Author Topic: Picking the right trans (Read 2121 times)

tbro · « **on:** July 06, 2008, 06:07:56 pm »

I am replacing my code ACH five speed trans in my TD caddy for something higher geared (taller) for fuel economy. I have found an AGS code trans in good shape, but the three charts I have researched are conflicting on the final drive ratio. One says 3.67 and two say 3.94. Also as the 5th gear ratios are the same, but 4th is a little taller on the AGS, would I be gaining enough difference in overall economy with the AGS to make the switch worth it? As this would make a big difference with my freeway driving, I would like to know if anyone has the actual final drive ratio for the AGS that are accurate? In the near future I may be able to get an FN code five speed with a .71 5th and 3.89 final. Would you wait and use the FN over the AGS if you were trying to accomplish a good freeway/overall compromise or go with the AGS? Thanks for your ideas, Tom

burn_your_money · « **Reply #1 on:** July 06, 2008, 06:35:20 pm »

I have a AGS transmission and based on my speed and RPMs I would say it has a 3.94 final drive.

What percentage of time do you spend driving in 4th gear? That is really the only way to tell if it will be a worthwhile swap.

Quantum TD · « **Reply #2 on:** July 06, 2008, 10:52:39 pm »

AGS final drive is 3.94. ACH is essentially the same as an AGS, with the minor difference in 4th as you note.

Personally, I'd wait for the FN, unless my trans was toast. I think it's the best of both worlds, especially if you want good highway MPG, but don't want to drastically alter driveablility at low speed/take-offs.

I'd put a late FF, or an FO/FN in every car I own, if it would just fit into the damn MK2 or 3 bodies without major mods!

Trust me, I think you'll love the FN. I really don't know why VW got away from those gear ratios and final drives.

tbro · « **Reply #3 on:** July 07, 2008, 07:22:24 am »

Thanks guys for the comments! That is what I needed to know. As having a taller 4th is nice, since I use 5th most of the time (lots of freeway driving out West), I will wait and get the FN as suggested. I will keep the ACH and AGS as backups for my NA rabbit. Tom

ilikevwdiesel · « **Reply #4 on:** July 07, 2008, 07:25:11 am »

What you want is a 7A from an 84 Rabbit or Jetta. FN transmissions suck because they don't have the spring packs behind the CV flange. FN transmissions have a slightly taller final drive but the lack of springs behind the CV flanges makes it junk in my opinion. I believe you can take the .71 fifth gear from an FN and install it in your current trans as well.

Quantum TD · « **Reply #5 on:** July 07, 2008, 12:07:31 pm »

Quote from: "ilikevwdiesel"

What you want is a 7A from an 84 Rabbit or Jetta. FN transmissions suck because they don't have the spring packs behind the CV flange. FN transmissions have a slightly taller final drive but the lack of springs behind the CV flanges makes it junk in my opinion. I believe you can take the .71 fifth gear from an FN and install it in your current trans as well.

A 7A has exactly the same gearing as an AGS, and is similar to an ACH. What then would be the gains at highway speed?

The FN has a lower final drive, that lowers RPMs, but does not make take-offs terrible. It also has the .71 5th, which is great for highway cruising. With a small (presumably stock) 1.6 turbo, then it's a nice match.

Overall, I'd expect to see fuel gains in the 1-4 MPG range, with some definite RPM drops that will help with the life of the motor.

If you're concerned about the "limited slip" aspect of the 7A, I don't imagine that there is too much gained with the springs. Especially since most used transmissions have really tired tension springs. If you want a "perfect trans" for any application, you'll have to build a trans yourself with a Quaife/Peloquin and the ideal gearsets. For a quick swap and go, this might be the best solution.