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How to use overdrive?


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Hi all, having joined the 'cracked diff club' I have been researching on here and came across a post that stated one reason for cracked diff mounts was poor use of overdrive. Would appreciate advice on using overdrive correctly, it was completely overhauled in 2011.

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Hi Christine (Is that you or the car?),

I remember a thread some months ago where the use and (perceived) abuse of overdrive was discussed - http://www.tr-register.co.uk/forums/index.php?/topic/51488-am-i-using-the-overdrive-properly-or-abusing-it/?hl=overdrive

 

Overdrive being relatively new to me at that time, it was a useful guide. However, there are differing opinions (as is often the case). Some feel that no or little special treatment is required, whereas others feel differently.

 

What I do nowadays is always disengage before changing gear, and I always try to hold the revs constant while I disengage (rather than coming off the power). It makes for a smooth change in my car, but for others it'll be different I'm sure.

 

Darren

 

P.S. Mike beat me to it with the link (above)!

Edited by TR5tar
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No need to disengage gear/clutch.

 

When engaging overdrive at a constant speed, lift the throttle slightly.

And on disengaging, press down, again slightly.

 

This will minimise the jolt going in either direction, due to a mismatch between the wheels' and engine speed as the o/d engages/diengages, and any strain on the drive train.

That's all!

John

Edited by john.r.davies
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Followed the previous thread on using overdrive, and see similar advice here (from some) about keeping some throttle on when coming out of overdrive, and understand the reasons. I always find that counterintuitive if changing down and using engine braking - where the natural inclination is to lift off the throttle, and probably apply the right foot to the middle pedal, as you come down through the gears (eg o/d 4th to 4th, to 3rd etc).

 

I appreciate that this can be overcome by heel and toeing, if you have the ankle dexterity to do it. If there is time on slowing I therefore tend to de clutch while flicking the o/d out.

 

Mike

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Hi Mike,

indeed changing down has that reverse logic of increasing engine speed.

However this is to allow the road speed and engine speed to match - otherwise you don;t have engine braking but gearbox breaking/braking instead.

 

Pressing the clutch is for cowards :P

 

Roger

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Roger,

 

Always more retardation effect if the engine speed and road speed don't match!

 

;-)

 

.....but pressing the accelerator while slowing down (and for most, not at the same time using the foot brake) feels odd, especially as drive is still engaged. Not the same as heel and toe, where the clutch is also used (most of the time).

 

Mike

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You should be reading the road better so you are in the correct gear for that particular situation.

Stuart.

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If overdrive is engaged with little throttle, there will be a jarring of the transmission - undesirable.

 

If overdrive is disengaged on the over-run, the rear wheels may be locked momentarily - highly undesirable!

 

To get smooth engagement of overdrive, have the throttle hard open - if the overdrive doesn't like this, it needs to be checked by a specialist.

 

To get smooth disengagement, just press the throttle very lightly as the switch is flicked.

 

Ian Cornish

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To get smooth engagement of overdrive, have the throttle hard open - if the overdrive doesn't like this, it needs to be checked by a specialist.

 

Although I think you need to be somewhat more gentle when not in 4th gear, which I've always understood is why some mfrs such as Jaguar only allowed o/d via top gear.

Edited by Superaquarama
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Early TR2s had the 22/1275 Laycock overdrive, which used 1.125" operating pistons and was on top gear only.

A change was made to the 22/1374 unit, which uses 1.375" pistons and operates on 2nd, 3rd and top gears.

 

The 22 refers to the fact that the overdrive fitted on TRs had a gear ratio of 0.82, and if you divide 1 by 0.82 you will get 1.2195, so an increase in output speed of 21.95%, which Laycock rounded to 22.

 

There were other versions giving as much as 32% increase in output speed - available on Standard 10, for example!

 

Ian Cornish

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To get smooth engagement of overdrive, have the throttle hard open - if the overdrive doesn't like this, it needs to be checked by a specialist.

 

Although I think you need to be somewhat more gentle when not in 4th gear, which I've always understood is why some mfrs such as Jaguar only allowed o/d via top gear.

The only one you really need to be a little careful of is with the IRS cars using 2nd overdrive, there can be a bit of a violent engagement if used at full chat! Jaguars use of overdrive only on top was because the amount of torque available and their use of the "Compact" overdrive unit, that said my 3.8 MK2 overdrive survived many years of heavy use of O/drive in 2/3/4 but that was with heavy re-enforcing of rear axle Panhard rod mount and torque arm mounts.

Stuart.

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my 3.8 MK2 overdrive survived many years of heavy use of O/drive in 2/3/4 but that was with heavy re-enforcing of rear axle Panhard rod mount and torque arm mounts.

Stuart.

Interesting, Stuart ! Can you recall you managed to wangle o/d in the lower gears, or did you just bypass the switch ?

 

I inadvertently did so with a Volvo P1800 once after a clutch change and it worked in all gears. Once put it into o/d reverse in the middle of Madrid, made a noise like wheels spinning and expected to have jiggered it, but fortunately no harm done !

Edited by Superaquarama
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Your right it was with the switch bypassed but on the Jag there is a large warning segment on the column that lights up and says "Overdrive" and we did all know about never reversing with it in back then as we raced them as well.

Stuart.

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After switching on the overdrive there is maybe a second's delay before it engages. At the exact moment it is due to engage I lift the throttle and re-apply it

Switching out of o/d I momentarily dip the clutch and blip the throttle, like a double de-clutch.

Have done that for decades. And never needed an od rebuild.

Peter

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Pete, It has worked like that for many years. My view is that rebuilds of anything classic-related carry their own risk these days, mainly to due with QC on repro parts. Some repro gearbox parts are I gather suspect. So if it anything on the 6 works "OK" I leave well alone. :)

Peter

And for good measure I've kept a spare od gearbox, gearbox was rebuilt around 1985.

Edited by Peter Cobbold
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I mainly use the overdrive on 4th as a 5th gear with clutch up and down.

Could see little point on using the overdrive on 2nd and 3rd with the type of driving I did in my 2.

Why would one need 7 gears for road use with a flexible engine?

Like a manual XJ6 S1 with overdrive I had for a while both overdrives engaged quickly and seemed to appreciate the cossetting use of the clutch.

Perhaps I was too old to learn to adapt to any new driving styles!

Regards

David

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Depends on which overdrive you have, early A type was instantaneous as it had continuous built pressure and engaged with quite a discernible thump, later A type still had continuous pressure but had a cushioned engagement which though still instantaneous didnt have the associated thump. J type overdrives as they have to build pressure everytime do take a second or so to engage and put less of a strain on the drive train.

Stuart.

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Depends on which overdrive you have, early A type was instantaneous as it had continuous built pressure and engaged with quite a discernible thump, later A type still had continuous pressure but had a cushioned engagement which though still instantaneous didnt have the associated thump. J type overdrives as they have to build pressure everytime do take a second or so to engage and put less of a strain on the drive train.

Stuart.

Thanks Stuart, Mine's a J type. So what I had been thinking was a fault isnt a fault after all. Its cured !! - after nearly 40 years.

Peter

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