Many many years ago when I was a young man I knew a old school hot rodder who was working on a car that had a rear differental that had an 'overdrive' unit in it. You could switch it from regular to overdrive.
Then I learned of a bolt on unit for the back of the transmission called a 'gear vendors' overdrive. Same basic concept, you change the gearing after the transmisson giving you some options.
I wanted one bad. Really bad. But they are expensive and while basically 'bolt on' they require some work to install. The other option is to go with an OD transmission. There are pros and cons to both and this post isn't meant to start a discussion on which is better. By now I already had a nicely built C6 with the extra clutch packs, shift kit and strong performance. I also had a weird carb setup that would have made the TV cable for an AOD even more weird. So set in my mind I kept an eye out for a GV overdrive.
In the second half of 2013 while poking around on craigslist a GV with a c6 tail housing showed up. The price was good so I picked it up. It was bare bones with just the unit, the housing and an older electronics setup. It lacked a yoke, gaskets, speedo cable and more modern electronics. So I tried a test fit. The tail housing and unit all fit inside the tunnel and what I call the 'torque plate'. But the exhaust was going to have to be rerouted since it runs inside the tunnel and above the 'torque plate'.
For you non-convertible guys there is some additional support on convertibles and this includes a big, thick plate. It basically resides just past the stock tail shaft and where the GV unit would end up so there was concern it would all fit.
I bolted it all in and it all cleared.
So it was decision time. Do I make what I have so far work, search out parts, make it happen or just buy one new and 'get it done'. I decided on the latter.
So below are some pics and some thoughts. Bottom lin:. If you have a convertible the GV overdrive will fit. You cannot run the exhaust above the 'torque plate' but otherwise there are no issues. It's an awesome upgrade that I am very happy with.
I run a 351c, hooker comp headers and a c6 that isn't stock (mine originally came with an FMX).
Here is a picture of the used unit I picked up:
It was in good shape and moved freely. Time for a test fit. To do this I had to pull the stock tail shaft off and to do that I had to drain the fluid. So drop the pan first and then pull the tailshaft. My first attempt at jacking up the car was from the rear.
While it seemed like a good idea I really didn't get much more clearance.
Here is a pretty good shot of what I call the 'torque plate' unique to the convertibles. You can see the exhaust it run above it and the tail shaft of the transmission ends just before it.
This is the GV tail housing that you put in place of the stock one. Note that there are transmission mounts.
And the unit itself:
To connect the stock output shaft to the GV unit there is an adapter.
here is a shot of it on the gv unit
You remove the tailshaft:
I was happy with the 'stuff' in my transmission pan. A little bit of material but overall nothing to be worried about.
Here are some pics of how it cleared in the tunnel:
So my next decision, do I make the used one work or just order a new one? These things are not cheap but I decided since I now knew it would fit that I would order a new one and sell the one I bought used. The new one had the modern electronics, all of the speedo stuff and my wife could just drive the car without messing with the OD.
What I learned is GV had gone from basically building adapters and electronics that would work with a Laycock over drive to building their own upgraded units.
The GV built units are more stout. Here is a side by side comparison of the two:
The GV has a larger output shaft.
Another nice feature of the later GV unit is it has a drain plug:
And here you can see it had a deeper oil 'pan' and a fill plug. You can also get a deep sump pan but I don't think it would clear the convertible torque plate.
Another side by side shot of the old GV and new GV units
The install was pretty straight forward. I was a little concerned about how the speedo cable fit into the adapter. You replace an O ring and the push it in. I don't have pics but I was surprised at how tight it fit.
The electronic hook ups are pretty easy. I used the supplied foot switch.
Operation is also easy once you know what to do. There is an 'auto mode' switch that is on or off. And a green/red indicator light.
They act differently depending on the switch setting.
If the switch is set to 'auto mode':
The red light means it is NOT in OD but will switch to OD at about 40mph. When it goes into OD the red light goes out and the green light goes on. When you get below about 20mph the OD switchs off and the light goes from green to red. If you hit the foot switch the unit goes out of OD (or stays out) and neither light is on.
If the switch is set to 'manual' mode the unit and lights act different.
If you hit the foot switch and you are stopped or not going fast enough for the unit to enguage the red light comes on and when you are at a speed
You stopped here to confirm the light action.
Then I learned of a bolt on unit for the back of the transmission called a 'gear vendors' overdrive. Same basic concept, you change the gearing after the transmisson giving you some options.
I wanted one bad. Really bad. But they are expensive and while basically 'bolt on' they require some work to install. The other option is to go with an OD transmission. There are pros and cons to both and this post isn't meant to start a discussion on which is better. By now I already had a nicely built C6 with the extra clutch packs, shift kit and strong performance. I also had a weird carb setup that would have made the TV cable for an AOD even more weird. So set in my mind I kept an eye out for a GV overdrive.
In the second half of 2013 while poking around on craigslist a GV with a c6 tail housing showed up. The price was good so I picked it up. It was bare bones with just the unit, the housing and an older electronics setup. It lacked a yoke, gaskets, speedo cable and more modern electronics. So I tried a test fit. The tail housing and unit all fit inside the tunnel and what I call the 'torque plate'. But the exhaust was going to have to be rerouted since it runs inside the tunnel and above the 'torque plate'.
For you non-convertible guys there is some additional support on convertibles and this includes a big, thick plate. It basically resides just past the stock tail shaft and where the GV unit would end up so there was concern it would all fit.
I bolted it all in and it all cleared.
So it was decision time. Do I make what I have so far work, search out parts, make it happen or just buy one new and 'get it done'. I decided on the latter.
So below are some pics and some thoughts. Bottom lin:. If you have a convertible the GV overdrive will fit. You cannot run the exhaust above the 'torque plate' but otherwise there are no issues. It's an awesome upgrade that I am very happy with.
I run a 351c, hooker comp headers and a c6 that isn't stock (mine originally came with an FMX).
Here is a picture of the used unit I picked up:
It was in good shape and moved freely. Time for a test fit. To do this I had to pull the stock tail shaft off and to do that I had to drain the fluid. So drop the pan first and then pull the tailshaft. My first attempt at jacking up the car was from the rear.
While it seemed like a good idea I really didn't get much more clearance.
Here is a pretty good shot of what I call the 'torque plate' unique to the convertibles. You can see the exhaust it run above it and the tail shaft of the transmission ends just before it.
This is the GV tail housing that you put in place of the stock one. Note that there are transmission mounts.
And the unit itself:
To connect the stock output shaft to the GV unit there is an adapter.
here is a shot of it on the gv unit
You remove the tailshaft:
I was happy with the 'stuff' in my transmission pan. A little bit of material but overall nothing to be worried about.
Here are some pics of how it cleared in the tunnel:
So my next decision, do I make the used one work or just order a new one? These things are not cheap but I decided since I now knew it would fit that I would order a new one and sell the one I bought used. The new one had the modern electronics, all of the speedo stuff and my wife could just drive the car without messing with the OD.
What I learned is GV had gone from basically building adapters and electronics that would work with a Laycock over drive to building their own upgraded units.
The GV built units are more stout. Here is a side by side comparison of the two:
The GV has a larger output shaft.
Another nice feature of the later GV unit is it has a drain plug:
And here you can see it had a deeper oil 'pan' and a fill plug. You can also get a deep sump pan but I don't think it would clear the convertible torque plate.
Another side by side shot of the old GV and new GV units
The install was pretty straight forward. I was a little concerned about how the speedo cable fit into the adapter. You replace an O ring and the push it in. I don't have pics but I was surprised at how tight it fit.
The electronic hook ups are pretty easy. I used the supplied foot switch.
Operation is also easy once you know what to do. There is an 'auto mode' switch that is on or off. And a green/red indicator light.
They act differently depending on the switch setting.
If the switch is set to 'auto mode':
The red light means it is NOT in OD but will switch to OD at about 40mph. When it goes into OD the red light goes out and the green light goes on. When you get below about 20mph the OD switchs off and the light goes from green to red. If you hit the foot switch the unit goes out of OD (or stays out) and neither light is on.
If the switch is set to 'manual' mode the unit and lights act different.
If you hit the foot switch and you are stopped or not going fast enough for the unit to enguage the red light comes on and when you are at a speed
You stopped here to confirm the light action.
