Replacing a differential for better performance

[Eric JS]

Hi, I am trying to improve the performance of my 1955 Standard 8.

Originally the car had (I think) an 850cc engine but has been retro fitted with an 1147cc Herald engine, mated to the original Standard 8 gear box joined to the cars ridged rear axle and its original differential.

I am told the differential is identical to the early Herald but the ratios are different, according to the stats the Standard 8 diff is a 4.88 and the Herald is 4.11.

Being a bit dim about this area of car mechanics can anyone tell me if changing the differentials will give a noticeable difference in performance.

Ric

[glang]

A diff ratio change can be a great modification especially if you now have more power. The most important factor is what its like at the moment pulling away in first? You have to consider this fully loaded on a steep hill as a lower ratio diff will mean its harder and you dont want to have to slip the clutch and increase revs a lot to do it...

I would guess the diff in the Herald is the same or at least the crownwheel and pinnion can be swopped over as the early ones were 4.88, then 4.55 before finally becoming 4.11 which will be the easiest to find.

[Clive]

Your existing diff will he quicker off the line, but you will quickly be in top gear. A 4.11 will allow you to stay in each gear to much higher speeds. And as the car has a similar weight to a herald, matching the factory gearing to the engine makes sense.

[JohnD]

Ric,  to help you appreciate the results of the changes you intend, the number attached to the different differentials refers to the gear ratio they apply.  It should be given in full as 4.88 ( or 4.11) to 1, often written as 4.88:1.

This means that for every 4.88 turns of the prop shaft, the rear wheels will turn once.   ( Or, vice versa) So you can see that the OE Standard had a lower "final drive" gear than the Herald.     Thus, moving off from stationary is easier, but you will reach top revs at a lower speed.

In cars as old as ours, that have been through so many owners, it is worth checking that they still have the original ratios.   Strip out your interior so that you or an assistant can watch the prop shaft.    Having marked a rear wheel next to some solid indicator point, push the cat through one wheel revolution, while counting the propshaft revs.  "4.88" will be  bit under five, while "4.11" will be a bit over 4.

Good luck!

John

[Clive]

http://www.mintylamb.co.uk/gearspeed/

Use that to compare a herald 948 (which I expect has the same gearbox ratios and diff as your car had originally) with the 1147 herald (later box and 4.11 diff)

You can see how may RPM for a speed in each gear. At 70 originally it would be doing 5000rpm, with a 4.11 about 4300. That is like an extra gear, far more relaxed. 

Worth noting there are 2 types of 4.11 diff, the early type with small 1/4 shaft splines, and later (1967ish?) with the bigger/stronger splines. I assume you would need the earlier type, but worth checking.

[Eric JS]

Hi Guys

Many thanks for your well written and informative replies and the information they contained, you have helped me a lot.

I knew some of the basics but I did not realise how small the differences needed to be to make a big difference when driving, to be honest, I had never thought about it!

I think driving a car has become so easy that many if not most of us have lost sight of how incredible the mechanical engineers and designers of the past and today are and in most cases we never give our cars a single thought - we just get in and drive.

Thanks again, the only thing I need to do now is get a differential.

Ric