cant be a simple case of dropping in a gear!

cheers

Al

[Gromit]

I`ve an MT 75 box in mine and never bother using first unless i`m putting the car in the garage.  Not an ideal solution but cheaper than a ratio swap.

Does mean it will get to 60 without a gearchange though.

[Edo]

My car had a new diff, and now the speedo runs low (assume this means I have a higher final drive?).

On my last Westie I did the "count the revolutions/push the car about" trick and sent the speedo to speedy cables..

Can I just get some GPS comparsion speds and tell them how much it is out by - or is this the only way?

[John Loudon - Sponsorship Liaison]

Agree with needing to know max engine revs and all that but you also need to know at what revs your max torque is so that you can calculate the the rev drops between gears to suit from the gearbox manufacturers range of ratios. IMO, this ismore important than anything else for good acceleration.

John