Why no more OD units?
Discussion
I suspect for anyone who's driven a 60s/70s sports car such as an MG or Triumph or even some of the saloons. You'll know just how great and how much fun the electronic over drive unit was and just how affective it was too.
Well in light of the 7 and maybe even 8 speed gearboxes. Why hasn't anyone though to bring back the OD unit? I can't believe they are that expensive to make vs developing whole new gearboxes with a million cogs inside.
In the day and age of needing good mpg and lower emissions, wouldn't having an "economy" set of gears and a "sporting" set of gears answer all questions and with out the need to be constantly waggling the gearstick about?
Well in light of the 7 and maybe even 8 speed gearboxes. Why hasn't anyone though to bring back the OD unit? I can't believe they are that expensive to make vs developing whole new gearboxes with a million cogs inside.
In the day and age of needing good mpg and lower emissions, wouldn't having an "economy" set of gears and a "sporting" set of gears answer all questions and with out the need to be constantly waggling the gearstick about?
I used to wonder about this too. The unit on my previous MG B was still working well with 350k miles on it and the one on my current B has about 270k on it and is ok provided I use the car regularly (seems to stick a bit in the freezing cold).
However, I know that when they first dropped in the V8 into the MG BGT, the police were keen to use it as a Z car. They found that the torque of the engine, especially when using the overdrive in 3rd at fully open throttle, was wrecking the overdrive. So I suspect that would be the case, although it may just be simple costs as it's now cheaper to manufacturer a 6 or even a 7 speed box than it is to make a heavier overdrive unit.
Last car I saw with one on a manual box was an early 80s volvo estate, although I did drive around in an auto toyota surf with an overdrive ~ 1990.
However, I know that when they first dropped in the V8 into the MG BGT, the police were keen to use it as a Z car. They found that the torque of the engine, especially when using the overdrive in 3rd at fully open throttle, was wrecking the overdrive. So I suspect that would be the case, although it may just be simple costs as it's now cheaper to manufacturer a 6 or even a 7 speed box than it is to make a heavier overdrive unit.
Last car I saw with one on a manual box was an early 80s volvo estate, although I did drive around in an auto toyota surf with an overdrive ~ 1990.
kambites said:
Is it particularly hard to fit one into a transaxle type gearbox? Has anyone ever made a FWD car with an overdrive unit?
That I don't know tbh.I just though that either an 8 speed (4 speed OD on each gear) or maybe a 5 speed with OD on 3rd thru 5th (8 speed) would offer up the ideal of large gear spacing for economy and emissions, tall top gear for low cruising rpms and a set of close ratio gears for performance.
doogz said:
I don't really see the point. Extra weight and extra complexity for what?
A single gearbox, with a single control/shifter, with all the necessary ratios in it just seems like a simpler solution.
But not simpler to use. 8 gears via a H gate would be horrid to use IMO and to make use of all the gears you'll have to have silly short gearing forcing you to change gear all the time.A single gearbox, with a single control/shifter, with all the necessary ratios in it just seems like a simpler solution.
Having only 4 positions would mean far less arm waving and you could have each set perfectly setup for performance or economy.
Also would it really be that much extra weight or complexity?
The MGB style Laycock De Normanville Overdrive is effectively a two speed automatic gearbox, sans torque converter, bolted to the back of a manual gearbox. It adds quite a bit of weight to the car (on smaller gearboxes the OD unit weighs almost as much as the gearbox) and the extra drag from rotating all those driveline components in oil sucks something like 3-4% of power if memory serves.
It came into use on lots of British cars because they didn't want to go to the expense of re-engineering their gearboxes to have an extra gear - bolting an extra gear to the back of the gearbox was an attractive alternative and pretty much free in terms of development costs.
Once they'd engineered a 5 speed box it became a bit irrelevant.
It came into use on lots of British cars because they didn't want to go to the expense of re-engineering their gearboxes to have an extra gear - bolting an extra gear to the back of the gearbox was an attractive alternative and pretty much free in terms of development costs.
Once they'd engineered a 5 speed box it became a bit irrelevant.
I remember a friend who had a Colt Cordia Turbo back in the 80's and that had a sort of overdrive unit. It had a 4 speed box but a seperate lever that operated a 2 speed final drive. From what I can remember it was pretty fast for it time although not that easy to change 2 levers at the same time especially if in 2 different direction!
doogz said:
Why do you really need 8 gears?
I don't think you do. In fact I'm not sure you really need 6, but the modern trend does seem to be to add more and more.doogz said:
And why would you only want 4 to use when you're driving enthusiastically?
With OD you could use as many or as few as you need tbh. But you could have them setup better for the task at hand.doogz said:
In a diesel/turbo, that might be alright, but in a petrol NA car, that's not really enough IMO, they'll be too far apart.
???Most/many cars are still 5 speeders today and even the ones with 6 speeds still tend to be 1:1 in 4th with a double OD on top for 5th and 6th. So in reality unless you are hooning on the autobahns you'll likely only use 4 gears at most. I suspect most modernish cars will do at least 70 in 3rd and somewhere between 90-100mph in 4th.
doogz said:
I've driven 8 gear H-pattern boxes, 4 and 4, with a switch. It's fun, although not really much use for enthusiatic driving I don't think, it takes that little bit more thought, and in the stuff I've driven at least, changes between FD's are much slower.
I've driven tractors with lots of gears. But I would say the OD unit in my MGB was hugely entertaining, fun and practical. And in truth probably found it more useful and more fun than a 5 speed box.doogz said:
Extra weight, well they generally were in seperate housings, weren't they?
True, but I don't think they are that heavy. Certainly they aren't as heavy as an LT-77 5 speed box and you can pick one of those up on your own without much trouble. So added weight yes (although gearboxes with more gears in probably weigh more too). But in terms of percentage vs a cars curb weight, I'm not sure they are exactly that heavy.alfa pint said:
I used to wonder about this too. The unit on my previous MG B was still working well with 350k miles on it and the one on my current B has about 270k on it and is ok provided I use the car regularly (seems to stick a bit in the freezing cold).
However, I know that when they first dropped in the V8 into the MG BGT, the police were keen to use it as a Z car. They found that the torque of the engine, especially when using the overdrive in 3rd at fully open throttle, was wrecking the overdrive. So I suspect that would be the case, although it may just be simple costs as it's now cheaper to manufacturer a 6 or even a 7 speed box than it is to make a heavier overdrive unit.
Last car I saw with one on a manual box was an early 80s volvo estate, although I did drive around in an auto toyota surf with an overdrive ~ 1990.
MGB with OD was fun for the instantaneous shift, but if you treated it btutally you could feel the mechanical pain as it changed up. So I expect ability to work with more torque was a big factor- it went same way as freewheels in the transmission which shared the problerm of sudden takeup of power. I think they dissappeared with the Saab 96!However, I know that when they first dropped in the V8 into the MG BGT, the police were keen to use it as a Z car. They found that the torque of the engine, especially when using the overdrive in 3rd at fully open throttle, was wrecking the overdrive. So I suspect that would be the case, although it may just be simple costs as it's now cheaper to manufacturer a 6 or even a 7 speed box than it is to make a heavier overdrive unit.
Last car I saw with one on a manual box was an early 80s volvo estate, although I did drive around in an auto toyota surf with an overdrive ~ 1990.
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