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Gear Vendors - Over/Underdrive Transmission
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Post Gear Vendors - Over/Underdrive Transmission 
Anybody running or considering running one of these tranmission O/U units? I have been intrigued with these for years.



Last edited by MOChev on Sun Jan 14, 2007 4:16 am; edited 1 time in total
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I have one on the Tahoe. It's a 22% Overdrive only. With this I can have 4.56:1 gear in the rear end and still have good freeway RPM too.

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how high does yours rev? I'm spinning around 2500 at 70 and 3000 at 80 with the 4.10's and 31's on da crapbox

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James B. wrote:
I have one on the Tahoe. It's a 22% Overdrive only. With this I can have 4.56:1 gear in the rear end and still have good freeway RPM too.



JamesB - You stated a 22% OD only. Is this only a final OD, or does it split all of the gears also?

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Y2KLimited wrote:
how high does yours rev? I'm spinning around 2500 at 70 and 3000 at 80 with the 4.10's and 31's on da crapbox


hmmmm, and just think what my 4.10's and 26" tires do?

i might have to pick one of those up as well, maybe then i can get a one piece driveshaft.

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the GV doesnt split every gear. It doesnt go 1low-1HI, then 2 lo-2hi then 3lo the 3hi then 4lo-4hi.
the GV controller has NO PCM/TCM input so it doesnt know what gear the trans is in.

There is a footswitch and an auto control. The GV will switch from UD to OD at 20+?? mph in auto mode OR it will stay in UD until you "Throw the Switch".

If you want to split a 4l80e 4 speed auto into an actual 8 speed auto, then you need the Jet 4l80e 8 speed valvebody.

peace
Hog

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Hog wrote:
the GV doesnt split every gear. It doesnt go 1low-1HI, then 2 lo-2hi then 3lo the 3hi then 4lo-4hi.
the GV controller has NO PCM/TCM input so it doesnt know what gear the trans is in.

There is a footswitch and an auto control. The GV will switch from UD to OD at 20+?? mph in auto mode OR it will stay in UD until you "Throw the Switch".

If you want to split a 4l80e 4 speed auto into an actual 8 speed auto, then you need the Jet 4l80e 8 speed valvebody.

peace
Hog


I believe it is the jet 6-speed conversion. It puts a 1.86 gear in between 1-2 and puts another gear in between 3-4 (1.21?).

I'm not really sure how they accomplish it. Anybody know?

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The GV Overdrive can split every gear, but it's operation is not automatic. About the only time I ever use it in any of the lower gears is during long hill descents. I can regulate my speed using only the transmission and the overdrive because there are enough final drive ratios there to work with.

Think of it this way: switching it on and off is like switching the rear end ratio on the fly. I have 4.56:1 gears. When I turn on the OD it's like having 3.55:1 gears.
If you have 3.73:1 gears, turning on the OD is like switching to 2.91:1 gears. You can make this switch in any gear the transmission is in as long as the vehicle is moving faster than 22 mph.

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Stealth97 wrote:
I believe it is the jet 6-speed conversion. It puts a 1.86 gear in between 1-2 and puts another gear in between 3-4 (1.21?).

I'm not really sure how they accomplish it. Anybody know?

The Jet kit is 6-speed. They get it by turning the overdrive clutches on and off. The 4L80-E has 3 planetaries, not just two like the 4L60-E. In the 4L80-E, every single shift is a single clutch event - there are no simultaneous clutch engage and disengage events for any one shift. Jet installs a custom valvebody to overdrive first, then overdrive second in addition the the factory overdriven third. Total = 6.

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James B. wrote:
The GV Overdrive can split every gear, but it's operation is not automatic. About the only time I ever use it in any of the lower gears is during long hill descents. I can regulate my speed using only the transmission and the overdrive because there are enough final drive ratios there to work with.

Think of it this way: switching it on and off is like switching the rear end ratio on the fly. I have 4.56:1 gears. When I turn on the OD it's like having 3.55:1 gears.
If you have 3.73:1 gears, turning on the OD is like switching to 2.91:1 gears. You can make this switch in any gear the transmission is in as long as the vehicle is moving faster than 22 mph.


Do you know what happens if you try to switch the OD under 22mph? Or at lets say a stop?

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Y2KLimited wrote:
how high does yours rev? I'm spinning around 2500 at 70 and 3000 at 80 with the 4.10's and 31's on da crapbox

At 70 mph I get about 2600 RPM with the OD off. With the OD on, that becomes 2000.

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Stealth97 wrote:
James B. wrote:
....You can make this switch in any gear the transmission is in as long as the vehicle is moving faster than 22 mph.

Do you know what happens if you try to switch the OD under 22mph? Or at lets say a stop?

Yes, nothing happens. The electronics prevent it. The GV-OD has a hydraulic pump driven from its input shaft. The electronics ensure there is sufficient line pressure to activate.

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It CAN split every gear, but it would be almost impossible to make a drag run splitting every gear. is this correct about teh GV O/UD?

Thanks for getting me straight.

I could see them being more useful if they would work WITH the trans to give you an automatic way to run up through the gears.

peace
hog

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Hog wrote:
It CAN split every gear, but it would be almost impossible to make a drag run splitting every gear. is this correct about teh GV O/UD?

Thanks for getting me straight.

I could see them being more useful if they would work WITH the trans to give you an automatic way to run up through the gears.

peace
hog



I believe GV offers the abiltiy to shift up through the gears, utilizing their electronics.

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They have a new electronics package designed that integrates with the transmission harness. It is designed to split all gears automatically. This is not for everyday common use though, it's for towing or for racing. Line pressure in the OD is not modulated at all - it's full line pressure always. When it engages it engages hard. Engagement under light throttle is mostly absorbed by the torque converter but under heavy throttle it will give you quite a jolt. Because of this characteristic, it can not provide a seemless run up through the gears.
I was given the option to purchase this controller. Like all things from them it is very expensive. I choose not to. I use the overdrive for highway speeds over 60mph and am happy doing so.

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