There's something extremely satisfying about blasting away from a light like a bat out of hell and then dropping down to sixth and cruising the freeway with the engine loping along at 1700rpm. 😎

I'm in the midst of a major engine/driveline overhaul. Torquey LS3 build; should be close to 500 wheel -- coming up from basically stock LS1 / 325rwhp. The car is street driven and sees the road course (HPDE) once or twice a year. I'll probably hit the drag strip with the new build just to shake it down and see what it can do, but that's not my thing and not at all a priority. I like being quick of course, but I don't see how that's going to be a problem. With this much more power, I'm more worried about having more power than traction, especially on street tires. One the road course, it's always nice to have enough power in reserve that you don't need to downshift, and nice wide gear spacing contributes to that.

I was pretty sure the LS3 would kill the 10-bolt, so as part of this project I bought a used Moser 12-bolt. It came with 4.10 gears in it, and I'm pretty sure that once I've had some time with that, I'll be interested in dialing back the gearing a bit.

My T56 was built up many years ago by Joe Huneycutt @ T56Rebuilds, which later became SixSpeedsInc, which later shut down. It's been a long time, and my new engine build is significantly increasing the power it'll be seeing. Still within the 700 lbft Joe said it'd be good for, but it wouldn't be a bad thing to have it gone over again.

So, with all that for context: if I send the T56 out for a refresh, I've low-key always wanted the shorter-ratio 1-3 gears from the GTO/C5Z/93F T56, with the 2.97 first gear. However, all of the cars that ever came with the shorter 1-3 also came with shorter 5-6, and I don't want that.

The idea here is to get the advantage of shorter gears for acceleration (an 11% difference in first gear -- similar to the 9% difference you get by going from a 3.42 to a 3.73 rear), while not sacrificing the low-RPM cruise capability of the fabulous 0.50 sixth gear. This would also give me a bit more spacing between 3rd and 4th gear, which means fewer times when I need to grab 2nd on the road course. There is even the 3.36 first gear that you got in a '93 with the 2.73 rear ratio. Compared to my 2.66 first, that would give 4.30-like gearing off the line with a 3.42 rear gear (the rest of the ratios are the same as the 2.97 first). Then I'd probably swap out my 4.10 gears for something in the 3.31-3.55 range.

Is it possible to swap out the 1-3 gearset while leaving the 5-6 alone? (4 is 1:1 no matter what, right?) What would be the cost/complexity of such a changeover? Are there downsides (e.g. notable loss of strength)?

Budget permitting, I could just purchase a T56 Magnum with whatever set of gears I want. It seems like Liberty's Gears sells a 2.97/0.50 Magnum set up for F-body. Can't seem to find anything with the 3.36.

To illustrate, here's what I had before the build (graphs built using the Speed and Gearing calculator I built on my website). This gearing was actually pretty good on the road course. I topped out around 130mph at my favorite tracks, and so I was running right up near the top of 4th gear. With the extra power, I'm thinking I might be gaining more than 10mph, which would mean I might need to grab fifth. Not ideal!


Here's what I'll have when the parts I have today are all put together. In terms of top speed on the road course, this is worse. Have to shift into 5th at 123mph -- that's practically guaranteed on every lap. I'd rather not! Plus, at least before the engine build w/ headers, I had a bit of exhaust drone around 2000rpm, so 77mph (my typical highway cruising speed) @ 2068rpm might be a bit unpleasant.


Here's what I think would be awesome. Having 4th gear go all the way out to 150+mph will be really beneficial on the road course -- no need to grab 5th on the straight! First through third gears get slightly more aggressive than stock, which should be fine paired with the extra power. And 77mph cruise RPM gets slightly lower than stock! This feels like the best possible compromise. I might play with this a bit -- I'm not totally set on the 3.31 gears. Maybe go back to 3.42? Those seem to be the only ratio options in this ballpark for a 12-bolt. I've seen 3.35 and 3.40 for some diffs, but not currently aware of any such ratios compatible with my 12-bolt.

 

I realize now that the 3.36 first gear might (probably does?) impart a much lower maximum input torque on the trans overall. I also don't see any evidence of the 3.36 ratio being an option on the TR6060, which probably means it's not available on a Magnum either.

Still, it would be cool. 😎

 

3-4 is closer in the 2.66 units vs. the 2.97 units.
Run your existing set up. If you're overrevving 4th, you'll know, and can justify 3.31. That's probably the direction you'll need to go.

The 2.97 unit has wider ratio drops, but evenly spaced 1-4. 2
66 units have closer gearing, but progressively closer ratio drops.

You're enjoying the OD. Maybe 6th will be a little less useful with 3.31. But leaving gearing alone.and having taller track tires might be everything that needs.chamged.
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