Classic cars forum & vehicle restoration.

[Timstopp]

I have the opportunity to swap my diff unit from a standard 5.37 to a 5.14 for reasonable sum of money.
The vehicle will currently go at about 55 flat out (or as fast as I want to rev the engine) and anything that will ease the cruising speed to just above or reduce the revs will make motorway dancing with lorries a less frightening prospect. It is possible to get standalone overdrive units but these are far too expensive. Tyres are 18 inch.
Does anyone have an opinion, subjective I know, if this will be much of an improvement or any other ideas (Wolseley/mg/mowog 4spd syncro box with 8 bolt mowog back axle and six spline halfshafts.
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North Essex,UK - Morris 8 tourer 1936 - Jaguar x350

[Timstopp]

Change of mind - whilst I find gearbox and final drive ratios of endless fascination, clearly the easiest route is a replacement engine and gearbox. A ford Sierra engine and box are available locally and will soon be in situ.
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North Essex,UK - Morris 8 tourer 1936 - Jaguar x350

[Rick]

[P3steve]

swap the engine and gearbox on your car and your ripping the heart out of the old girl why not just go out and buy a Ford Focus or something like that and cut out the middle man, at least changing the diff is a subtle change that will if not making a Dynamic change at least will keep the car in character. I have just picked up a Rover P5b diff (V8 saloon) to fit in my 1949 P3 which should lower the revs at crusing speed and make the first gear more usable, the club say its a straight swap with only a spacer in the prop shaft to take up the gap from the slightly shorter P5 diff so I guess Im lucky to have such an easy swap but I hope you can look at all the options befor taking out the original running gear
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If the world didn't suck we'd all fall off

[Timstopp]

fear not, I was just poking the tiger with a stick
Out of interest, what are the ratios of the old and new diff?
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North Essex,UK - Morris 8 tourer 1936 - Jaguar x350

[Phil - Nottingham]

I would be interested to hear whether dropping to the 3.54:1 P5B ratio will make the car more drivable - it has been done and it does just need the spacer (or a long prop-shaft) but will 1st gear take the extra use as it was not intended to always start off in it and will it make acceration too sluggish - is it a 60 or 75?

There is an overdrive conversion now to fit in place of the freewheel as well which seems to be the best of both worlds - though I do find my 1938 P2 16 to be quite OK as standard though and I like the freewheel. The P3 used the same diff 4.7:1 as standard but later P3's did have 16" wheels in place of earlier standard 17" and these may have had the lower P2's 4.88:1 diff. A 3.9:1 or 4.3:1 from P4 of P5 may be better?

Apart from ratios the diffs that were used from the 1930's to the 1980's are what was basically the same diff on Rovers?land Rovers & Range Rovers at moment
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Rover P2
Rover P4
Rover P5 & P5B
Land Rover S2 & S3
Morris Mini Traveller Mk2

[P3steve]

It’s a 75 Phil and I’m going with the P5b diff as it came my way for next to nothing and as the mod is revisable I just feel like giving it a go, first gear is so low on the 75 that I often start in second and unless I come to a complete stop will often not come down below third. Also I want to keep the free wheel if I can so don’t really want to fit an overdrive as I say its not permanent so I might give it a go.
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If the world didn't suck we'd all fall off

[peter scott]

Hi Tim,

I think your current axle is giving you about 16.9 mph/1000 and changing to the 5.14 will give about 17.7 mph/1000.

Lorries with tachos travel at 56 mph so for you not to impede them you are currently running at 3300 rpm. If you change to the 5.14 axle then you can run at 3160 rpm.

This reduction doesn't really seem worth it unless there is minimal cost and effort involved.

Another factor that might influence you is how well your car deals with motorway gradients. Can you maintain 56 mph in top gear on most gradients with your current axle? If not then you won't have an easier life by upping the axle ratio.

If you don't like running at 3300 rpm then another thing that could make a big difference would be to get your crank, rods, pistons and flywheel balanced. This could dramatically reduce vibration levels and make you happier running at higher rpm.

See: http://www.nostalgiatech.co.uk/new%20page%2015.htm

You need professional help for crank and flywheel balancing. I got mine done for £30 / item. So crank, flywheel and clutch only cost £90. Rods and pistons you can do yourself. Having balanced my engine it is very much happier running in the 3500 to 4000 rpm range.

Peter
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http://www.nostalgiatech.co.uk
1939 SS Jaguar 2 1/2 litre saloon

[clan chieftain]

Looking for our o"pinion" on diffs........brilliant
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The Clan Chieftain

[Timstopp]

Peter
Yes I fear you are correct, I had come to the same conclusion, that it just was not worth the effort. I had the engine apart (in the car) in the spring, and balanced the pistons and rods - I used the article on your website which I found on a web search!! - great article.
The car is now quite a bit smoother at higher revs, but as it is over square it still feels like I am abusing the engine, which I don't really like to do.
Perhaps I should save up for something with a larger engine and sportier pretensions
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North Essex,UK - Morris 8 tourer 1936 - Jaguar x350

[Jim.Walker]

As most gerboxes give a direct drive in fourth gear (i.e. 1, Changing the gearbox will make no difference to "cruising revs.". Though it might influence acceleration and engine braking in intermediate gears.
A higher rear axle ratio (i.e. A lower number:1) may reduce road speed as well as maximum engine revs., While a lower ratio could increase both.
A lot of cars etc. fitted with overdrives are actually faster in fourth gear!
Jim.
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Quote from my late Dad:- You only need a woman and a car and you have all the problems you
are ever likely to want". Computers had not been invented then!

[Timstopp]

Jim
Thanks, we were thinking about the diff , not the gearbox (as you say - all gearboxes end up 1:1 usually)
Pre war cars have unusually high ratios ie mine is 5.37:1 This is due to whole raft of reasons , some stemming from the treasury rating, some from the desire to get to top and stay there. As you effectively state - the gearing overall has to match the engine power and torque, so if I went for a modern(ish) 3.41:1 diff, I probably couldn't use forth gear - even on the flat!
Really not sure about your comment that a lot of overdrive cars are actually faster in fourth gear.....do you mean top speed able to be pulled by the engine?, acceleration? An overdrive is just a set of epicyclic gears bolted in a unit to the back of the gearbox to increase the output shaft ratio to above ratio n plus a bit. What I an trying to say is that the gearbox remains standard and the overdrive is switch able - in the same way as a firth gear is used - or not depending on driver selection
You have given me an idea though:D .........what if I could find an overdrive that could be mounted separate from the gearbox - I have plenty of room under the body, hidden away. Maybe a 4x4 transfer box if they are switchable - It is winter - I have a new workshop...........
Thanks
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North Essex,UK - Morris 8 tourer 1936 - Jaguar x350

[Jim.Walker]

An overdrive reduces engine revs and for a givven road speed that means the engine is running further down the horsepower curve. Hence it may not be abl;e to reach the same maximum revs. and that can mean a lower maximum speed (but not always).
Curiously a 5:1 ratio is a LOWER ratio than (say) 4:1.
Jim
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Quote from my late Dad:- You only need a woman and a car and you have all the problems you
are ever likely to want". Computers had not been invented then!

[MVPeters]

A cheap way to test whether your car will 'pull' a higher gear satisfactorally is to temporarily fit bigger wheels &/or tyres - perhaps borrowed from a friend with a similar make/model?
Most cars will take 1 or 2 sizes bigger than original for a 5-10% change.
Your speedo will read lower than actual - a GPS/SatNav is accurate enough to show the difference.
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Mike - MVPeters at comcast.net
2002 MINI Cooper 'S'

[Rick]

Years ago when I ran a Spitfire, I borrowed a couple of wheels that had very tall town and country tyres on them and ran them on the back for a few days. The difference was very noticeable on this non-o/d car.

RJ
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