Carlton differential ratios - BobbyDazzler
Hi all

I'm in need of your valuable assistance yet again.

As I've previously mentioned on this forum my diff is howling (2.0 litre 1993 Carlton CDX with cat 143,000). I have the chance of a replacement diff (some sortin' out to do yet on just how good it is)'s from a 3.0 litre Carlton without a catalyser.

Apparantly the final drive ratio is 3.70:1. On mine it's 3.90:1.

Will this difference be a problem? I know nothing about this sort of thing so am totally in your good hands.

If it's possible to fit this replacement, with the differing ratios, what would change? i.e. mpg? performance? would the trip computer still 'compute' correctly in terms of consumption etc.?

Incidentally, I notice that cat and non-cat versions of exactly the same engine differ in their final drive ratios....why would the cat necessitate this?

Lot of questions I know :)

As usual thanks for any help, advice or guidance

Many regards to all

Carlton differential ratios - Hugo {P}
OK, as an engineer but unable to answer which part of the figure relates to the input from the gearbox or output to the wheels, here goes.

Final drive ratios are chosen by car designers taking into account the BHP of the motor and the weight of the car, and what it is likly to carry or tow.

Assuming the 3.70 or 3.90 relates to the input from the box, you will find that the total gearing will be slightly higher, the 3.0 litre would kick out more power and you would not need a lower geared diff to enable the car to achieve optimum performance and economy, in the same way that someone with stronger legs may be able to start cycling in a higher gear than someone else with weaker legs.

Hence your acceleration will be slightly slower and the power to the back wheels may a bit more difficult to obtain. As a result you would find that you may need to rev the engine higher in each gear before changing up. The power to the rear wheels could be more critical if you are carrying or towing heavy loads, although your main problem here would be pulling away, if it were an issue.

On the flip side, for ordinary driving, your fuel economy may be increased whilst maintaining speed, however, whereas now you may be able to accelerate say from 50 to 70 fairly quickly without changing gear, with the higher gearing you may need to change down to do this.

One other thing to whach out for - Insurance! You will need to inform your insurance company if you make any mods to the car. This could result in increased premiums.

On your last Q, the reason CAT and non CAT varients have different final drive ratios is because, the presence of the CAT reduces the power available from the engine.

For my money, look for an identical unit to the one coming out.

Carlton differential ratios - jc
3.7 to 3.9 will make a little difference as already stated but the other thing to check before you go ahead are the flange sizes on propshaft and drive shafts.I don't know Vauxhall diffs but Sierra used three sizes of halfshaft,two sizes of propshaft and a small and large diff. cover all their range of engines.
Carlton differential ratios - BobbyDazzler
Thank you Hugo and JC for your very helpful and informative replies.

I agree with the suggestion of replacing the diff with the exact same type unit. Too many potential problems here i think.

Cheers guys :)

Carlton Diff problem sorted - BobbyDazzler
Thought I'd just keep you informed.

I sorted my differential problems out at last!

The howling was definitely getting worse, and after taking advice from this forum, decided NOT to buy one off Ebay (from a 3.0L Carlton....mine's a 2.0L).

I used one of those broker type (no pun intended) setups on the web who, after you enter your requirments, distribute them to various dismantlers around the country. I must admit that I had no real hope of anything coming up trumps (cynical I know...but past experience and all that) but the next morning I received two emails and telephone calls quoting very reasonably for what I needed.

Less than twenty four hours later I received (from Lea Valley Motors, London...highly recommended!!!) a good 2nd hand diff for £120 including VAT and delivery to the North East!

A day and a half later (and one kackered owner) and the car is running as quiet as the proverbial church mouse!

The original diff was well and truly goosed, turning the rubber flexible drive joint by hand when it was off felt all notchy and gravelly....stiffened very noticably after being left for a few minutes too (bearing?)

Thanks to all who helped.

Best regards


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