Any guidance on my 1970 Duster plans

poconos_power said
Hey everyone! I was recently lucky enough to pick up a 340 TA engine that has been off the road for about 40 years (going to need to get sent out, but it's complete). It belonged to my Dad's friend and his wife finally convinced him it would go better in a car than sitting in their garage. So I did what any reasonable person would, I bought a car to put it in. I ended up finding a 1970 Duster that was originally a slant 6 automatic. I got lucky the interior is great and it only needs small rust patches in the rear quarters, then I may send it out for paint.

In figuring out what I want this car to be, I have never done a performance build before, and was thinking autocross seems like something I want to do. Though I also want the car to be able to cruise and do road trips and such. So I'm looking for it to be good at multiple things but not great at one thing I suppose, I'm sure my limitation will be my driving skill instead of the car in the beginning, I'm just something that works and lets me get into it and would be an interesting project to get to my goal. I did some research and there are plenty of great options for full coil over conversions on the front, and I'm sure I could do that eventually, but I find the prospect of trying to tune in a t-bar suspension more interesting. I figure I can always upgrade and change in the future. So after a few weeks of lurking and reading here is what I'm thinking, and please let me know what you think about this.

For the rear, I was able to score a new 8.75 rear at Carlisle this past summer to upgrade the previous rear. I was thinking of putting in the Hotchkis geometry corrected leafs.

For the front, I was going to do the Firm Feel reinforcement kit for the k-member, reinforce the lower control arms, get the Firm Feel tubular uppers with the big ball joint (if I replace the spindles), and adjustable strut rods. Poly bushings for everything. Also the car has a pretty big front sway bar currently, though I haven't measured it, but I don't think it's original.

For torsion bars I was thinking 1.08 or 1.12 based on what I'm reading here. However the question is, how do I address the 1" drop from the Hotchkis leafs? Do I get drop spindles or just drop the front using the torsion bars?

And finally I'll replace the power steering box with a better ratio. Perhaps at some point I'll also add frame rail connectors and torque boxes, but I'd like to get driving down the road first at least.

Any thoughts out there about this? I've read a lot of posts saying if you set up a torsion bar suspension correctly, it can be on par with a coil spring setup, but I haven't found the full breakdown or how that can be achieved. This is what I've pieced together from various posts. Thanks!

View attachment 1716334719

Welcome!

You are correct, there's no reason you need to go to coil-overs to handle well. The geometry of the torsion bar suspension is actually very good, especially if the car is lowered a bit. In fact, the only coil over geometry I've ever seen published was worse than mildly modified torsion bar suspension geometry out of the box. Obviously if you commit to modifying geometry either system can be made to work well, but other than a rack and pinion and some header clearance there's nothing you can do with a coil over conversion that can't be done with torsion bars, there are plenty of aftermarket components available to put the torsion bar suspension on any handling performance level you'd like.

Before I really get into it, what wheels/tires are you planning on running? Because tire choice will absolutely change how you set up the suspension, so if you plan on keeping 15" rims that will be a different plan than if you use 17" or 18" rims- for example, your torsion bar choice will be different. There is precious little in performance tires that are good for AutoX and street use in a 15" wheel. To the point that I would say if your AutoX aspirations go further than just having a little fun here and there you will want your plans to include 18" wheels, because that's really the size you need to be at to get a large enough wheel in the front with a good performance tire for autoX.