rear diff ratios

[tone]

ok for a dvid readers of this forum you will be aware my car is now sort of fixed and working well.

Someone said I should consider a diff change from 5.1:1 to 4.7, apparently will give me better/ quitter cruising?

Any thoughts

[frenchblatter]

And much slower acceleration.

[Bob Schapel]

I have a road TC and a supercharged race TC. I run a 4.2 or 3.7 ratio in the race TC (depending on the track) and a 4.55 ratio in the road TC.

Ideal diff ratio depends on several factors. The power of the motor (stage of tune), the size of the wheels and the type of use the car gets. If you usually drive on level roads you can use a taller ratio than if you usually drive in hilly country.

My 4.55 ratio in the road going TC is ideal as I have 16" wheels (MG part number A1338 as listed by the factory in their june 1949 Technical literature), a fairly powerful motor (1366cc, high comp, 168551 cam, big valves, balanced, polished etc) and do most of my driving on fairly level roads. With that ratio I can cruise at the 110kph (70mph) speed limit and feel comfortable in modern traffic.

I use Morris Minor/Spridget diffs as they can be modified to fit TCs without chopping original bits. They are available in 5.375, 5.125, 4.875, 4.555, 4.222, 3.9, 3.727 (and 3.5 with special CW&P from Germany).

A 4.7 ratio sounds good but how is your TC used and what is the engine power and wheel size? Is the 4.7 a special CW&P for an original TC diff or is it an adapted diff from something else?

Bob Schapel

[Gene Gillam]

I have 2 TC's, one with a 4.55 (supercharged) and one 4.22 (supercharged). Neither engine is quite a "built" as Bob's, in fact the one running the 4.55 is a stock bore, but both easily pull the higher ratio with the supercharger. My problem is I have 5 speed transmissions in both of them and the 4.22 never really lets me get into 5th gear at the speeds I run.

[Steve Simmons]

I run 4.875 gears as found in MG TA, and it's a great ratio for a mostly stock engine. I do plan to try 4.625 as soon as I get around to rebuilding my spare diff. If you have the power, a 4.3 is great but much too high for a stock engine unless all you do is flat land driving.

[tone]

Crickey, this needs a bit more thought than I realised!

Basically I have a supercharged over bored +40 with a polished, but otherwise standard, head converted to unleaded.
I have a 5 speed type 9 hi gear box
Diff is currently as it came from the factory at, I think, 5.1
A rolling road gave a top speed of 79mph, not sure I am brave enough to do that on a road though especially as I have the original steering box, albeit re-built.

It is much better in 5th, i.e. a bit quitter, but I thought a change of diff may allow a higher, and still quiet, cruising speed.

I live in London and we do have a few hills, maybe not Pikes Peak, Box Hill in Surrey is quite daunting at 675 feet with an average gradient of 5%!

[frenchblatter]

Tone,

All that work on the engine for 1 MPH faster! The book says the standard car will do 78 MPH, so unless that's wrong, surely yours will go faster than 79.

We lived in the Peak Park before moving to France and we lived at Low Bradfield, the hill to High Bradfield is 1 in 4 in the middle... now that's a hill.

[tone]

As I said previously I wasn't expecting any/ much top speed improvement, its all about mid range with a supercharger, and that is dramatically different, so I am a very happy chappie!

[frenchblatter]

I've resisted all the suggested "improvements" to my engine. I have a 220 BHP, 500 Kgs Caterham for speed and acceleration. The TC is bog standard (apart from 0.030" oversize pistons).

It's nearly finished now, I won't get it done in under the year but it should be on the road by the end of May.

Anyway, good luck with yours, the cars I've supercharged in the past have all had higher top speeds as long as the heads are ported to allow better breathing.

[Steve Simmons]

Tone, with the modifications you've described, you should be running a 4.625 rear end. It will greatly lower RPMs on the highway and allow at least 10MPH higher top speed. And you should have plenty of power to push the higher ratio.

[Kameron M]

Does anyone have a recommendation for a stock TC engine?

[Steve Simmons]

On a stock engine, in hilly terrain, stock gearing is good but you won't find yourself very comfortable over 55MPH. 4.875 works also, with some loss of climbing ability in 3rd gear. On flat terrain, 4.875 works a treat and 4.675 can be used as well but don't expect much power in top gear. Just my impressions.

[Duncan M]

Any number of "calculators" to give you an idea of the RPM differences at 60mph, etc. Need tire diameter.
http://www.advanced-ev.com/Calculators/TireSize/
Of course, for power changes in different driving situations, the comments above will give you an indicator.

[ian theobald]

My car has a later type diff from either a TD.TF or something similar with axle ends cut off and re welded to original TC mounts. I have often wondered how you tell what current gearing now is as speedo is driven from gearbox .I was thinking of using my NavMan to give me a speed at a known revs such as 3000 on the road .
My car sits happy at 3500 revs but have no clue as to correct speed as speedo shows about 50 odd mph but then looking across from other side is misleading.
At 4000 revs is happy for short periods
Would love to fit a 5 speed Toyota Celica gearbox .Any brochures would be great and could keep original if ever required as still ok but noisy on the over run.
Definitely not a car built for Aussie conditions
I heard of a TC that had been fitted with an early Holden 6 cyl grey series motor and would be interested to know how performed as these 6 cyl motors were rather small and similar in design.
Would be similar to an MGC compared to MGB or 6cyl Healey .

[Bob Schapel]

Tone,

If the fifth gear in your five speed gearbox is an overdrive, (and fourth is "direct"), it completely changes the diff ratio you need. In that case you will probably need the standard diff ratio because a ratio for high speed cruising will be provided by fifth gear. Sorry, I don't know if this applies to your type 9 box.

Bob Schapel

[John Frederick Cockrem]

OK my 2 bobs worth on this issue. My TC has a 4.875 diff with 16 inch wheels and 550/16 tyres. The car was revving its ring out at highway speeds even with the 4.9er so I fitted a High Gear Engineering type 9 gearbox, 5 speed from a Ford Sierra. The result is fantastic with the long distances we travel in Australia. The good part is there is no mods necessary to the chassis and reversion would not be a problem. Ian I bought a Celica box and looking at its size reckoned it would be a butcher`s job to fit it so I sold it. I live just out of Melbourne in Victoria and trips to Winton raceway up near Benella are a breeze now,can`t recommend it enough despite the Originality Police having a go at me from time to time!

[ian theobald]

In all honesty I would not want to travel much faster than 55 mph [90kph] in my TC and guess it is reving at about 3500 or in that vicinity.
I have no idea how mine compares to a standard TC as my diff is not original but looks identical to TD ,TF or similar with modified ends to suit my TC [That's how it came ]
Most of the roads in my part of the world are single lane and hilly suiting current gearing except for the main highway which fortunately has overtaking lanes every so often until it links up with dual lane heading north to Sydney and higher gearing would be a benefit ,even at 55mph.
Passing a massive Semi trailer passing at 100kph+[60mph ] just about blows you off the road and in your rear view mirror is usually a conga line of impatient motorists waiting to pass
!6 inch wheels are nearly a must with there wider tyres .
A friend of mine once fitted a Laycock ? overdrive off an MGB or similar to his 1.5 ltr Riley for similar reasons and was a dream on the motorways and have often wondered if could mount one to rear of standard gearbox
One day will set up a Nav Man out of curiosity as never driven a standard TC and compare actual speed but given the cars that over take might not be much different to standard gearing.
60 mph is scary and think is just over 4200 revs and rarely take car up to that although motor seems willing
I discovered about a year ago that my motor was running only on 1 carbi as the piston was frozen on the other but not much improvement except vapour lock was eliminated and more pulling power in top up hills ,would never of known otherwise .Perhaps the low gearing helped ?

[Steve Simmons]

I'm currently in the middle of a 3-day excursion with the TC Motoring Guild, and spend most of my highway time at 4000-4200 RPM, about 60-65 MPH. The cars handle it just fine! TA gearing in back, stock wheels and tires, Datsun steering. Don't be afraid to drive them flat out! Just make sure the steering and alignment are set up correctly.

[John Frederick Cockrem]

Steve I certainly agree with your comments re TA diff and 19inch wheels.With my 5 speed and 16s with 550/16 tyres at 60mph just under 3500rpm which is very relaxed motoring in a TC.Ian I am familiar with the roads around Nowra and some like the one through Kangaroo Vally are pretty bad, others like the road up to Sydney are great. I recon your TC is well set up by reading your posts and should be able to handle 60mph. Borrow a set of 16s off a friend and see how it feels with them, I think you would be pleasantly surprised.

[ian theobald]

G,day Steve ,My car has Datsun steering and steers straight and have had stub axles crack tested for piece of mind
.On a good smooth road have had run at that rev range but when you see a huge Kenworth or Mack hurtling towards you on single lane Highway it,s time to slow down or risk being blown off road which seems nearly every other vehicle is a huge truck on our busy highway .Enjoy your trip.
My car is fully registered so gets driven as often as I can