Overdrive spec

[Dazzer]

Hi Guys

 

Most literature seems to spec the TR4 with overdrive 2nd,3rd & 4th, however there's also conflicting spec in some Haynes manuals and other sources stating just 3rd & 4th.

 

Did this change during the production run, or would all TR4s have had overdrive 2nd, 3rd & 4th as the original spec?

 

Cheers

Darren

[BrianC]

AFAIK, all TR4s would have had an A-Type overdrive (if fitted - it was an optional extra) from new, with overdrive on 2/3/4.

 

However, some originally non-overdrive cars will have had overdrives added by later owners and these could be a very early A-type which had overdrive on top only, or a later J-type as fitted to later TR6s, which had overdrive on 3/4 only.

 

There are good explanations of the history and differnces of overdrives in the Moss catalogues.

 

The Haynes manuals cover multiple models and are not a reliable reference source. Get a genuine workshop manual and parts catalogue (available from the TR Register office or web shop).

Cheapest option is the Heritage CDROMs which have workshop manual, parts catalogues and owners handbooks on the same CD.

[TR 2100]

AFAIK, all TR4s would have had an A-Type overdrive (if fitted - it was an optional extra) from new, with overdrive on 2/3/4.

 

Brian's comments agreed.

 

The difference between overdrive on top only, on 2, 3 and 4 and the

later option 3 & 4 only was down to the top cover.

All the top cover does is provide the earth using isolator switches,

activated by the gear selectors, that only provide the earth when

the appropriate gear is selected.

 

This means, of course, that you can bypass the top cover gearbox

selectors and take the lead directly to earth, and you get overdrive

on ALL gears.

 

But BEWARE - if you forget and leave overdrive engaged, then try

to drive in reverse (as many have done in the past) you can end up

with a very expensive repair bill.

Been there, done that, although I realised the problem in time to

avoid a repair bill.

 

AlanR

[Dazzer]

Thank you Brian and Alan for the comprehensive replies, which I think clear up the cunundrum of why an A Type overdrive would only operate on 3rd and 4th.

 

It must be the top cover.

 

The overdrive was replaced in 2005 for an uprated racetorations unit (original beyond repair) and still works flawlessly on 3rd and Top, immediate seamless engagement... but not 2nd.

 

Dont feel I need overdrive on second, never seem to hang around long in 2nd gear territory anyway! Is there a practical advantage?

 

3rd overdrive is very close to fourth ratio, but much more convenient and quicker to engage. I personally find 4th overdrive to be of greatest benefit making cruising at motorway speeds a joy.

 

Just having a J Type fitted at Jigsaw racing to our 2000 mk1, and I presume with a different top cover/wiring you could engage a J type on 2nd, 3rd & 4th, interesting.

 

Thanks once again

 

Darren

[Tom Fremont]

Dont feel I need overdrive on second, never seem to hang around long in 2nd gear territory anyway! Is there a practical advantage?

 

You probably don't need it, but once you've had one you'll never go back

 

With a 3.7:1 axle and original diameter tyres you can exceed 60 mph in 2nd o/d ( make that 72 mph on a P.I. car ), with one gear change from rest. It's the acceleration factor in 2nd which is such a joy. You can have 3rd o/d though I do use it when accelerating into highway traffic.

[BrianC]

I'm not sure why an A-type overdrive would allow operation on 3/4 and not 2. Gearboxes with the early overdrive units only had a single isolator switch in the top cover, controlling use of overdrive in top. I always thought the addition of the second isolator switch provided overdrive on 2nd and 3rd, but perhaps the changes to selector forks affect the use with 2nd. I will have to recheck the details in the workshop manual and on the TR Register Technicalities CD.

 

However, amongst my spares, I have an overdrive gearbox with a single isolator and a spare top cover with two, which was intended to upgrade to give the full 'seven-speed box'. Note that this should only be done if the overdrive unit has the uprated clutch operating pistons (1_3/8" diameter instead of 1_1/8"). This is denoted by the serial number on the plate attached to the top of the overdrive cover. Serial numbers 22/1275/--- are not up to use of overdrive on the lower gears. The uprated pistons were in units numbered 22/1374/---.

 

I have to admit, I have in the past made full use of overdrive on 2nd, but tend to be a little more gentle on the cars nowadays - these are expensive items to have rebuilt.

[TR 2100]

The overdrive can only take so much torque through the system.

 

With a standard engine at maximum revs in 2nd, the torque already

pushes the overdrive near to the limit.

A Racetorations engine is likely to be modified beyond standard output,

so the overdrive is even nearer to/past the limit.

 

You can, of course, upgrade your overdrive to take the extra power,

but of course you are now talking about more money.

 

AlanR

[Tom Fremont]

The overdrive can only take so much torque through the system.

 

No doubt So this raises a few questions for me, since the A-type o/d was used on Jag MkIIs ( wasn't it? ) and AH 3000s as well. What gives? Also, if I understand correctly the spring-engaged cone clutch in the o/d transmits the torque in normal mode ( or does the one-way clutch augment this function too? ). Does anyone recommend stiffer springs for uprated applications?

 

I've done 75,000 miles with 150+ BHP / 165 lb-ft and no o/d failures to date ( though a few layshafts have gone bad - since gone uprated ), with liberal use of 2nd o/d. Boxes I've had rebuilt by Quantum Mechanics have had higher HP spec'd, and I thought it was a relatively simple matter of using a stiffer spring/ shim set up in the accumulator to achieve higher hydraulic pressure. Note that Quantum don't advise engaging 2nd o/d under full load yet mine have withstood it. I confess I am always conscious of the risk

[stuart]

No doubt So this raises a few questions for me, since the A-type o/d was used on Jag MkIIs ( wasn't it? ) and AH 3000s as well. What gives? Also, if I understand correctly the spring-engaged cone clutch in the o/d transmits the torque in normal mode ( or does the one-way clutch augment this function too? ). Does anyone recommend stiffer springs for uprated applications?

 

I've done 75,000 miles with 150+ BHP / 165 lb-ft and no o/d failures to date ( though a few layshafts have gone bad - since gone uprated ), with liberal use of 2nd o/d. Boxes I've had rebuilt by Quantum Mechanics have had higher HP spec'd, and I thought it was a relatively simple matter of using a stiffer spring/ shim set up in the accumulator to achieve higher hydraulic pressure. Note that Quantum don't advise engaging 2nd o/d under full load yet mine have withstood it. I confess I am always conscious of the risk

Tom Jaguar MK 1 & 2s/S types and MK10s all used the "A" type overdrives but they were only switched for top gear use only (although I had several wired for all gears and used the reversing light switch to prevent reverse gear engagment) they didnt tend to last too long if you shoved the entire 250 HP that a well set up triple 2" SU carbed 3.8 could produce through the gears. They did have different internals though. Overdrive services would probably be able to tell you the exact spec.

Stuart.

[Rhodri]

Tom, I think the one-way clutch only transmits the drive for that split second when changing into or out of overdrive, and the cone clutch is not engaged.

[Dazzer]

As a footnote, I spoke to racetorations and they confirmed that it was just the top cover that had been changed to inhibit 2nd engagement, primarily to avoid excessive torque in 2nd, an easy mod to reverse but not recommended.

 

They didnt feel 2nd OD was particularly beneficial, although in the thread there are a number of you having great fun with it.

 

Thanks for all the comments

 

I'll always feel now that I'm missing out on something.

 

Yours 2nd OD less.

 

Darren

[TR 2100]

2nd Overdrive is great for overtaking and sounds fantastic, but I fully

understand Racetorations wisdom in disconnecting it from 2nd if the

engine is tuned and the overdrive is not uprated.

 

BUT - with normal setup, OR with modified engine and uprated overdrive,

flicking out of 2nd O/d and foot down at the same time to overtake, then

back into O/d as you blast past is one of the real experiences of TR motoring.

 

AlanR