Yeah, there are online calculators, but you need to know the exact dimensions, and I can't really seem to find those published.
It would be 8-12% larger in diameter, but IDK if that would make a noticeable difference.
The torsional stiffness of a bar varies with the 4th power of its diameter, so seemingly small changes in diameter do actually result in meaningful increases in stiffness. For example, if everything else remains the same, the 4.5% increase to 23mm diameter from the stock 22mm rear bar would result in over 19% greater stiffness.
You also don't need to have any of the bar dimensions to compare the effect of these diameter changes, since as long as you're looking at a single car application those dimensions will solve out of the comparison, being near enough identical between bars. Otherwise they wouldn't fit.
Unfortunately this simple estimating method only holds for solid bars, whereas the R has a hollow stock rear bar. For that construction you calculate a reference stiffness first for a solid bar of the same outer diameter, then for a solid bar corresponding to the inner diameter of the hollow bar, and subtract one from the other.
But do we know the inner diameter? No we don't, but there is a way to get some idea of it. On the stock bar, the end tab where the links bolt on is made by just stamping the original tube in a press until it's flat. If you look at that area on the stock bar you can actually distinguish the two walls of the tube at the end. Measure that double thickness with a Vernier caliper, subtract it from the bar's outer diameter and you now have the inner diameter — more or less. It's going to be an inexact number, because the material thickness is likely to have been reduced somewhat in the stamping together process, so what we measure may be a bit less than the native wall thickness of the steel tube.
I measure the stock bar at 22mm OD, and the tab thickness at 7.4mm, suggesting an ID of 14.6mm. That would provide equivalent stiffness to a solid bar of 20.8mm OD. If you hypothesize that the wall thickness of the raw tubing as purchased might likely have been 4mm, the solid equivalent OD comes out to 21mm — not too different.
Now we have a way to compare the 22mm hollow stock rear sway bar, which is roughly equivalent to a 21mm solid bar (but much lighter), to aftermarket solid bars of known diameter.
What we can't necessarily do is compare it to a hollow aftermarket bar. My APR rear bar is 1 inch diameter (25.4mm) and hollow. However its link mounting tabs aren't made by crushing the bar ends, instead consisting of welded-on tabs cut from steel plate. Short of sawing the bar in half to take measurements — yeah, not doing that! — there's no easy way to know its ID. (Hmm, guess I could take a crack at that by measuring its total tubing length, weighing the bar, and using the known density of steel to figure out how much is "missing", but that seems like a whole lot of work, and probably not too accurate anyway.)
BTW, feel free to flag me on any math errors in the above. Stuff happens...
I can tell you that my APR bar, when set on the stiffer of two (arguably three) settings, has a dramatically beneficial effect on handling. However it does want aftermarket links, adding some extra to the project cost.
Neil
Note: If you want to get more accurate about the stiffness of a sway bar you'll also take into account the bending of its two arms under load. The better online calculators do that. However the effect of diameter changes on bending is
much smaller than on the torsion of the bar's main span, so I'm intentionally ignoring it.
