[Wil Ferch]

Dickster:
What complicates this whole thing is that Porsche also altered the anti-roll bar sizing..which can be considered an "overlay" to this whole scheme. The front/rear balance is not totally borne by the torsion bars. Good practice, however, would say do the rough tuning / balancing with the torsion bars...and "fine tune" additional understeer/oversteer characterisitcs with adjustable anti-roll bars.
I didn't want to complicate matters more than than they are..so I focused on the TB's. As to the numbers you quote.. I must say you're correct in your math...but you need to look at the whole spectrum to see where you're headed. Stock is known. "Commonly quoted" upgrades are known ( 21/27... 21/28). All these maintain "roughly" the same balance. Going progressively stronger in the rear ( increasing the rear size an additional 1-2 mm) is the ticket to dial-out a bit of the original car's understeer. Just look how much change one size of rear bar compares to another 1-2 mm larger. It's a big increase because the stiffness increases to the FOURTH power with diameter ! You don't need a sledgehammer approach to get the rears fully 2x as stiff as the front to then say.."Yessir....only *now* do I feel a difference!". You can attack this in a less aggressive manner. Don't forget, you have the added complication ( with *any* of these sizes chosen)...that the car will understeer more at low speeds...and less at high speed corners. IMHO...a 21/31 set up will get you too close to high speed oversteer, which would be an irresponsible recommendation unless you were REAL experienced AND real good !! Maybe Jack Olsen ...with his bountiful track experience...is such a guy...but I wouldn't do this for a street car that may be driven at highway speeds.
I'm not trying to scare you...just asking you to realize where this set up is heading. You know..if you *think* you're driving your car very hard, but in actuality are not...this set up might not be as bad as I make it out to be. Certainly I thought I was driving aggressively ( on the street), until my first track experience and found out that my street driving and getting to the "limits" was actually pretty lame !!

---Wil Ferch