B5 Audi/A4/S4:
The chassis on the Audi A4 Quattro is no slouch in terms of chassis rigidity but by modern Audi standards it is quite flimsy. On a luxury car like this most people aren't going to go out and stick a road racing cage in there for the sake of a stiffer chassis, but there are little things that you can do.
The factory S4/A6 shock tower bar is the easiest upgrade for the B5 A4. There is a how to for that on Audiworld so I wont repeat that here. A drivetrain stabilizer is the next easiest stiffener, available from AWE tuning or AutoSpeed.
Beyond that it is going to be custom fabrication. You can weld roll cage tubing across the rear shock towers or fit a custom bar of some kind. Also if you are planning on paint, like I mention at the bottom, get a race shop to spot weld along seams and around doors.
The B6 chassis is stiffer still (and heavier) and doesn't need much in the way of bracing. Audi's are heavy for a reason.
Accords/Hondas/Everything else:
The chassis on my Accord was probably pretty stiff for its day. It was a two door coupe without a sun roof. By today's standards it might as well be made of cheese.
That doesn't mean that we're stuck that way, but if you want a solid chassis you must choose your modifications wisely. Most of the shock tower bars you can buy for the CB7/hondas are garbage. Any time you box in a flexing structure like that, if you don't create a 3rd or 4th diagonal brace it will still be moving horizontally. Think of a cheap metal shelf before you put the cross braces in, it will nearly fall over under load, even though it's boxed in.
So how to add a third and fourth bar? And where? You can buy 3 and 4 point strut/shock tower bars for more popular tuner cars but you generally have to make them for most. If you cannot weld then i suggest going to a roll cage fabricator, a good welder, or a machinist. You want your front bar to go between the shock towers, either welded to them or bolted to them with solid, hingeless, brackets, then two bars that reach diagonally to the firewall to be bolted or welded there.
Once you have a four point bar on the front you need something in the rear. Here is where my opinion differs from most people. In the rear you really don't need something you can take off like a STB. The engine isn't in the way like it is in the front. So my solution: weld a bar across the strut towers like you would for a roll cage, then weld in a cross bar to the floor (or two) to stop movement completely. If you still think this is overkill check the rear of the RX8 chassis (or the TRD rear STB for a Celica GT-S) and you will see exactly what I just suggested. You'll need a roll cage fabricator to do this as you're going to want it to be structural, not just welded to the visible body (there are non-structural metal sheets over the rear strut towers in the accord). Steering, handling, and ride should all be much improved.
Integra GS-R
The integra chassis is not by any means a rigid one. Hatchback design is notoriously weak. A rear shock tower bar, cage, or some kind of four point brace in the hatch area should help immensely. There are c-pillar bars available too but that to me seems like it would be dangerous in a crash and unless it is triangulated elsewhere, not going to do much functional bracing.
There are front and rear underbody braces available as well which i have not tested, they could be of use if their mounting points are at a weak spot.
The Integra Type R came with a thicker body shell at the C pillar and a lot of extra chassis braces from the factory. Some of those you can add to a regular integra.
MR2
The Gen 2 MR2 comes with quite a lot of bracing from the factory in the usual spots. Front and rear strut tower bars, both of which are well made. TRD makes even better ones. Beyond that there isn't a whole lot availble. The T top is going to be considerably less stiff than the hardtop or sunroof models but besides a roll cage there isn't much you can do about it.
urS6
The urS6 also comes with a very solid strut tower bar in the front. There isn't anything else in available as far as I know.
EF Civic
The EF Civic chassis is pretty flimsy. There are likely dozens of shock tower bars available, you want to look for one without any joints in it and prefereably a triangulated one going to the firewall. The bar off of the Integra may fit, IDK. A rear shock tower bar would probably help out a lot too but that would definitely mess up your cargo capacity. Subframe braces are surely available as well. This car would definitely benifit from a full drivers seat back roll cage but again there goes your cargo space.
