Just to clarify, Powerflex doesn’t currently make an offset bushing.
The issue I’ve experienced in the past with the aftermarket eccentrics is wear and overly soft materials. If you are using a adjustable eccentric it probably needs to be a pressed interference fit to really work long term. If the bushings are soft and you tighten the mounting bolts too much they can twist in the arm and change your settings. The harder ones are better but they can wear over time losing precision and possibly thier ability to hold adjustment. I’d recommend keeping a close eye on them, or tac welding them in place once perfectly adjusted or they stand a chance or coming out of adjustment. We stopped selling adjustable bushings several years ago because they are hard for street guys to keep in adjustment over time.
The OE eccentrics are reasonably hard and small so you don’t lose that much performance. If you know the direction you need to move them, they are pretty easy to setup initially. I’d recommend putting them in, getting your setting in the right ballpark and forgetting about them until worn. They are softer than most ‘race’ poly or delrin. This leads to more passive rear steering (flex causing the car to feel squirley because of the rear toeing out more than intended in the corners and braking). Even if you have more friction the car drives nicer because it’s more consistent. Hard bushings make the car easier to drive on the limit. The real reason factorys used fixed rubber bushings isn’t friction or the lack thereof at all. It’s cost for one, and just as importantly noise! Most new cars have bushings designed so the suspension components move and stretch or sheer rubber during suspension movement. This prevents the parts from moving on the rubber mountings so there can be no squeaking! Drive an old torsion bar suspension pick up and you’ll see what I mean, they squeak like crazy. It’s nice for noise but typically service life is shortened greatly. Those that own E46 cars and have to deal with thier front bushings will know what I mean (those cars barely make it out of warranty before there are issues). E36/46 rear trailing arm bushings are the same way. For racers this means the bushing is soft and stretchy by design. Under high loads the suspension deflects greatly and this reduces your performance potential.
I do not use adjustable ‘bushings’ in my car. I used a weld on kit with eccentric tabs. I think this is superior because the mounting points are both reinforced, and the toe and camber can be adjusted separately. It’s also torqued and stays adjusted like a factory part would, with any factory fit bushing. I ‘race’ prepped two subframes incase I was ever protested on that, but I figured nobody would notice and it’s not a performance advantage. On my car I ended up needing to raise the rear ride height back up some to make the car handle better. This actually put my camber in a good spot so I really only needed toe adjustment.
In regards to Spec Cars, I used delrin in my one subframe. It drives the same as really hard adjustable poly in the other. The side flanges on my delrin bushings cracked after about 5 weekends. This resulted in some side to side play which hurt the car a little. The flanges are very thin and probably not robust enough. I intended to replace them with Poly on the next shot as it’s flexible and shouldn’t crack. I don’t think most people need tons of adjustment in these cars so any of the adjustable options should provide adequate range.
The subframe bushings are really more critical. Go as solid as you can to prevent it from shifting side to side and moving around. If you don’t mind the mounts being a one shot deal do aluminum, if you want to be able to remove the mounts and reuse them get urethane. If you get bumped and bend the subframe you could probably remove the urethane carefully and put it in the next subframe.
