M46 vs M90 - how does the overdrive work + fuel efficiency

[tofufi]

Hi folks,

Just idly thinking this morning... how does the overdrive on the M46 gearbox work? I've tried googling but can't see anything specific and understandable in idiot-proof terms .

Secondly, I've seen a few people mention that the overdrive unit isn't that efficient. Would I be likely to see a gain in MPG by switching to an M90 from M46/OD? My car spends most of its time on the motorways in OD/5th..

[Laird Scooby]

The overdrive unit on the M46 box is a Laycock de Normanville unit and a description of what it is and how it works is here :

http://www.thorden.dk/Teknik/A%20Type%20Overdrive.pdf

More on the actual history of it is here :

https://en.wikipedia.org/wiki/Overdr...ics)#In_Europe

In simple terms, it's a 2-speed epicyclic gearbox (think most automatic transmissions) where the standard ratio is 1:1 or direct drive, the other is in the region of 0.8:1 and is switchable.

This is the overdrive ratio and 0.8 turns of the gearbox output shaft will result in 1 turn of the overdrive output shaft.
Another way of saying that is for every 1 turn of the gearbox output shaft/overdrive input shaft, the overdrive output shaft will turn 1.25 times or has a 25% overdrive.

In other words if you're on the motorway and have 4000rpm showing on the rev counter, engaging overdrive will drop that to about 3200rpm resulting in less noise, engine wear and a saving on fuel consumption.

As long as the overdrive is well maintained i don't see the cost of converting to a 5-speed box as being viable. You're unlikely to improve fuel economy, perhaps even make it worse so doubtful that you've ever recover the cost of converting it.

If you're looking to reduce your running costs then think seriously about an LPG conversion, with prices of LPG at 50-60p/L it's about half the cost of unleaded and less than half the cost of super unleaded. There are many different ways of achieving an LPG conversion so don't dismiss it out of hand as being too expensive.

[tofufi]

Originally Posted by Laird Scooby

The overdrive unit on the M46 box is a Laycock de Normanville unit and a description of what it is and how it works is here :

In simple terms, it's a 2-speed epicyclic gearbox (think most automatic transmissions) where the standard ratio is 1:1 or direct drive, the other is in the region of 0.8:1 and is switchable.

In other words if you're on the motorway and have 4000rpm showing on the

As long as the overdrive is well maintained i don't see the cost of converting to a 5-speed box as being viable. You're unlikely to improve fuel economy, perhaps even make it worse so doubtful that you've ever recover the cost of converting it.

If you're looking to reduce your running costs then think seriously about an LPG conversion, with prices of LPG at 50-60p/L it's about half the cost of unleaded and less than half the cost of super unleaded. There are many different ways of achieving an LPG conversion so don't dismiss it out of hand as being too expensive.

Thanks for the help

Surely having a second, epicyclic gearbox would add further losses to the powertrain? I understand that an epicyclic gearset is typically around 96-97% efficient...

I believe the M90 gearbox has essentially the same gear ratios as the M46, just is a 5 speed gearbox rather than 4 + OD. With that in mind, I don't see much case for the fuel economy worsening as the engine would still be doing the same RPM at any given road speed...

LPG wouldn't work on my engine, it's a D24T . Currently averaging just over 45MPG.

[Laird Scooby]

In the direct ratio, the power losses would be negligible as the epicyclic gear train is non-functional.
In overdrive then yes, you'd probably get 3-4% transmission losses but i doubt that is any more (or less) than a conventional, helical cut, 5th gear set.

Without checking the ratios of 5th gear and overdrive, i can't say for sure but if the 5th gear is slightly different then it could adversely effect economy.
For example, a friend of mine recently built a custom gearbox for his car. The standard 5th gear was something like 0.82:1 and he replaced it with a much hgher one from a diesel version of the same car. This was something like 0.7:1 which doesn't seem that much different.
However, up to about 40-45mph, 5th gear is no longer usable so round town, through villages etc he has to drop into 4th so is losing out on economy. On the open road and motorways, cruising is much more relaxed and economical so it's swings and roundabouts for him.

Or at least it was, he recently changed jobs and no longer needs an economical long-distance cruiser so treated himself to a Lexus LS430. Round town he's lucky to see 12-16mpg but that wasn't what he bought it for, long distance he gets 34+mpg from it even when he's "playing" but did achieve 37mpg on the long run home from buying it.

Back to the Volvo though, if overdrive and 5th gear ratios are the same, unless the overdrive is giving trouble and can't be fixed with the usual methods then there's no sense in changing it as you'll gain nothing.

I know on some cars, you could engage overdrive on 3rd gear as well as 4th effectively giving you 6 forward gears. Usually overdrive 3rd was in between 3rd and 4th in terms of how the ratios worked which was handy for overtaking.

There are kits available to add LPG to diesel vehicles, however it's an additional rather than an alternative fuel. I don't know what the mix is but for arguments sake, if it was 50/50, you'd use half diesel, half LPG thereby reducing you cost per litre of fuel to half a litre of diesel (60p~) and half a litre of LPG (28p~) so a litre of fuel would cost 88p instead of 120p so would potentially save some money.
However i think it works best on the common rail diesel injection systems that are electronically controlled, i don't know enough about adding LPG to diesel to say anything else and to be honest, i'm not even sure on what i've said there, all i really know is it's possible on certain vehicles.
Someone that probably could tell you more is Dai (ClassicSwede) but whether it would be a viable conversion for yours i couldn't say.

[TonyS9]

At motorway speeds the drive train efficiency is pretty much swamped by overfueling due to the unknown load (or slow response of the AFM). Diff ratios vary too between turbos, LPTs and autos.

If you can keep it below 60 I think there is a chance of getting into closed loop using the O2 sensor, but its pretty fragile and the slightist acceration will kill it off. However I'm not even sure it will do closed loop outside of idle.

Max fuel efficiency is achieved during pulse and glide, minimising the time the engine is above idle. Accelerate briskly 80% throttle and coast in neutral. I can average 40mpg this way on a long journey. 30mpg is possble with 50-60mph constant and making sure throttle is accurately minimised, preferably to a slight speed reduction. Feels like 0.5mm on your foot can drop 10mpg.

[Laird Scooby]

Originally Posted by TonyS9

If you can keep it below 60 I think there is a chance of getting into closed loop using the O2 sensor,

Originally Posted by tofufi

LPG wouldn't work on my engine, it's a D24T . Currently averaging just over 45MPG.

I thought it was a petrol as well until i spotted the remark about the LPG!

[rogerthechorister]

An M90 is not easy to drop in in place of an M46 - apart from anything else the propshaft is different length. I think the clutch differs too - I know that if I want to put an M90 onto a B234F+T I will need the diesel Sachs clutch, the splines are different. What will go straight in is an M47. But the main issue is strength. An M90 is quite hard to break, and the best M46 will be marginal if you can get to about 230 bhp (and remember there are the three different M46 types, the P, the J and the P/J which is for the twin-cams - I can never remember which of the P and J is for the turbos and which for the wimpy N/As and it is only the turbo one that will go even as far as 230 bhp).

However as I understand it the M47 is weaker than a turbo M46.

[TonyS9]

Originally Posted by Laird Scooby

I thought it was a petrol as well until i spotted the remark about the LPG!

Ah, I see

[Joe of Loath]

Originally Posted by rogerthechorister

An M90 is not easy to drop in in place of an M46 - apart from anything else the propshaft is different length. I think the clutch differs too - I know that if I want to put an M90 onto a B234F+T I will need the diesel Sachs clutch, the splines are different. What will go straight in is an M47. But the main issue is strength. An M90 is quite hard to break, and the best M46 will be marginal if you can get to about 230 bhp (and remember there are the three different M46 types, the P, the J and the P/J which is for the twin-cams - I can never remember which of the P and J is for the turbos and which for the wimpy N/As and it is only the turbo one that will go even as far as 230 bhp).

However as I understand it the M47 is weaker than a turbo M46.

The M46 and M47 are identical until you get to the back half - the M46 has an overdrive, the M47 has an extra bit of housing for the 5th gears to slot into. No one online seems to agree on which one is stronger, so with that in mind I'd say they're near enough identical. However, they're both still really weak gearboxes by modern standards.

I swapped my old M46 box for an M47, and saw about 2-5MPG increase in fuel economy on motorway runs. Here's my fuel log.

However, that's not entirely representative as I replaced some fuel tubing and the stuck thermostat at the same time (although K-jet isn't affected so much by engine temperature). The biggest reason for me is that my overdrive broke and over the last couple of hundred miles before I removed it, started sticking on in all gears and slipping out in 1st (involving wheelspins in embarrassing situations, like passing a stopped police car).

In my eyes, the sheer number of parts involved in the M46 overdrive mean I just cannot be bothered with it. The M47 is lighter, nicer to use (no more trying to put the M46 into 5th...), and much less likely to go wrong (and if it does go wrong I'm more inclined to open it up and have a look). It also seems more efficient too, which is a bonus.