CGT80
1/2 ton status
I have one on my 79 C20 with a built th400 and built gen I 406 sbc 10.2:1 aluminum heads, 264 roller cam, and all the other works, but built for CA smog
(estimated 400+hp 525tq) and tow a toy hauler that weighs up to 8500 lbs. I installed it in 2003 and after 100k+ miles lost reverse and exchanged it for a rebuilt unit, for $1,000.
It works well and when towing I can use all 6 gears. The rear is an open 14bff with 33" tires. 70 mph is 2400-2500 rpm. Fuel mileage only went up a little and driving nicely never helped mileage much, so I just beat on it with that kind of power. I average 8 mpg and doing all highway really doesn't help any. Towing, it averaged around 6 mpg and the trailer (2001 fleetwood wildernes toy hauler) sits high with the rear of the roof at 11.5', 21 foot trailer that is 23.5" ball to back.
The biggest thing was to get away from being at 3000+ rpm on the freeway and then when I got the trailer, being able to split gears. 2nd over is used a lot, but after going to the 406, I burned up 2nd gear. Now it has 4 clutch packs from a 4l80e, it already had 32 or 34 element sprag, shift kit, and all the other good stuff, along with a torque converter for an allison transmission (RV application). My cooler is also not big enough for the trans. My uncle owns a trans shop and actually builds th400 for one of the top major off road teams, as well as many other racers, but he had another guy who worked with him who normally did the trans builds. Somehow, despite me telling him to do any upgrade worth while, he didn't do the 2nd gear clutch upgrade and failed to mention that my cooler was an old design and not really up to par. I also had a line that was slightly kinked which may restrict flow. Now I have a guage on the trans pan, but have not done the cooler upgrade as I need to get a newer truck this year and will go with a duramax and tow with that.
The install is easy on the GV, but you do have to shim it to match the input shaft to the trans, per the directions. The tunnel had to be bashed in a bit on my relatively stock truck and the 2 piece shaft was shortened, but I would try a singe piece instead as the front shaft is very short and vibrates some. It was right at the cut off for going single vs. shortening the original.
I have never heard of a GV automatically shifting in and out as you go through the th400 gears, but it could be done if there is a way to setup electronics and figure out what gear the trans is in. The auto feature on the computer will shift into OD at about 40 mph or so and out at maybe 20. I never use it and instead shift with the floor switch. I will shift into OD as low as 30mph if I am not in a hurry and usually use it at 40 or above. Even if it is clicked early, it won't shift until it builds enough hydraulic pressure (about 25 mph). To shift out at speed, it is best to give it a little throttle to feather it, otherwise it will bang into direct drive. There is no problem shifting into OD at full throttle, even at 5000 rpm and 1st gear, if you wan't.
Sometimes the computer or solenoid can screw up and it will come out of OD on it's own. That would be really bad if you are at a very high rpm. I think I have only run it up to maybe 3800 rpm in 3rd over, so it would still probably be under the 6k redline if it popped out and I have only run it that fast once or twice, but when towing I'm not afraid to push it to 5k rpm.
The GV is great for older vehicles. I have been looking at 2017-2020 duramax trucks and the 2020 has a 10 speed allison auto and 910 ft lbs of torque. That is just crazy to think about vs. what has been available in the past. In the 2000's when the 5 and 6 speed auto transmissions showed up it did away with most of the late model market for GV. Their customer base is likely getting smaller as people move to later vehicles.
(estimated 400+hp 525tq) and tow a toy hauler that weighs up to 8500 lbs. I installed it in 2003 and after 100k+ miles lost reverse and exchanged it for a rebuilt unit, for $1,000.It works well and when towing I can use all 6 gears. The rear is an open 14bff with 33" tires. 70 mph is 2400-2500 rpm. Fuel mileage only went up a little and driving nicely never helped mileage much, so I just beat on it with that kind of power. I average 8 mpg and doing all highway really doesn't help any. Towing, it averaged around 6 mpg and the trailer (2001 fleetwood wildernes toy hauler) sits high with the rear of the roof at 11.5', 21 foot trailer that is 23.5" ball to back.
The biggest thing was to get away from being at 3000+ rpm on the freeway and then when I got the trailer, being able to split gears. 2nd over is used a lot, but after going to the 406, I burned up 2nd gear. Now it has 4 clutch packs from a 4l80e, it already had 32 or 34 element sprag, shift kit, and all the other good stuff, along with a torque converter for an allison transmission (RV application). My cooler is also not big enough for the trans. My uncle owns a trans shop and actually builds th400 for one of the top major off road teams, as well as many other racers, but he had another guy who worked with him who normally did the trans builds. Somehow, despite me telling him to do any upgrade worth while, he didn't do the 2nd gear clutch upgrade and failed to mention that my cooler was an old design and not really up to par. I also had a line that was slightly kinked which may restrict flow. Now I have a guage on the trans pan, but have not done the cooler upgrade as I need to get a newer truck this year and will go with a duramax and tow with that.
The install is easy on the GV, but you do have to shim it to match the input shaft to the trans, per the directions. The tunnel had to be bashed in a bit on my relatively stock truck and the 2 piece shaft was shortened, but I would try a singe piece instead as the front shaft is very short and vibrates some. It was right at the cut off for going single vs. shortening the original.
I have never heard of a GV automatically shifting in and out as you go through the th400 gears, but it could be done if there is a way to setup electronics and figure out what gear the trans is in. The auto feature on the computer will shift into OD at about 40 mph or so and out at maybe 20. I never use it and instead shift with the floor switch. I will shift into OD as low as 30mph if I am not in a hurry and usually use it at 40 or above. Even if it is clicked early, it won't shift until it builds enough hydraulic pressure (about 25 mph). To shift out at speed, it is best to give it a little throttle to feather it, otherwise it will bang into direct drive. There is no problem shifting into OD at full throttle, even at 5000 rpm and 1st gear, if you wan't.
Sometimes the computer or solenoid can screw up and it will come out of OD on it's own. That would be really bad if you are at a very high rpm. I think I have only run it up to maybe 3800 rpm in 3rd over, so it would still probably be under the 6k redline if it popped out and I have only run it that fast once or twice, but when towing I'm not afraid to push it to 5k rpm.
The GV is great for older vehicles. I have been looking at 2017-2020 duramax trucks and the 2020 has a 10 speed allison auto and 910 ft lbs of torque. That is just crazy to think about vs. what has been available in the past. In the 2000's when the 5 and 6 speed auto transmissions showed up it did away with most of the late model market for GV. Their customer base is likely getting smaller as people move to later vehicles.
