o/d tranny

[burly94gt]

just 2 simple questions...what does o/d do, and what is it for?

[stang_racer20]

Not quite sure how to answer. Its 4th gear in your tranny. Overdrive

[GTStang]

O/D stands for overdrive. It's not really an extra gear. It's a clamp with brake like material which locks the primary drive hub, which then do to they way planetary gears work changes the gear ratio of the tranny.

That is how most O/D's work but then there are OD's like the 4+3 etc..,

[Mercracer]

Originally Posted by GTStang

O/D stands for overdrive. It's not really an extra gear.

Actually, it is an extra gear. Most if not all automatic overdrive transmissions utilize an additional planetary gearset for overdrive. For example.....Ford AOD, A4LD, E4OD all utilize additional rotating gears in one configuration or another.
In the example of the AOD, a modification for drag racing is to remove the OD specific gearset for less rotating mass and quicker acceleration.
You are correct in the fact that when overdrive is selected, a band holds the drum in place, and with the other corresponding clutches applied, due to the planetary gearset drives the output shaft at a ratio greater than the normal 1:1 of a conventional high gear. For an example, the AOD overdrive is at approximately a .67:1 ratio. Meaning that for .67 revolutions of the engine, the driveshaft will rotate 1 revolution. 1985-1993 T-5 standard transmissions operate overdrive at a similar ratio.

[GTStang]

It's not an extra gear in an AOD... if it was truly an extra gear there would be a 4th drum.

[68chevelle]

O/D stands for overdrive. It's not really an extra gear. It's a clamp with brake like material which locks the primary drive hub, which then do to they way planetary gears work changes the gear ratio of the tranny.

what your describing seems more like a TCC for an auto trans, atleast to me,(torque converter lockup). overdrive is what it actualy implys. like mercracer stated. now i dont know ford nearly as well as chevys but it should all be the same. im going to go off of chevys and all that i know cuz it should be the same otherwise it should have a different name. overdrive is an actual gear. its either your last 1 or 2 gears. 4 speed autos its the 4th gear. 5 speed mauals that have overdrive its 5. and 6 speed overdrives its 6 but then there is also double overdrives sutch as a T56, both 5th and 6th are two different overdrive gears. your gears in your trnas will be something like (example only, im using actual chevy 700R4 trans numbers) 3.06 first, 1.63 second, 1.00 third, and .70 fourth. the 4th is the overdrive because your crank is turning less than a 1:1 ratio from your driveshaft. thats what makes it an overdrive. its spins the driveshaft faster than the crank. in this case it would be a 30% overdrive. it means that the driveshaft is spining 30% faster than the crank. saving you gas and engine wear. you accelorate slower because the force you are working agenst is much greater, useing a ratio agents you as oposed to help the car move easier. the overdrive was just designed to help economy. you should not need it when racing because then you would be going so fast that you probably wouldnt be racing and it pulls alot less than you would want. the overdrive concept should be the same for all transmissions. its always another gear that lowers the RPM's below the crank speed. now the TCC does lower the RPM's a couple hundred all by itself but its not an overdrive and never is concidered the overdrive to my knowledge.

GTstang, if it is something different please show me some text somewhere or something explaning it.

[GTStang]

Originally Posted by 68chevelle

what your describing seems more like a TCC for an auto trans, atleast to me,(torque converter lockup). overdrive is what it actualy implys. like mercracer stated. now i dont know ford nearly as well as chevys but it should all be the same. im going to go off of chevys and all that i know cuz it should be the same otherwise it should have a different name. overdrive is an actual gear. its either your last 1 or 2 gears. 4 speed autos its the 4th gear. 5 speed mauals that have overdrive its 5. and 6 speed overdrives its 6 but then there is also double overdrives sutch as a T56, both 5th and 6th are two different overdrive gears. your gears in your trnas will be something like (example only, im using actual chevy 700R4 trans numbers) 3.06 first, 1.63 second, 1.00 third, and .70 fourth. the 4th is the overdrive because your crank is turning less than a 1:1 ratio from your driveshaft. thats what makes it an overdrive. its spins the driveshaft faster than the crank. in this case it would be a 30% overdrive. it means that the driveshaft is spining 30% faster than the crank. saving you gas and engine wear. you accelorate slower because the force you are working agenst is much greater, useing a ration agents you as oposed to help the car move easier. the overdrive was just designed to help economy. you should not need it when racing or anything because then you would be going so fast that you probably wouldnt be racing and it pulls alot less than you would want. overdrive should be the same for all transmissions. its always another gear that lowers the RPM's below the crank speed. now the TCC does lower the RPM's a couple hundred all by itself but its not an overdrive and never is concidered the overdrive to my knowledge.

GTstang, if it is something different please show me some text somewhere or something explaning it.

Ok here is where the mention of the 4+3 tranny comes into play. A T-56 is not an OD manual. Althought the 5 and 6th gear are low numeric ratios it is not an OD it's just a tranny with 2 extra gears that are a low numeric gear ratio.

The Doug Nash 4+3 used in mid 80's Vettes is a true OD manual tranny. Here is how it works you have a 4 speed tranny. There is an OD unit which is engaged and disengaged by a switch. This unit can be engaged for 2nd, 3rd, and 4th gear lowering the number value of each. These are made up numbers but if 2nd= 2.6 3rd= 1.9 4th= 1.2 with the OD unit engaged 2nd=2.4 3rd=1.6 4th=.8. Now do you have a transmission with 7 gears.... no you have a tranny with 4 gears and a OD unit which can reduce the last 3 gears ratios.

I hope this explians to you why....
#1 Wether or not it is an OD, is not determined by what numeric gear ratio it is.
#2 That an OD is not an extra gear.

[68chevelle]

LOL. wow i dont know how you thought of that, it was kinda confusing to me but anyways. that is just like a gear vendor overdrive unit.but anyways here read this it should help. also im not saying your wrong but im not either. and yes the T56 is an overdrive, that part your are mistaken about.
this is what i ment by showing me some text. this was take straight from a mustang magazines websight. i did a simple search for a mustang magazine ane then did a search on there sight for overdrives.

" The automatic overdrive (AOD) transmission first appeared in new Fords and Mercs in 1980 and has steadily improved since. It’s a four-speed automatic transmission where Fourth gear is Overdrive—actually the reverse of what your engine experiences in gears 1 and 2 in conventional automatics."

here the link if you would like to read even more. it explanes more. http://www.mustangmonthly.com/howto/33598/index.html

edit: and the doug nash 4+3 is not activated by a switch. it is a 4 speed maual with a automatic overdrive. it is probably controled similarly to a automatic trans detent/kickdown/T.V. cable. it disengages the overdrive unit an a certain % throtle. i think you are canfusing an overdrive and a torque converter lockup or something and maybe combining the two. because many TCC untis have a switch that when activated locks up the torque converter in 2nd and 3rd gear as oposed to only automaticaly in 4th, and it is primaraly used for a towing only situation, and like i stated above when the lockup is ingaged it does actualy lower the RPM's by a couple hudred RPM's. on a chevy car or truck the TCC is tied into the brake light switch and if you are driving along in 4th gear just crusing and the torque converter is locked up you can bearly touch the brake pedal so that the lights would come on but not apply the brakes and you can watch the tach or listen and the RPM's should come up a few hundred.

[351wStang]

Originally Posted by GTStang

Ok here is where the mention of the 4+3 tranny comes into play. A T-56 is not an OD manual. Althought the 5 and 6th gear are low numeric ratios it is not an OD it's just a tranny with 2 extra gears that are a low numeric gear ratio.

I disagree, Look at a T5; You have Gear Reduction (1-3)
Direct Drive (4), and Overdrive (5). You have a smaller gear turning a larger gear in underdrive or gear reduction ex. 3:1. You have two gears of equal size (gear on input shaft turning gear on output shaft) for direct drive 1:1. You have a larger gear turning a smaller gear in overdrive .6:1.

Overdrive is any time the driveshaft makes more revolutions than the crankshaft.

[Mercracer]

Originally Posted by GTStang

It's not an extra gear in an AOD... if it was truly an extra gear there would be a 4th drum.

Dude, you are clearly hung up on the bands (and drums) and obviously ignoring the planet gear arrangement as a whole. Have you had both a C-4 and a AOD apart? Do you at least have a shop manual detailing the geartrain? Do yourself a favor and check it out. The C-6 3 speed transmission has only 1 drum (intermediate) being held by a band.

[Mercracer]

Originally Posted by GTStang

#1 Wether or not it is an OD, is not determined by what numeric gear ratio it is.
#2 That an OD is not an extra gear.

Overdrive is defined by nothing more or less than the numerical gear ratio. Having a gear reduction unit in addition to the internal transmission gears does not exclusively define an overdrive.
Overdrive is indeed an extra gear in both the T-5 and T-56 Manual transmissions in addition to other manual and automatic transmissions.
The T-56 actually has 2 overdrive gears.
The Richmond Gear/Doug Nash 5 speed is an example of a transmission which has steep 1st and second gears with a direct drive (1:1) 5th gear. No Overdrive there.

[redhorseV6]

overdrive is for increasing fuel economy. to answer your other question.