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Explanation of the overdrive button


iain_spen
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Originally posted by iain_spen

Hey, I'm still pretty new to Supras and was wondering...

 

1. Do you ever take the overdrive off?? because i dont :p

 

2. Does the TT have the same gearbox as the Turbo and N/A??

1. Yeah I do, how do you expect to piss on other road users without doing that...

 

2. Unsure, they other guyz are more knowledgable.

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Originally posted by Snowman

If you don't take the overdrive off you obviously aren't going fast enough!!!!

 

Not sure I would want to do over about 120 in 3rd gear! If you want to stretch its legs are make some big speed you are going to need 4th!

:conf: huh?

Turning overdrive OFF would mean the car didn't get into 4th gear

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Jake

 

my bad wording perhaps would make more sense if said if you have the button pressed in and the o/D off light on then you aren't going fast enough. Need this O/D Off button to be "OFF"...not illuminated to engage 4th gear and thus have the higher speeds

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Originally posted by Jake

I don't think that's right Ian. I think you'll find that 'Overdrive Off' just stops it getting into 4th gear.

Indeed it isn't. All overdrive does is allow the 4th (aka overdrive) gear to be engaged. Turning overdrive off (so you get the o/d off illuminated on the dash) just prevents the box from using the o/d gear. Most useful when preparing to overtake I find as most overtake procedures can be done at less than 120mph! :D
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Originally posted by Snowman

my bad wording perhaps would make more sense if said if you have the button pressed in and the o/D off light on then you aren't going fast enough. Need this O/D Off button to be "OFF"...not illuminated to engage 4th gear and thus have the higher speeds

Ah, gotcha now mate.
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Originally posted by Aerotop Dave

I never understood how it worked (even with the explanations in this thread) so I just leave it on so I don't get the annoying light on the dash.

That's probably cause a lot of people seem to think it's a magical button that makes the car go faster. It's really simple. An auto box goes 1st -> 2nd -> 3rd -> O/D (4th), having the "O/D OFF" turned on just causes the box to go 1st -> 2nd -> 3rd. That's it. Instead of 4 gears you now only have 3.
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Okay, but I still don't get what it's actually for.

 

3 gears spread across the speed range suggests the car will rev higher in each gear with it turned off (3 gears spread across the rev range versus 4 gears means each gear has to handle a longer mph range?). But with my turned on it still revs to the redline when I mash my foot into the floor.

 

So if it revs to the redline with it turned on, why do I want to turn it off? If the light comes on with it's turned off, that suggests it should generally be left on.

 

Basically - I don't understand what it does, I don't understand why I'd want to turn it off, and the car seems to be telling me that I shouldn't turn it off!

 

:)

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It doesn't affect the rev range at all. That's why with O/D Off you can't go above 120mph, cause that's only achievable in 4th gear.

 

Example 1

 

Amagine this, you're in a manual crusing along a nice stretch of single carriageway A-road. You come up behind a couple of lorries doing a far from acceptable speed. So you decide to overtake. You watch the on comming traffic, you see a gap about to approach so you drop it into 3rd to bring the revs/boost up, and as the last car passes you pull out and boot it. A perfect overtake.

 

Now amagine doing it in an automatic. You do nothing, as the last car goes past you pull out and kickdown. Can be very dangerous especially in the rain. You're pulling out in a high gear, then the car is changing down a gear, then you're shooting up the rev range probably engaging the 2nd turbo in the process. Then finally you have full power to overtake, by which time you've already (slowly) done half of the over taking. The reason I say it's dangerous is for the same reason you try to avoid the car changing down whilst going around a roundabout, you could lose the back end in the wet.

 

So, the what you do is turn off the overdrive. That drops the car into 3rd, which means you'll be higher up the rev range and hopefully have the 2nd turbo engaged in a controlled maner. When you pull out you instantly have all the power there at your disposal. No waiting for gear changes, boost increases etc.

 

Example 2

 

Imagine you're on some twisty roads and having some fun. In a manual you'd probably keep it in 3rd, only chaning down to 2nd for tight corners and only going into 4th+ for long straights.

 

What happens in the auto, you accellerate up to a corner, take your foot off to go round the corner and it changes up a gear. You put your foot down coming out of the corner and it changes down gear up, then up, then down, always changing between 3rd and O/D. Turning O/D off prevents this, it also give some engine breaking when entering the corners, and means you have more power there when coming out of them. If you reach a slow corner then it will still drop down into 2nd, just like you would in the manual.

 

Example 3

 

Touched upon above really. You're going down a steep hill. In a manual you could put it into 3rd to get lots of engine breaking to make the decent at a safer speed without over heating the breaks. In an auto you can turn off O/D to achieve the same thing.

 

Does that help?

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Licked document by Piran

But if you disable the overdrive (will be indicated by a light in the dash) then the car is allowed to rev further before the box changes up a gear.

I'm sorry but that's just wrong, unless someone can prove otherwise?
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Originally posted by Piran

I'm just quoting some Supra resource I found on the web somewhere

That's fair enough mate. :) I remember the last time this was discussed I went out and tested it and it's not true. I can hit red line in standard auto mode with O/D on, so the suggestion that O/D off allows higher revs is obviously wrong.
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Yeah, that's what I said above - I leave it on all the time and it still goes to the redline.

 

Cheers mawby, that's about the clearest explanation I've ever seen for the overdrive, and it clears up a few old wive's tales in the process.

 

One for the FAQ section methinks.

 

:thumbs:

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Maybe its my imagination but when i've got o/d off my car seems to change just before redline 1 -3 normally and around 5000rpm when o/d is on normally take car right up to redline in 3rd put the o/d on and can feel it kick into 4th when i'm really gunning it down a dual track however in normal town driving it doesn't seem to make any difference apart from not selecting 4th. But as I said it might be my imagination so will make a point of looking at rev counter next time I do it

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Originally posted by Ian R

Maybe its my imagination but when i've got o/d off my car seems to change just before redline 1 -3 normally and around 5000rpm when o/d is on normally take car right up to redline in 3rd put the o/d on and can feel it kick into 4th when i'm really gunning it down a dual track however in normal town driving it doesn't seem to make any difference apart from not selecting 4th. But as I said it might be my imagination so will make a point of looking at rev counter next time I do it

 

it's just how hard you press the accelerator. The harder you want to accelerate, the later it'll change up. If you're being gentle it'll change dead early.

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