Overdrives

Paul Spencer · Posted: Thu Jul 03, 2014 8:15 am Post subject: Overdrives

This thread http://www.bdcl.org/forum/viewtopic.php?t=3459&sid=9df8fdfc9232e657dfb16064a0db355b drifted onto discussion of overdrives. I know of overdrives on several WOs. I have one on my 3 litre with no problem. I have a low ratio rear axle and find the overdrive is fine from about 40mph on a flat road, 45mph on an undulating road. It tranfoms the car when touring. I bought it from Jock McKinnon in 2005 and it has given no problems. Jock's car has a higher ratio axle and wouldn't pull the ovredrive. It takes about 1 litre of ATF. I originally ran it on engine oil (after all, they share the engine oil in a car like the Triumph GT6 I had many years ago). Then someone at Tim Payne Overdrives suggested changing to ATF (which apparently Volvo used in the same Laycock unit). When I asked Tim, it said they could be used with anything. Mine works only on top gear.

I know of another on a 4.5 that does huge mileages touring. This car has an aftermarket deep sump on the overdrive. A third is on a blower. I think this might be pushing it for torque, but it seems to work OK. Let's hope so as it is currently on its way to Le Mans. The other I know of is on an 8 litre. I saw the previous one from this car ripped to shreds on the bench.

I know some people carry spare prop shafts. Tim Payne does a nice little unit that fits within his overdrive mounting bracket and replaces just the overdrive unit itself. It takes less space in the car when carried as insurance.

Stephen Blakey · Posted: Thu Jul 03, 2014 8:41 am Post subject:

Very interesting. Thanks for that.

Chris Card · Guest

I have an overdrive on my 4.5L that was fitted when the axle ratio was 3.53:1, and that worked very well. Subsequently, I had to have the diff rebuilt, and could only get a 3.3:1 cwp. This is great for comfortable touring, but the overdrive is now a bit over geared.

However, as long as it is not very hilly, I can drop it in at about 40mph and it will take me up to 80mph - and beyond if I'm feeling brave! The engine temperature goes up by about 10F after a few miles, so it is obviously having to work a bit harder to pull the gearing. Also, the unit has a small sump and it starts to drop out at lower speeds as things get hot - a bigger sump sounds like a good idea.

The benefits are comfortable and much quieter cruising - as I hit 2,000 revs, the exhaust note changes to a roar, which is hard on the ears of Liz, although I rather like it! So, in overdrive, I rarely get over 1800 revs (68mph) in normal road driving.

Overall I get about 19mpg, but whether the overdrive helps this or not, I don't know.

The overdrive only works in top gear, a micro switch in the gate turns it off if you come out of top. This switch became jammed recently, so that I had overdrive on every gear. I tried this for a while, to see if there was any benefit, but found it more of a nuisance, having eight gears was too much for my ageing brain!

Chris

Laurie Fox · Guest

Chris

Whether you get better mpg with overdrive is not easy to find out. The rise in engine temperature that you mention is due to the fact that the engine is dealing with a different situation once overdrive as been engaged. Usually the engine rpm will be lower which means that the water pump is not turning so fast for a given bhp output. The coolant will thus be circulating more slowly.

So for a given number of BTU's per minute to be removed from the engine the number of coolant gallons per minute will have dropped. Each gallon will thus have to remove more BTU's than previously.

The radiator will also be working under different conditions and the coolant outlet temperature may have dropped a bit. But if it has not dropped much then the inlet temperature will be higher.

Laurie

Laurie Fox · Guest

Chris

Second thoughts. Is the thermostat working properly?

Laurie

Bob Shanks · Joined: 11 Mar 2005 Posts: 5

Any views on the approproate oil to use in the overdrive units?? Engine, Automatic or axle 90?? Various different opinions.

John Murch · Posted: Sun Mar 05, 2017 10:11 am Post subject:

I had an overdrive fitted to my car last year, I'm embarrassed to admit that I don't know what oil is in it, but the unit was supplied by Tim Payne who does a lot of overdrives, so a chat to him may be worthwhile.

Tim Payne Overdrive Designs Ltd
Oakfield Industrial Estate, Stanton Harcourt Road, Eynsham, OX29 4AW
Phone: 01865 882 299
Email: sales@overdrives.co.uk

Christopher Carnley · Posted: Sun Mar 05, 2017 11:43 am Post subject:

The most appropriate lubricant is ATF for its high temperature and low foaming characteristics,depending on the application.
The overdrive was originally part of the gearbox and used the same oil so the whole of the case acted as a heat exchanger.
The small, less than 1litre capacity, makes for over heating, but then many swear by SAE 30.
There have been many failures due to inexperience of the rather more delicate mechanism, over the WO box, and overdrive should only be selected when in 4th speed as an additional 5th gear.
Any similarity of the arrangement with regard to the use, as opposed to the already factory fitted O/D in say a TR6, is inapproprite.

Bob Shanks · Joined: 11 Mar 2005 Posts: 5

For the record I have used Valvoline 20/50 racing oil for 11 years with no problems. It is on a 3 1/2 Derby and it is only used in 4th then only when it is 2000rpm in overdrive. Also I just touch the clutch on engagement and disengagement to ease the shock of 1 1/2 tons meeting a large rotating mass. Oil changed and filter checked regularly. Some advise a 90 oil but there are experiences cited inevitably on the web of overdrives jamming with this.

David Cocking · Posted: Sun Mar 05, 2017 6:25 pm Post subject:

I've just installed a BDC overdrive in my 3.5 Derby and filled with SAE30. As per Chris C's previous advice, I made a (crude but effective) switch that will only allow engagement when already in 4th. As per Bob, I always use the clutch when going up from 4th and back down, avoiding the considerable thunk that results otherwise. Really chuffed just how much the OD has transformed cruising.

As a matter of interest, if my OD oil does overheat, what would I most likely see / hear / feel / smell first?

Chris Card · Guest

In my WO 4.5l, when the overdrive gets hot, it just drops out of it's own accord at a higher than normal speed.

Chris

Christopher Carnley · Posted: Mon Mar 06, 2017 9:07 am Post subject:

The considerable "thunk" has damaged many an O/D, but surprisingly, to me,
Tim Payne suggests constant circuit energising and to flick the O/D in and out to settle it!?

David Cocking · Posted: Mon Mar 06, 2017 7:24 pm Post subject:

Thanks for the helpful responses Chris & Chris. I think I'll stay with the clutch approach!

Chris Card · Guest

Yes, I always dip the clutch, both up and down. I have a rubber "doughnut." Joint - Ford Granada - between the gearbox and overdrive. This has needed replacing a couple of tmes.

Chris

Paul Spencer · Posted: Mon Mar 06, 2017 10:23 pm Post subject:

I have come late to this thread as I have been away planning a tour. When I got the overdrive for my 3 litre in 2005, I filled it with gearbox oil as the unit was designed to share gearbox oil. Then I heard that another manufacturer (Volvo?) uses ATF. I spoke to one of Tim Paynes' staff, and he recommended ATF. When I spoke to Tim, his view was that it didn't matter - chip fat would do. I switched to ATF.

I also have overdrive only enabled on 4th gear and dip the clutch up and down. You can get big sumps to allow for more oil, but I haven't bothered.

Chris (Card) - I dislike the single u/j with no spline between my gearbox and overdrive. I suspect that this has caused the noisy gearbox bearing I now have. I had wondered about a doughnut instead but people have said that it isn't necessary. Interesting that you have this. Any advice?

Paul

John Murch · Posted: Tue Mar 07, 2017 6:09 am Post subject:

I'm feeling rather ignorant, why should the od oil overheat? Would this be a sign that the internal clutches were slipping?
John

Bob Shanks · Joined: 11 Mar 2005 Posts: 5

A few years ago there was a increased capacity sump that was produced in USA. I cannot find it now.
At the risk of being a bore see the following on oil viscosities etc

The confusion may come from gear oil typically being SAE 80 or 90 whereas engine oil is typically SAE 20W/50, and some people describing those numbers as 'weights' which implies viscosity. They aren't, they are just arbitrary number ranges for engine oils and gear oils that don't overlap with each other. The reasoning behind this non-overlapping is to avoid confusion and people using engine oil where they should be using gear oil or differential oil which would result in rapid wear. The opposite may be equally damaging as the additives in gear and differential oils may harm the soft metals of the bearings. However it is equally important to know the difference between gear oil and differential oil, of which more later.

The actual viscosity of SAE80 gear oil is 7cSt, which falls between the viscosity of 20W engine oil at 5.6cSt and 25W at 9.3cSt. SAE90 gear oil is 13.5cSt which falls between SAE40 engine oil at 12.5cSt and SAE50 at 16.3cSt. So you can see that SAE90 gear oil is in exactly the same viscosity range as 20W/50 engine oil.

To add to the confusion it is usual these days for manufacturers to refer to gear and axle oils by a GL number from 1 to 6. Usually GL4 and GL5 are used for passenger cars, and in most cases GL4 will be used for gearboxes and GL5 for axles, although in some modern cars GL5 is specified for gearboxes, perhaps where they contain the diff as well. GL5 oils are intended for hypoid-type applications most commonly found in axles, but a GL4 oil may be specified for axles that don't have a hypoid action. Some GL4 oils may be suitable for the rear axle, check they have statements to the effect that they are 'extreme pressure' or 'mild extreme pressure' and are suitable for hypoid axles. GL5 oils have additional anti-wear additives that can be harmful to some of the components in a gearbox designed to take GL4. Gear and axle additives are designed to coat the metal surfaces, and are 'sacrificial' in that it is the oil coating that is stripped away (replaced when the teeth go back into the oil bath or are otherwise sprayed) during use rather than the metal being worn away. But GL5 additives stick more tightly to the metal than GL4, and if used in GL4 gearboxes, when the additive is stripped away from softer metals it sticks so well that it actually takes a microscopic layer of metal away with it, even though there has been no metal to metal contact. This gives rise to another misconception of GL5 axle oils when used in a GL4 gearbox - that they in some way 'attack' the softer metals. But it's not a chemical reaction that dissolves the metal merely by being in contact with it, but the stripping action in use as described.

Paul Spencer · Posted: Tue Mar 07, 2017 9:15 am Post subject:

You could write a whole book on oils. In fact, someone has - see https://www.amazon.co.uk/Which-Oil-Choosing-greases-collector-ebook/dp/B00CP9IW4U/ref=sr_1_1?ie=UTF8&qid=1488877770&sr=8-1&keywords=which+oil. I have a copy on my bookshelf and it is very interesting, although I can't say it helped me choose which oils to use.

Chris Card · Guest

Paul - These pictures show the Ford Granada doughnut in place, the one that was breaking up but, amazingly, still providing drive, and a new one complete with clamping band. The band has to be kept in place until the doughnut is bolted into the drive line, otherwise the bolt holes won't line up.

Chris

Christopher Carnley · Posted: Tue Mar 07, 2017 10:49 am Post subject:

It is a Giubo, derived from the Italian word for joint, "Giunto" and the inventor," Boschi". First used on the Alfa Romeo in 1960 I think, it absorbs vibration and flexes to minimise mis-alignment; it is a development of the old "rag joint".
John M,as mentioned above the O/D was always part of the gearbox assembly using the same, usually 20/50 oil, and never intended as the modern reclaimed independent unit. The heat sink is therefore much reduced.

The comment regarding using chip oil in such a mechanism must be regarded as a "throwaway" as it is risible, and anyone knows that vegetable oils become oleo-varnishes.

It may be seen from the above that the viscosities are measured at either 100C or 100F, as gear oil at 10C is about as thick as runny honey, when 20/50 is like milk.
The mechanism of reaction of anti scuff additives is still uncertain, but what is clear is that at peak contact, without the ZDDP, the tip pressure evaporates the oil and the iron becomes an oxide, flaking off.
Although the ZDDP becomes a very sticky boundary layer, at extreme temperatures it too becomes gaseous, and a smaller amount of iron combines to becomes a Ferro-Sulphur-Phosphorous compound.
There is a lot of rubbish talked about "active sulphur", and if the sulphur were inactive it would never have a beneficial effect as an anti-scuff additive. The Zinc and sulphur/phosphate compounds are very stable and have no effect on phosphor bronze in the corrosive destructive sense,the old lead and sulphured oils are long gone.
GL4 gear oil contains about 3.5% of ZDDP, GL5 contains about 6.5% ZDDP, and as its coefficient of friction is reduced it is unsuitable for the gearbox synchromesh, but should be used in hypoid rear axles.
The original specification for the post war differentials was Castrol Hi- Pres SC, which is the same as the modern Castrol EP140.
Finally, this 2nd motion(lay) shaft came out of DOC9. a Bob Burrell special, and a blown R-R Phantom 3 (V12) engine in a Derby Bentley chassis,(!!) with a Chrysler rear axle (!!)and the first of the Jaguar built 4 speed synchromesh E Type gearboxes, the only defect being 2 missing teeth. The question is, if the corresponding 3rd speed constant mesh driven gear is undamaged, and the teeth were in the bottom of the case , how did it happen?????
I have (hopefully) located the last (S/H) layshaft Pt No C20500, anywhere, but I know that there area lot more MK VI parts than JE series Jag gear box parts!

Stephen Blakey · Posted: Tue Mar 07, 2017 2:10 pm Post subject:

Paul Spencer · Posted: Tue Mar 07, 2017 2:14 pm Post subject:

I have felt the oil sump casing after a long drive, and it was warm, but not especially hot.

Stephen Blakey · Posted: Tue Mar 07, 2017 2:22 pm Post subject:

Anecdotally, the problems seem to be significantly worse with the larger engined cars and there are good reasons why that might be the case.

John Murch · Posted: Tue Mar 07, 2017 3:00 pm Post subject:

Thank you Stephen, I will never be an engineer but am fascinated!
Similarly with the efficiency of the 4.5ltr (WO) engine compared with the 3ltr which I understand is due to the better shape of the combustion chamber and the ability to raise compression ratio much higher than possible with a 3.
My 3 was fitted with a 4.5 block last year, but retains the long stroke phoenix crank. I was amazed by the increase in power and torque, with fuel consumption only affected if I cruise at over 70. Below that I have a 10% improvement. Overdrive is also now fitted.
John

John Murch · Posted: Tue Mar 07, 2017 3:14 pm Post subject:

There is an alternative to the usual overdrive.
I was very tempted to go this route, no electrics, less to go wrong!

Christopher Carnley · Posted: Tue Mar 07, 2017 3:27 pm Post subject:

The Borg Warner overdrive as fitted to the 6 cylinder BMC cars.

However mechanically efficient the epicyclic gear train may be,do you remember the experiment of Joule, for heating the water?

Stephen Blakey · Posted: Tue Mar 07, 2017 3:48 pm Post subject:

Christopher Carnley · Posted: Tue Mar 07, 2017 4:00 pm Post subject:

The conical synchromesh was patented in 1919 by Earl Thompson, in a form not readily recognised today.
A Californian hydraulics engineer, he took his idea to GM who developed the first synchromesh system for gearboxes. All synchromesh systems are based on the braking cones patented by Thompson.
That is if anyone knows any differently.
He was also part of the team that developed the first successful automobile automatic gearbox, in turn being inspired by the Wilson epicyclic gearbox and fluid flywheel in a 1935 Daimler car bought by GM.