Convince me to put a TKX in my Dart

[Arthur Sperotto]

For context, my car is a '71 Dart with a 360, hp somewhere around 450-500. 727 trans.
There is ONE thing keeping me from purchasing a TKX PerfectFit kit from Silver Sport Transmissions. The cut you have to make to the upper part of the torsion bar crossmember. They supply a crossmember for the TKX, but that only replaces the lower part. Unfortunately, I don't have numerical data to analyze how much that upper section is actually used by the torsion bars, but from my eye calculator, it looks to be important. Here is a picture of how it would look like: (pic from @jcolman )

Short of fabricating a replacement piece (which I'm not very comfortable doing with my current skills...), I was hoping to get suggestions of alternatives. I was wondering if frame connectors and torque boxes from US Car Tool would be adequate? Again, using the "eye calculator", it looks like they would give the twisting forces from the torsion bars more places to go. I contacted ToddRon Restorations but unfortunately no offerings for the TKX there.
Thank you in advance for any suggestions!

 

[jcolman]

I'll let you know how my car handles soon. I should be driving it by Friday. To be honest, I think that the very beefy lower cross member supplied by Silver Sport should do the job just fine. However my car only makes around 350-370 hp so you have to take that into consideration.

But I also added subframe connectors (not shown) to help stiffen up the car. I have torque boxes that I was going to install, but I'm taking a "wait and see" approach to see if I really need them.

 

[Dana67Dart]

I would not do it. It was designed the way it was and for a lot less power.

 

[dano]

For my TKO swap you had to cut it back about that at far anyway. You can always re-weld the replacement hoop back in. My kit is the defunct Hurst kit but Holley still sells the loop. Below show what they had you do. The new .188" thick hoop was way stronger than the factory stamped one. We sub-framed my car at the same time which also helps

Install Overview: Hurst Driveline Conversions Tremec TKO Kit for 67-76 Mopar A-Body

 

[72bluNblu]

I dunno. Looking at the pictures of the TKX install, I think I would still install a T56 instead. It appears you have to do about 90% of the cutting anyway, and if you're going that far, why not get the extra gears and a transmission that has massive aftermarket support? It would be different if there was a 5 speed that fit without all the cutting, but even with a TKX there will be no mistaking the tunnel or crossmember for original.

On my T56 install I used a ToddRonn fabrications crossmember, it includes a complete upper hoop that's even more stout than the original. I can't say that I like the idea of running torsion bars without an upper section to the crossmember. Maybe it would be fine with that beefed up lower section of crossmember, but, when you drop the transmission you remove that whole section of crossmember.

Here's some pictures from my T56 install, there's a TON more and a complete list of every part I used with a link in my build thread My "new" '74 Duster- or why I need a project like a hole in the head

I did the T56 install for $6,620 in parts, I did the install myself. That was in 2019 though, so I'm sure it would be more expensive than that now. I love the transmission though!

 

[Craig Burriss]

Go with a T56 Magnum and don’t look back. The best thing I’ve done to my car by far.

 

[RustyRatRod]

Best thing to do is find someone in your area who has one and go drive it. You'll convince yourself.

 

[Arthur Sperotto]

I'll let you know how my car handles soon. I should be driving it by Friday. To be honest, I think that the very beefy lower cross member supplied by Silver Sport should do the job just fine. However my car only makes around 350-370 hp so you have to take that into consideration.

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But I also added subframe connectors (not shown) to help stiffen up the car. I have torque boxes that I was going to install, but I'm taking a "wait and see" approach to see if I really need them.

I'm looking forward to it! I've had a couple experienced people leading me to believe that the beefy SST lower section should do the job as well, especially when frame connectors are added to the equation. Please do let me know how it drives.

I would not do it. It was designed the way it was and for a lot less power.

I agree, I'm trying to avoid just doing the kit as-is. I want to do SOMETHING for the top section... just trying to figure out what. Something along the lines of the ToddRon piece would be perfect, but he doesn't offer one for the TKX and I don't feel confident fabbing one myself.

For my TKO swap you had to cut it back about that at far anyway. You can always re-weld the replacement hoop back in. My kit is the defunct Hurst kit but Holley still sells the loop. Below show what they had you do. The new .188" thick hoop was way stronger than the factory stamped one. We sub-framed my car at the same time which also helps

Install Overview: Hurst Driveline Conversions Tremec TKO Kit for 67-76 Mopar A-Body

I tried to find the Holley loop but couldn't! Do you happen to know what they call it, or maybe have a part number? When you say re-weld the replacement hoop back in... the SST kit comes with a section of floor to replace the floor section you cut out, but it's thin metal and doesn't compare to the upper torsion bar crossmember part you have to remove. Let me know if you meant something else by that and I misunderstood. At the link you posted, I did see a CHUNKY upper crossmember section which looked awesome, but giant compared to the space the TKX needs haha. Is that what you were referring to?

I dunno. Looking at the pictures of the TKX install, I think I would still install a T56 instead. It appears you have to do about 90% of the cutting anyway, and if you're going that far, why not get the extra gears and a transmission that has massive aftermarket support? It would be different if there was a 5 speed that fit without all the cutting, but even with a TKX there will be no mistaking the tunnel or crossmember for original.

On my T56 install I used a ToddRonn fabrications crossmember, it includes a complete upper hoop that's even more stout than the original. I can't say that I like the idea of running torsion bars without an upper section to the crossmember. Maybe it would be fine with that beefed up lower section of crossmember, but, when you drop the transmission you remove that whole section of crossmember.

Here's some pictures from my T56 install, there's a TON more and a complete list of every part I used with a link in my build thread My "new" '74 Duster- or why I need a project like a hole in the head

I did the T56 install for $6,620 in parts, I did the install myself. That was in 2019 though, so I'm sure it would be more expensive than that now. I love the transmission though!

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72bluNblu, you've been an inspiration to this undertaking. I think I've read almost every page of your build thread over the years. I certainly appreciate all your contributions! I can understand why some ppl would lean towards the T56, but I just truly don't need it. The .68 overdrive you can get with the TKX is exactly what I want, and I like the fact it's lighter and more compact (yes, even though the cutting is very similar). They're also about a grand apart. Your pricing is very accurate, nowadays a T56 swap (from auto) runs right around 7k, while a TKX is right around 6k.

Go with a T56 Magnum and don’t look back. The best thing I’ve done to my car by far.

I have the feeling it'll be the best thing I do as well! I just have absolutely no need for the T56 over the TKX. It'll be a waste of an additional $1k in my particular case. The .68 od from the TKX is perfect for me.

Best thing to do is find someone in your area who has one and go drive it. You'll convince yourself.

Oh I know I want it! I just wanna go to sleep at night knowing my car still has a safe amount of structural integrity lol. I was chatting with a Mopar friend yesterday who thinks it might be okay... to quote him, "it's probably not the end of the world. But if you introduce something like bumper wheelies into the equation... you're asking a lot of what is in a roundabout way... your spring seat"

 

[dano]

I tried to find the Holley loop but couldn't! Do you happen to know what they call it, or maybe have a part number? When you say re-weld the replacement hoop back in... the SST kit comes with a section of floor to replace the floor section you cut out, but it's thin metal and doesn't compare to the upper torsion bar crossmember part you have to remove. Let me know if you meant something else by that and I misunderstood. At the link you posted, I did see a CHUNKY upper crossmember section which looked awesome, but giant compared to the space the TKX needs haha. Is that what you were referring to?"

Here is a link to the Holley/Hurst hoop and transmission cross member.

Hurst 67520001 Hurst Transmission Crossmember & Floor Brace for 1967-1976 Mopar A-bodies

Tthe "Hoop" I refer to is missing in your photo and 72bluNblu shows going over his T56. The sheet metal they refer is the patch panel to replace the areas you cut out to fit the transmission in the tunnel.

My TKO swap was $3350 (install by my dad and I) about 5 years ago mostly because I started with a 4 speed car and didn't need a drive shaft (stock length was perfect), Flywheel, or pressure plate (I just needed a disc to match the spline) and retained the stock clutch linkage. I will the aluminum bell they sent liked the 72-up Z-bar pivot if you're going that route. I love my TKO600 and had a Tremec 3550 before and both shifted smooth and were fun to beat on.

That TKX isn't saving any work over a TKO or T56 on an A-body. It was probably made for the cookie cutter 1 wire Chevy crowd.

 

[Arthur Sperotto]

Here is a link to the Holley/Hurst hoop and transmission cross member.

Hurst 67520001 Hurst Transmission Crossmember & Floor Brace for 1967-1976 Mopar A-bodies

Tthe "Hoop" I refer to is missing in your photo and 72bluNblu shows going over his T56. The sheet metal they refer is the patch panel to replace the areas you cut out to fit the transmission in the tunnel.

My TKO swap was $3350 (install by my dad and I) about 5 years ago mostly because I started with a 4 speed car and didn't need a drive shaft (stock length was perfect), Flywheel, or pressure plate (I just needed a disc to match the spline) and retained the stock clutch linkage. I will the aluminum bell they sent liked the 72-up Z-bar pivot if you're going that route. I love my TKO600 and had a Tremec 3550 before and both shifted smooth and were fun to beat on.

That TKX isn't saving any work over a TKO or T56 on an A-body. It was probably made for the cookie cutter 1 wire Chevy crowd.

That's awesome, thanks for the link. I hadn't come across that product yet, it's a pretty trick piece, I like it. Not sure it would work for the TKX but I'll definitely keep it in mind.
And yes, I agree, it's not saving any work over a TKO or T-56, it's saving money! From what I've learned, it seems to be a better overall transmission, with several little annoyances fixed.

Edit: upon further research, the TKO and TKX have identical length and mounting points, making their crossmembers interchangeable! You might have just given me exactly what I needed.

 

[TT5.9mag]

There is no reason at all to cut out the top of the torsion bar cross member and not replace it with something else. I would not rely on just the bottom trans support to maintain strength in the crossmember. You can build (or buy) a hoop across the top that will maintain the strength of the original and give you room for a larger trans. The subframe connectors and torque boxes strengthen the frame in the wrong plane to help the situation. Seriously think about the weight of the entire front end of the car and that crossmember supporting it.

 

[CudaFactHackJob]

C body mopars had a torsion bar crossmember that didn't go across the top at all, You had to take the weight off the front wheels and back off the tb adjusters before removing the crossmember or the tbs would twist the ends down. I had one come in that had the trans out and the car had been sitting on the ground. The tb ends were dug down into the ground. I could see those ends twisting down if the crossmember isn't really tight. I would remove the carpet and weld strap across the floor. Maybe drill down thru the floor to tie/weld them together.

 

[Topless69]

Dude I know installed a TKX and he had women crawlin’ all over him!

 

[DC Dart]

The original torsion bar cross member combined with transmission mount for its weight is very strong. The top is compressed as the mount is pulled if looked at as a truss. And the torsion bars add to that. Now cut that top section off.
My simple mind thinks the top piece is important.

 

[Ironracer]

Best thing to do is find someone in your area who has one and go drive it. You'll convince yourself.

Rob the Realist! Love it Man!

 

[Arthur Sperotto]

Figured it's my duty to update this, since there is now a TKX in my A-Body lol!
Silver Sport Transmissions have been fantastic to deal with, top notch customer service. No regrets, highly recommend!

As far as cost, the complete kit was right around $6k, with everything I could ask for and more. This is including a nice dial indicator to measure bellhousing run-out, a better clutch than what they include in the kit (I requested a 700hp unit), and their hydraulic concentric slave cylinder which is a $650 upgrade.

As far as ease of installation, my mind is still blown. I was able to do 90% of the work in 2 days. From 727 out, to TKX in. Now only waiting on FedEx to deliver the 31 spline 1350 slip yoke from Strange, and 1350 pinion yoke so I can measure for a new driveshaft and get that going.

As far as the cutting, it is WAY LESS than a T-56, or TKO! Like Silver Sport mentions in the instructions, those instructions were developed for the TKO, so the TKX may require less cutting. What I did was, I completely ignored the instructions and cut little by little, to figure out the least necessary amount. Check it out:

As you can see, apart from the obvious opening for the shifter, the tunnel only needed that minor split in the middle. That gap is so small I could even just fill it with weld lol!

Now, the part that hurts. Yes, the upper section of the crossmember does need to be cut. I did everything in my power to avoid it, but I couldn't. So I still need to figure out a way to re-join both sides. There is no way in hell I'm just leaving both sides disconnected like that. Here is another picture for a slightly different perspective:



I will update this thread when I have that solved.

 

[jcolman]

I’ve put around 300 or so miles on my TKX equipped car since I had the conversion done. No top brace. No issues yet. I did add subframe connectors which really helps.

 

[Arthur Sperotto]

Awesome, glad to hear! I'm looking forward to driving the car.

The subframe connectors really help in chassis stiffening, and I'm sure they give you a tighter feel when driving, but they do nothing to distribute the torsion bar forces. But we already know we stand on different sides of the fence when it comes to this subject

 

[CRUZE 418]

It's interesting that one would be concerned about structural integrity, and then take a wait and see on torque boxes? Or the US Car Tool subframe connectors.

 

[jcolman]

It's interesting that one would be concerned about structural integrity, and then take a wait and see on torque boxes? Or the US Car Tool subframe connectors.

If you're referring to me, I did install subframe connectors....except I went with Hotchkiss.

 

[CRUZE 418]

Yes, if you need torque boxes.

 

[72bluNblu]

So, I've been doing some research into further improvements for my car and part of that was tracking down some of the out of print books from Steve Smith (thanks @autoxcuda for your contributions to that old "reading list" on Moparts) because they included pictures and explanations for some of the modifications on the old Petty Nascar builds and some of the kit cars.

One of the books I managed to get ahold of was "Building the Race Car Picture By Picture" by Steve Smith, S110, 978-0936834108. All the catalog numbers are here Amazon product ASIN 0936834102, I didn't pay anywhere near this price for mine FYI.

Anyway, one of the gems in that book are pictures of the Petty Enterprises torsion bar crossmember, which had a part number and everything I assume for purchase for the kit cars etc. Anyway, it weighed 125 lbs and was used because of how much it added to the torsional rigidity of the cars. Keep in mind, those were cars with full cages, and they STILL used a 125 lb reinforced torsion bar crossmember, in a race car where weight was crucial. Think about that.

Bottom line is, I wouldn't have done it before, but there is now absolutely, positively, not a snow ball's chance in hell I would run a torsion bar car without the upper hoop of the torsion bar crossmember. Makes me feel much better about going with the heavy duty T56 crossmember I used, and much less like that was "overkill".

 

[Arthur Sperotto]

Fantastic contribution, @72bluNblu , thank you! If I could buy that Petty crossmember today, I would seriously consider it!

 

[72bluNblu]

Fantastic contribution, @72bluNblu , thank you! If I could buy that Petty crossmember today, I would seriously consider it!

Yeah I wish the picture wasn't so washed out, that's actually the "fixed" version that I did some photo editing on, the picture in the book is pretty terrible. Now that I know a bit more about it I can look for better pictures out there of one, or maybe an old Petty Enterprises parts catalog? Obviously with the T56 in my car the original version wouldn't help, but I'd love to have more details on the construction of it.

 

[autoxcuda]

So, I've been doing some research into further improvements for my car and part of that was tracking down some of the out of print books from Steve Smith (thanks @autoxcuda for your contributions to that old "reading list" on Moparts) because they included pictures and explanations for some of the modifications on the old Petty Nascar builds and some of the kit cars.

One of the books I managed to get ahold of was "Building the Race Car Picture By Picture" by Steve Smith, S110, 978-0936834108. All the catalog numbers are here Amazon product ASIN 0936834102, I didn't pay anywhere near this price for mine FYI.

Anyway, one of the gems in that book are pictures of the Petty Enterprises torsion bar crossmember, which had a part number and everything I assume for purchase for the kit cars etc. Anyway, it weighed 125 lbs and was used because of how much it added to the torsional rigidity of the cars. Keep in mind, those were cars with full cages, and they STILL used a 125 lb reinforced torsion bar crossmember, in a race car where weight was crucial. Think about that.

Bottom line is, I wouldn't have done it before, but there is now absolutely, positively, not a snow ball's chance in hell I would run a torsion bar car without the upper hoop of the torsion bar crossmember. Makes me feel much better about going with the heavy duty T56 crossmember I used, and much less like that was "overkill".



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Glad those old reading list could help.

I used to have that list on my old website: 68 Cuda Autocrosser. And I had a scanned in version of the "Trans Am and Corvette Chassis" book on that old aol.com site decades ago.

The torsion bar cross member is in the 1974 Petty Circuit Car catalog (Cup cars, not Kit cars) It probably was available earlier than that. And could very well be derived from a Nichols Engineering part.

Nichols had the contract for building Mopar nascar parts and cars before 1970. That’s when Petty switched back to Mopar and got the parts and car building business. That was part of the deal.