My "new" '74 Duster- or why I need a project like a hole in the head

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Ok, so for several years now I've been talking about a T56 swap. And for the last few years I've had a T56 Magnum collecting dust in my shop while I used my Duster as my daily driver. And I collected all the parts I thought I would need, and then a few more, so it would be a pretty smooth deal. Of course it wasn't, but that's how it works. Anyway, I did the swap a few months and about 1,300 miles ago and just haven't gotten around to writing any of it up.

Before I get started, I'd like to thank @Tjfly4pay at ToddRonn Restorations, his crossmember was great and one of the reasons I finally decided to just get this thing done. And @goldduster318 for some of the technical advice once I got rolling with the swap and hit a few snags, because there are always snags. And my buddy Bryce, who actually installed the crossmember in his car first and gave me a hand with the install on mine.

So here we go. I started with my '74 Duster, previously converted from an auto to an 833 4-speed. I covered everything with FatMat when I did that, which was fun to remove.
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I only attached the tunnel with sheet metal screws though, because I wanted access. So this went quick
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This part, not so much. Lots of heat gun and scraping.
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And then I drilled the spot welds to get at the original crossmember.
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From there the easy part of the build, installing the ToddRonn crossmember. A test fit, followed by some welding
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In this picture you can see the tube spacer I added, the version I had of the A-body crossmember didn't have them but the E-body version I also have did. So I added the tubes to keep anything from crushing when I tightened the bolts. Probably overkill, but I did it anyway.
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Now for the modifications to fit the T56 in what's left of the original tunnel. The pinch weld gets folded flat, and I mean FLAT. I trimmed a couple sections so it would fold completely flush

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And then some real progress, an actual transmission test fit
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Which of course showed more problem areas to fix, like I want to be able to get to this bolt with the tunnel in
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So I made some more cuts to flange the tunnel up a little further
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At some point I dialed in the bellhousing. This was a total pain, lots of learning experience here on my part. Let's just say that removing the original bellhousing studs while the engine is in the car is no fun AT ALL. After some trial and error, a couple of ruined pins, and a day or two of delay waiting on more pins I got it aligned with a set of .014" offset pins.

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On a T56 Magnum there's a bracket that sticks out on the side of the housing that doesn't get used for anything. On my initial install it hung up in the crossmember, requiring some angling of the trans and some colorful language. Before the transmission went back in I removed it.

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Taa-daa! With it gone the transmission just rolls straight in while at the proper height. Much easier!
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With that done it was time to finish up the tunnel fabrication. I did this a little differently than I've seen on other installations, I used most of the original tunnel. I sectioned it up into about three pieces per side and bent the original tunnel back. Yeah, it was kind of in the way up until this point, but rather than roll a new tunnel, or buy one, I just reshaped the original tunnel by making some relief cuts.

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Yeah, I kept the original sections I cut out for the 4 speed conversion too from a few years back. Which probably explains why my shop is so cluttered. Found them though didn't I?
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So just a section down the middle to add, plus some new metal at the tail
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Fabrication on the tunnel finished. The section behind the shifter comes off so the transmission can slide straight back without hitting anything. Plus it gives access to some of the electronic connections in case I ever need to get at them.
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72bluNblu said:
Plus it gives access to some of the electronic connections in case I ever need to get at them.
Click to expand...
nice thing about working on these long enough, is knowing to plan ahead for the things you hope you won't need to do
 
I shortened the 71-74 B-body shifter by about 1.5". I just drilled new holes and cut the bottom of the stick off. The body plugs allow access to the electronic connections and my O2 sensor in the back and the ports for the hydraulic clutch at the front. There's one on the other side for that tight clearance bell to transmission bolt too, so everything is easy to access to drop the transmission.
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This is the final arrangement for my hydraulic clutch lines. This took some trial and error, the first couple of iterations had interference issues. The throw out is from a C5 corvette, NOT a Viper like I had previously planned. And purchased. This is important, because the Viper and the T56 magnum do not have the same diameter input shaft. The Viper slave will slide over the input, but it DOES NOT have enough clearance. As in, less than the free play in the input. I narrowly avoided disaster on this one, and it did involve removing the transmission more than once. The C5 slave required a 1/2" and a .156" spacer to set the slave where I wanted it, which also changed the clearance on the lines. I have a remote bleeder installed as well, not gonna do that at the bellhousing. So this took a few days, involved a few different waits on AN lines and fittings with different angles, etc.
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It still required some clearance on the quicktime bellhousing
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Which got me to "test and tune", everything installed except the carpet. (there's that other body plug!). And yes, those body plugs made the multiple installs and removals MUCH easier. That hydraulic set up gave me fits.
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From the bottom. I had to make some modifications to my exhaust routing to clear the crossmember. That's not something everyone might encounter, but because I had mine tucked right up against the floor previously I had to move it around a little to clear the T56 crossmember. Also, my brand new 3.5" driveshaft with 1350 joints.
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I installed the clutch master on the firewall, much like 72BBswinger did on his car. I made a bracket that fit into the strengthening rib on the firewall. This was early, I tack welded the bolts into the bracket so it was easier to install.
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Obviously that's not everything. But that's most of what I took pictures of. There was a reverse module from SilverSport transmissions that I wired up and connected to the speed sensor. The T56 has a spring in it with like a 60lb pressure to keep you from shifting into reverse when you mean to downshift into 5th (good idea right?). There's a module that overides that spring so you can shift easily which goes to the ECU on the newer applications. On a classic some places call for that to be wired to the brakes. Which kinda works, unless you're braking and downshifting at the same time. So I spent the money on the SilverSport module, which uses the VSS on the T56 to override the spring only when you're going less than 5mph. But wiring is boring, so, no pictures. Also I didn't need the VSS. That was one of the considerations buying the T56 Magnum vs using a viper or other T56. The Magnum has both a VSS output and a cable drive for a standard speedometer, so I could still use my 150 mph rallye speedo. :D

And then there's this. The final list. Everything I used along the way. No, not the cheapest way to do it, but that's what I did.
T56 Swap List:

American powertrain T56 Magnum TUET-11009: 3,050 (+$109 freight)
American Powertrain Tremec T-56 Magnum Manual Transmissions TUET-11009

Quicktime Bellhousing #8074: 684.95
QuickTime Bellhousings RM-8074

Summit Racing® Bellhousing Dowel Pins .014 27.99
Summit Racing® Bellhousing Dowel Pins SUM-192007

ARP Bellhousing Bolt Kits 144-0902 18.99
ARP Bellhousing Bolt Kits 144-0902

ARP Chromoly Bolts 672-1004 (T56 to bell) 15.99 x2
https://www.summitracing.com/parts/arp-672-1004

McLeod 130-tooth flywheel #464102 313.36
http://www.summitracing.com/parts/mcl-464102

McLeod B&B pressure plate #360153 193.01
http://www.summitracing.com/parts/mcl-360153

McLeod 26-spline clutch disk #260570: 216.32
http://www.summitracing.com/parts/mcl-260570/overview/

Ram slave cylinder 525 (C5 corvette) 211.99
https://www.summitracing.com/parts/ram-525

Wilwood 7/8” master cylinder #260-3376: 69.93
http://www.summitracing.com/parts/wil-260-3376

Hydraulic clutch line (MC to slave) 16.98
http://www.summitracing.com/parts/aer-fbpa0049-36

Fragola Adapters 650402 3/8-24 to -4 AN (MC) 8.99
https://www.summitracing.com/parts/fra-650402

McLeod Adapter Fittings 139027 (slave) 41.60
https://www.summitracing.com/parts/mcl-139027

Hydraulic clutch line FBPA0039-36 (remote bleeder line) 14.95
http://www.summitracing.com/parts/aer-fbpa0039-36

Fragola Brake Adapters 650308 10x1.5 to -3 AN (slave) 4.99
https://www.summitracing.com/parts/fra-650308

Russell Brake Adapter 641291 -3 AN to 3/8 24 inverted (bleeder) 7.67
https://www.summitracing.com/parts/rus-641291

Strange Transmission Yokes U1659 80.99
https://www.summitracing.com/parts/str-u1659

3.5” x.083 steel driveshaft, 1350 joints, 45 3/8” 293.85

RAM Slave Cylinder Spacer 78511 77.97
http://www.summitracing.com/parts/ram-78511

RAM Slave Cylinder Spacer 598: 17.99
http://www.summitracing.com/parts/ram-598/overview/

Prothane Transmission mount 42.57
https://www.summitracing.com/parts/ptp-6-1605-bl

Neutral Safety Switch Connector 7.83
http://www.summitracing.com/parts/rfw-pg-097

Reverse Lockout Connector 7.83
http://www.summitracing.com/parts/rfw-pg-098

Hurst Speedometer Cable with gear M-243-69-RED 95.00
Hurst Shifters - Automatic, Manual, Racing Shifters, Hurst Exhaust, Hurst Wheels, Shifter Handles and Knobs

Advance adaptors 716156GM Pilot bushing adaptor(1.818 x .590) 30.99
https://www.summitracing.com/parts/add-716156gm

SilverSport Transmissions T56 reverse module w/ speed sensor 119.90
Tremec 6-Speed Reverse Lockout Control Module

Synchromesh Transmission fluid #88900333 17.97x4
http://www.summitracing.com/parts/nal-88900333

ToddRonn Restorations T56 Magnum A-body Crossmember 750

Total: 6,619.52
 
i know you spent a long time planning and collecting, but, easier, harder or about what you expected?
 
kursplat said:
i know you spent a long time planning and collecting, but, easier, harder or about what you expected?
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Kinda depends. The fabrication part of it was easier than I thought it would be. Some of that was definitely the ToddRonn crossmember, which was great. The tunnel wasn't that bad at all. Dial indicating the bellhousing took longer than I thought, and that would have been a lot easier and a ton faster had I not been working under the car at the time.

The hydraulic clutch part of it was harder than I thought. I lost a lot of time to that too. Part of that was I did all my test fitting with the line routing for the bellhousing with the viper slave. So when I had to switch to the corvette slave everything was in the car. I probably should have just pulled the bell again at that point and set it up off the car, but I didn't, so that took some trial and error and every error meant trying to reinstall the transmission only to find an interference issue. So that part wasn't "hard", but it was super frustrating. Especially knowing I was that close to the end. The spacers needed for the slave made the difference, spacing it out further because of the shorter overall length of the corvette slave moved the lines closer to the bell and required the re-route.

So it was kinda funny, I had been holding off on doing the swap because of the amount of fabrication it needed. But in the end the fabrication was probably the most straightforward part. The mechanics of the hydraulic clutch conversion was the worst part of the whole deal by far. And some of that was just the parts I chose, guys had used the viper slave before but they'd used them with Viper transmissions, and I didn't realize how different the T56 magnum was from the Viper version. And then I couldn't just buy an aftermarket slave, because I also used a Borg & Beck pressure plate which has a smaller inner diameter compared to the diaphragm clutches, so there was a clearance issue with some of the aftermarket slaves.

It was totally worth it though. I swapped over to 4.30 gears from 3.55's, which works a ton better with my cam. And I still get to cruise the freeway at 1,000 rpm less than before. The shifting is a ton easier and faster too, much shorter throws on the shifts.

As it sits now. I reused my carpet, so, yeah, there's an extra hole. I also made a set of console brackets for when I install my A-body console. I did the test fit already, but I have a little refinishing to do on the console still. The hole will be under the console once it's installed.

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72bluNblu said:
Kinda depends. The fabrication part of it was easier than I thought it would be. Some of that was definitely the ToddRonn crossmember, which was great. The tunnel wasn't that bad at all. Dial indicating the bellhousing took longer than I thought, and that would have been a lot easier and a ton faster had I not been working under the car at the time.

The hydraulic clutch part of it was harder than I thought. I lost a lot of time to that too. Part of that was I did all my test fitting with the line routing for the bellhousing with the viper slave. So when I had to switch to the corvette slave everything was in the car. I probably should have just pulled the bell again at that point and set it up off the car, but I didn't, so that took some trial and error and every error meant trying to reinstall the transmission only to find an interference issue. So that part wasn't "hard", but it was super frustrating. Especially knowing I was that close to the end. The spacers needed for the slave made the difference, spacing it out further because of the shorter overall length of the corvette slave moved the lines closer to the bell and required the re-route.

So it was kinda funny, I had been holding off on doing the swap because of the amount of fabrication it needed. But in the end the fabrication was probably the most straightforward part. The mechanics of the hydraulic clutch conversion was the worst part of the whole deal by far. And some of that was just the parts I chose, guys had used the viper slave before but they'd used them with Viper transmissions, and I didn't realize how different the T56 magnum was from the Viper version. And then I couldn't just buy an aftermarket slave, because I also used a Borg & Beck pressure plate which has a smaller inner diameter compared to the diaphragm clutches, so there was a clearance issue with some of the aftermarket slaves.

It was totally worth it though. I swapped over to 4.30 gears from 3.55's, which works a ton better with my cam. And I still get to cruise the freeway at 1,000 rpm less than before. The shifting is a ton easier and faster too, much shorter throws on the shifts.

As it sits now. I reused my carpet, so, yeah, there's an extra hole. I also made a set of console brackets for when I install my A-body console. I did the test fit already, but I have a little refinishing to do on the console still. The hole will be under the console once it's installed.

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As usual, thanks for sharing. I've often thought of switching from my TKO-500 to a 6 speed, but the thought of hacking the tunnel always deterred me from doing that. I like the way you did it. You didn't end up with a tunnel that looks like it belongs in a 4x4 truck! That being said, I still think I'll stick with the 5 speed because the cost/labor benefit just isn't there for me. Besides that, I am still running manual clutch linkage and have thought of switching to hydraulic. Based on your issues, I'm sticking with the linkage. I may just make it better by using some small heims to get rid of the flimsy clips that hold the rods together.
Do you find first gear to be too short with the 4.30 gears? What ratio is 1st in that trans?
 
racerjoe said:
As usual, thanks for sharing. I've often thought of switching from my TKO-500 to a 6 speed, but the thought of hacking the tunnel always deterred me from doing that. I like the way you did it. You didn't end up with a tunnel that looks like it belongs in a 4x4 truck! That being said, I still think I'll stick with the 5 speed because the cost/labor benefit just isn't there for me. Besides that, I am still running manual clutch linkage and have thought of switching to hydraulic. Based on your issues, I'm sticking with the linkage. I may just make it better by using some small heims to get rid of the flimsy clips that hold the rods together.
Do you find first gear to be too short with the 4.30 gears? What ratio is 1st in that trans?
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The hydraulic clutch thing was more about set up than anything else, and most of it was just my fault because of parts compatibility. If I’d just bought a complete kit I wouldn’t have had the issues that I did. And if I’d checked the clearances for the new set up off the car it would have been easy. Now that’s it’s on the car it’s great, and I wouldn’t hesitate to do it again.

1st is 2.66. It’s a little short with the 4.30’s, but not bad at all. With the close ratio 6 speed all the gears are pretty equally spaced, and the first 3 or 4 go by pretty fast!

mopowers said:
Nice work. Just curious- with the short 4.30's, do you even use 1st gear?
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I use the HELL out of first gear! Honestly it was one of the reasons I did the swap. I was running a 3.09 1st gear 833 with 3.55’s before, so I already had a 10.97 drive ratio. The Lunati 60404 cam in my 340 is supposed to have at least 3.91’s based on Lunati’s recommendations, and I understand why.

The T56 Magnum I’ve got has a 2.66 first gear, so I had to change the rear gears. With the 4.30’s my drive ratio is 11.44, which still isn’t crazy. It’s actually one of the nicer things about the swap, it’s so much easier getting the car going. With the 3.55’s it was taking some slip of the clutch to not lug the 340 down during normal driving. It was fine if I was driving it like I stole it, but that doesn’t work in traffic and it’s my daily, so it sees plenty of that.
 
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Great work!
I love the detailed pictures and the step by step write up as well.
Sounds like a lot of fun............
 
Dartnut said:
Great work!
I love the detailed pictures and the step by step write up as well.
Sounds like a lot of fun............
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Thanks! :thumbsup:

You know, the majority of it wasn’t that bad. The fabrication went way better than I expected. The part that made everything harder was doing most of the work in the car. When I do this swap for my Challenger I’ll dial indicate the bellhousing and do all the fitting and routing of the hydraulic clutch and lines with the whole assembly out in the shop instead of with the engine installed in the car. That alone would have made the process much faster and easier. I just didn’t want to tear the car down that far.
 
Awesome glad you finally got the 6 speed in. Been following for quite awhile, really enjoy your posts very informative. How much did your gas mileage improve so far? Dustin
 
Stupid question, but what's the benefit of a 6-speed over a 5-speed? I'm assuming it's the added over-overdrive that allows the use of a shorter rear gear? Are they stronger, shorter throws, tighter gear spacing?
 
Dartsun said:
Awesome glad you finally got the 6 speed in. Been following for quite awhile, really enjoy your posts very informative. How much did your gas mileage improve so far? Dustin
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Gas mileage? What's that?

Um, around town I haven't really noticed much of a change. I think the "fun factor" of having the 4.30's probably costs me as much fuel around town as I save at cruise on the freeway. I would imagine that on a longer drive, where I spend the entire tank at cruise, I probably picked up a couple of mpg's. But I've pretty much only done mixed driving since I finished the install. And to be honest I don't track my mileage that closely. I have a wideband A/F gauge to monitor my engine tune and as long as my typical commute doesn't suddenly start taking a few extra gallons I probably wouldn't notice.

mopowers said:
Stupid question, but what's the benefit of a 6-speed over a 5-speed? I'm assuming it's the added over-overdrive that allows the use of a shorter rear gear? Are they stronger, shorter throws, tighter gear spacing?
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An extra gear? :rofl:

Seriously though, I did look at doing a 5 speed conversion. A big part of it was the actual transmission. The TKO 500's and 600's get "ok" reviews IMO. There's quite a few people out there that don't like the feel of the TKO's shifting. It seems like that was the swap to do for awhile with the B/E bodies, it doesn't take a ton of tunnel fabrication on those cars. But I read a lot of lukewarm reviews on that swap. On the flip side, I haven't found anyone that installed a T56 in a Mopar that didn't love it. I mean like rabidly love it. So that was a really big factor for me.

The next part was the cost and fabrication. After I priced out the 5 speed vs the 6 speed swap, the difference was only about $1k. Which I know is a lot, but if I'm already going to spend $5,600 spending $6,600 isn't a crazy difference. And the fabrication was about the same. Yeah sure, for the TKO's you don't have to do as much tunnel fabrication or quite as much crossmember removal and fabrication. But it's like 80% of the way there. If I already have to cut the tunnel and crossmember, then going a little further isn't as big of a deal. If I could have done it without cutting that might have been different.

And yeah, the gearing is just easier. For the TKO's you get gears of 2.87, 1.89, 1.28, 1.0 with a 5th of either 0.64 or 0.82. For the TKO 500 you get 3.27, 1.98, 1.34, 1.00, 0.68. So the 500 has a big drop from 1st to 2nd. And the 600 either has a big drop to 5th, or only a .82 overdrive gear.

The T56 Magnum I installed has 2.66, 1.78, 1.30, 1.0, 0.80, .063. So the gear splits are all tighter, and with a 4.30 gear I improved my 1st gear drive ratio and still dropped almost 1k off of my rpm at cruise speed compared to the 3.55's which was great. They have a T56 magnum XL now with a 2.97 first gear and the .8 and .63 overdrives, that's the one goldduster318 installed. If that one had been around when I bought mine I could have done like 4.10's or 3.91's instead and lowered my cruise rpm a little. But my 340 doesn't like to pull much below 2,200 anyway. So setting my 70mph cruise at 2,400 was just fine and dandy.
 
Glad it's done. I like how in general we made the same thing in different ways (my conversion for others reference is https://www.forabodiesonly.com/mopar/threads/t56-magnum-conversion.416558 ) My kit came with the sleeves for anti-crush and also I didn't cut anything off the transmission, my tunnel is slightly wider, I can pretty much install every bellhousing bolt from the bottom with some minor engine tilting.

My transmission isn't an XL, its the same case as yours, just the TUET-16885 which is 2.97:1 1st and 0.63 OD. I have driven mine for a couple tanks and I'm getting 15 mpg on short trips so its about 1.5 better than no OD. The Cruising factor is incredible though. Now that its broken it it shifts like butter. No kidding it's incredible, I doubt you could make a better upgrade to one of these cars. In my case with the 25.6" dia tires and the 3.23 is 1900 rpm at 70, mine pulls it awesome.

The other thing is this transmission holds 700 lb-ft of torque so you will have to try to break it.

In my case, good to know about the corvette hydraulic throwout bearing, I'm not a super huge fan of the aftermarket shimmed ones.

Curious about the choice of Syncromesh fluid, considering the giant Dexron III only sticker on the side
 
goldduster318 said:
Glad it's done. I like how in general we made the same thing in different ways (my conversion for others reference is https://www.forabodiesonly.com/mopar/threads/t56-magnum-conversion.416558 ) My kit came with the sleeves for anti-crush and also I didn't cut anything off the transmission, my tunnel is slightly wider, I can pretty much install every bellhousing bolt from the bottom with some minor engine tilting.

My transmission isn't an XL, its the same case as yours, just the TUET-16885 which is 2.97:1 1st and 0.63 OD. I have driven mine for a couple tanks and I'm getting 15 mpg on short trips so its about 1.5 better than no OD. The Cruising factor is incredible though. Now that its broken it it shifts like butter. No kidding it's incredible, I doubt you could make a better upgrade to one of these cars. In my case with the 25.6" dia tires and the 3.23 is 1900 rpm at 70, mine pulls it awesome.

The other thing is this transmission holds 700 lb-ft of torque so you will have to try to break it.

In my case, good to know about the corvette hydraulic throwout bearing, I'm not a super huge fan of the aftermarket shimmed ones.

Curious about the choice of Syncromesh fluid, considering the giant Dexron III only sticker on the side
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Good to know! Yeah I had a fairly early version of the crossmember, there were some changes made along the way and even though you did the swap before I did I had the crossmember before that. Drawback of collecting parts over a few years I guess.

Shifting the T56 is so much nicer than the 833. Even though the 833 I was running was fully rebuilt by Passon it's just not the same as a new transmission. I think my biggest issue is dialing back the effort for the shifts, the T56 shifts so easily and the long B body shift handle is a bit much, it's easy to overdo it. The 340 in my Duster has always been a little light below 2,200. Which honestly makes sense to me, the cam in it is rated for 2,200 to 6,400. It's not like it's a total dog, but the whole package just works so much better above 2,200 it just makes sense to drive it there. It pulls like a monster from 3k up. But it's a hydraulic flat tappet off the shelf cam, I'm sure if I went to a roller I could pick up some more bottom end.

I never liked the aftermarket throwout bearing deal, the shims and everything, the generic application, etc. And once I went with the B&B clutch plate I was pretty much committed to not running one because of clearance needed on the fingers. The C5 throwout seems to be working great.

As far as the syncromesh goes, that was straight from Tremec, and it's right in the description- "Chevrolet Performance Synchromesh transmission fluid is designed for GM manual transmissions and is the recommended lubricant for all Tremec brand manual transmissions". It is after all supposed to be an LS1 transmission.
 
The fuel injection helps out at low RPMs a lot too, you get the boggy stuff going on with a big DP carb when you tip in, kind of the nature of the beast really. I can pretty easily pull a low to medium sized hill at 1200 rpm with the EFI after some tuning.

Its hard to know, but this transmission is actually a TR-6060 (Like in a Hemi Challenger, 10-up Camaro SS, Mustang GT350, etc) not actually a T56, but with a custom "Ford" slip yoke output shaft and tailhousing since all of those vehicles have "divorced" shifters. If you look up "2010 Camaro TR-6060", the main case is identical. Not sure the exact Syncromesh application because all the earlier applications came with Dexron III and it appears that at least the challenger and camaro for 2010 came with "pick your flavor" of ATF. GM's fluid specs are all over the place, the OE fluid on my 04 Colorado's Aisin is 85w-90 GL4, my Cruze had 70W GL-4 (I went to 75w-85 GL4), and I had a 1990 Cutlass Supreme that used 10w30 motor oil, so who knows. I also don't know why they'd put a giant Dexron III sticker on the side if that wasn't what should be used, but I'm sure there isn't much weight difference, that stuff is light weight.
 
goldduster318 said:
The fuel injection helps out at low RPMs a lot too, you get the boggy stuff going on with a big DP carb when you tip in, kind of the nature of the beast really. I can pretty easily pull a low to medium sized hill at 1200 rpm with the EFI after some tuning.

Its hard to know, but this transmission is actually a TR-6060 (Like in a Hemi Challenger, 10-up Camaro SS, Mustang GT350, etc) not actually a T56, but with a custom "Ford" slip yoke output shaft and tailhousing since all of those vehicles have "divorced" shifters. If you look up "2010 Camaro TR-6060", the main case is identical. Not sure the exact Syncromesh application because all the earlier applications came with Dexron III and it appears that at least the challenger and camaro for 2010 came with "pick your flavor" of ATF. GM's fluid specs are all over the place, the OE fluid on my 04 Colorado's Aisin is 85w-90 GL4, my Cruze had 70W GL-4 (I went to 75w-85 GL4), and I had a 1990 Cutlass Supreme that used 10w30 motor oil, so who knows. I also don't know why they'd put a giant Dexron III sticker on the side if that wasn't what should be used, but I'm sure there isn't much weight difference, that stuff is light weight.
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GM stopped licensing DexIII in 2007. There was a TSB somewhere on it, they went from calling it DexIII to Synchromesh and now they use DexVI in all the new stuff. When I bought the transmission the stuff I bought was what Tremec recommended at the time (there’s more than one type of Synchromesh too). As for the sticker, you’d have to ask them.

It shifts great, I’m not worried about it.

Yeah my next “big” upgrade will probably be EFI. The 750 Ultra DP I’ve got works well but EFI would make my daily driving life a lot easier. I’ve got a few things to do before I spring for that one though.
 
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