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Advice on Diff Ratios please.


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Hi Everyone,

 

I note people speaking about different ratios for diffs on TR4s. Does this mean that they came out of the factory with different ratios?

 

Or, was there is a different ratio for overdrive cars, as opposed to none overdrive?

 

My TR4 is a four speed box without overdrive. What would be the correct diff ratio for this car?

 

If anyone can spare the time to explain a bit about these ratios and why they differ, I would be very grateful.

 

I guess, it all has something to do with acceleration versus high speed cruising at lower revs, but maybe it's all to do with how quickly the kettle boils at the top of the Stelvio Pass!

 

Thanks for your help.

 

All the best.

 

Dave.

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The factory fitted the 4-speed cars with a 3.7 axle and the OD cars with a 4.1 axle.

 

For racing you also had 4.3 and 4.55 form the factory.

The current retailer also provide a 3.9 as a non factory fitting which many people run in racing with a 4-speed box.

 

Cheers

Chris

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It has all to do with acceleration versus top speed, Dave! Excuse me if you already know much of it but it goes like this:

 

The TR4 is quoted as having a ratio of 3.7-to-1 as standard (usually shortened to '3.7' in common speech), but with a 4.1:1 available as an optional ratio. The number of crownwheel teeth is divided by the number of teeth on the pinion to determine these figures. So the pinion (and engine, via the prop, in direct drive fourth gear) has to turn 3.7 times to achieve one complete revolution of the crownwheel (and rear wheels). Or 4.1 times at the same road speed for the lower diff ratio. Jack up one back wheel and count the number of times the propshaft turns for one revolution of the rear wheel with the gearbox in neutral to see what your car has fitted! Naturally the gearbox gear selected changes the engine speed/rear axle ratio equation so for the purpose of the exercise we assume all ratio considerations in 4th gear, which is a 1:1 direct drive therefore the engine is turning at the same number of revs as the propshaft.

 

So the higher the ratio (= lower numerically) the less revs the car needs to maintain higher speeds but the acceleration will not be particularly vivid; this is the 3.7 option. The TR4 will accelerate fairly well and cruise at speed acceptably, a good everyday compromise for a sports car.

 

The lower the diff ratio (= higher numerically) the better the acceleration of the car but the engine will be revving more at speed. This means top speed at the rev limit will be lower. In practice, it will be quick off the mark than the 3.7-equipped car but the engine will be noisier at 70 on the motorway.

 

Please notice that a higher numerical ratio is a lower differential ratio, and vice versa. This is one of the most common mistakes in the car enthusiasts' world!!

 

So if you wanted to be quicker off the mark and were prepared to have the car a bit noisier and less economical at speed - perhaps to do the odd bit of light competition - you would order your new TR4 with the optional lower 4.1 ratio axle instead of the usual 3.7 fitting. Naturally the addition of an overdrive would help here and could give you the best of both worlds, but I believe the two options could be ordered separately so maybe you didn't automatically get the lower ratio diff if you chose an overdrive, there should have been no reason why you couldn't order a 4.1 without an overdrive.

 

Example; My TR4 had an overdrive and a 4.1:1 rear axle ratio when bought and it had been used for road rallies. I converted it to do special stage rallies where acceleration is paramount so changed to the optional competition 4.55:1 crownwheel and pinion (CWP) which gave it much better acceleration but reduced the top speed. I also removed the overdrive for reasons of weight saving and reliability which didn't help on the motorway either! However the equation has been further altered by having the engine highly tuned including full balancing which gives me an extra 1000 rpm to use, so while the car accelerates fiercely it can now still hit high speeds, but it's damn noisy in there!! But it's purely a rally car so I put up with it. There was an even lower ratio of 4.8 available as a Standard Vanguard (?) part which fits TR4's but it's not been available new for many years and is very rare second hand. TR4 Tony has one in BST but he still uses an overdrive as well.

 

Incidentally, speedos are calibrated for the specific rear axle ratios so change the ratio and your speedo will be under- or over-reading.

 

Phew. That's enough for a Monday morning!

 

Nigel.

Edited by Nigel 628
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Nice presentation, Nigel.

In a nutshell, after 42 years of TR4 driving with a 3.7 axle and no OD, I am perfectly happy with this situation. I know many members are fans of the OD, but the same persons frequently complain about the darn gizmo playing tricks.

My cruise range goes from 3200 to 4500+ rpm and I must confess I enjoy the growl at 4500, even on long distances. Just get a good 2" exhaust with proper silencing. Drive casual and now and then, floor it for the howling.

 

Badfrog

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  • 6 years later...

Can  a 3.7 diff be installed in a overdrive equipped TR4 to give her longer legs? Are there recon pumpkins available set up for a bolt in app? Thinking of buying a TR4 with overdrive and plan on using her for 3 hour drives to the beach. Very popular conversion in the Healey world. 

On 9/30/2013 at 3:34 PM, ianc said:

As is so often the case, there's useful information in the Technicalities CD, Section E2.

All members of the Register should have a copy of the CD.

Ian Cornish

 

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15 minutes ago, Derek Snead said:

Can  a 3.7 diff be installed in a overdrive equipped TR4 to give her longer legs? Are there recon pumpkins available set up for a bolt in app? Thinking of buying a TR4 with overdrive and plan on using her for 3 hour drives to the beach. Very popular conversion in the Healey world. 

 

Yes

my 3a had 3.7:1 and 22% A type OD worked well.

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4 hours ago, Derek Snead said:

Can  a 3.7 diff be installed in a overdrive equipped TR4 to give her longer legs? Are there recon pumpkins available set up for a bolt in app? Thinking of buying a TR4 with overdrive and plan on using her for 3 hour drives to the beach. Very popular conversion in the Healey world. 

 

It quite probably already has a 3:7 diff.

Stuart.

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Any of the TR2/3/4 cars will happily pull a 3.7 diff with the A-type overdrive (22%).  Gives a good combination of cross-country performance, reasonably relaxed cruising on motorways and some improvement in economy.

It's easy to check which ratio has been fitted - refer to the CD.

Ian Cornish

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Very interesting post.

Is an OD conversion possible on a non-OD gearbox?

 

Nigel H

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1 hour ago, NigelH said:

Very interesting post.

Is an OD conversion possible on a non-OD gearbox?

 

Nigel H

Of course, you just need a mainshaft and an adaptor plate and an overdrive unit.

Stuart.

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