Hmmm, I thought it would have been a Camry motor. Or was it the Scion tC that have that? The Civic SI does have an upgraded suspension system actually. LSD's make a pretty siginificant difference when channeling power flow to the front wheels --something the XRS does not have. Part of the new steering/suspension combo also includes the all new drive-by-wire steering system, with much improved responses. Standard tires are the larger P215/45R17 Michelin Pilot HX MXM4 all season treads, with same sized Michelin Pilot Exalto PE2 performance tires available as an option. With this optional tire, Honda says the Civic SIís traction and handling is much improved on wet pavement as is the same to the standard rubberís grip in dry conditions. It's basically stickier to put it in easy terms. To match the new setup, the 2006 Honda Civic SI includes a beefed up MacPherson strut front and double wishbone rear suspension, strengthened by a 28mm front and 17mm rear stabilizer bar, boosted spring rates, and less compliant dampers for a roll stiffness improvement of 30%. The front discs has been increased in size to allow faster stopping response. To utilize that, again, I'll mention the new helical limited-slip differential....
And on the racetrack, that differential makes all the difference. The SI would handle well without it, but it may be the best handling FWD car around because of it. Carry too much speed into a corner and the SI understeers like any other front wheel drive car, but you can easily correct your mistake by burying your foot in the throttle. Yes, you can floor it. The SI's differential transmits more torque to the outside tire which has the most grip. This immediately tightens your line and gets you around the turn faster. This combination gives a firm ride without sacrificing comfort on the street. Grip is high, body roll is very well controlled.
Aside from the suspension systems, the Civic SI gets a new motor. Well, it shares its platform with the same motor as the Acura RSX Type S, but has a balance shaft for additional smoothness and "throttle-by-wire" that are unique to the SI's engine. Honda also designed a special short, single cast aluminum intake manifold for the Si with an extra large 70mm intake duct for the i-VTEC, with power boost you can feel at 3,000RPM and again at 6,000RPM. The only bad thing about the new CIvic is that the engine tend to hold onto revs when you close the throttle due to the electronic throttle control now. Originally, the SI was rated at 200HP, but the new SAE ratings system cut that number back to 197 hp at 7,800 rpm. Torque is up to 139 lbs/ft at 6,200 rpm.
As for the XRS, Toyota tries to beef it up a little more than the average sport compact sedan. THe XRS does handle fairly well with a slightly stiffer suspension system. Although only rated at 164 horsepower at 7600 rpm and 127 lb/ft of torque, it hopes to use it's weight as an advantage among other sport compact cars. It's specifically tuned for more mid-range torque and has the new VVTL-i lift system (as compared to the SI's i-VTEC) for a power boost from 6000 to 7600 rpm. The XRS features a sportier tuned suspension with higher rate coil springs and shocks and a lower ride height (a half-inch lower). The strut tower brace the XRS incorporates is not a big whoop. Anyone can slap on a strut brace. Hell, even a Toyota Echo has one available aftermarket. The larger 16 inch wheels come with Michelin performance tires doesn't impress that much as compared to the SI's option. The revised steering system of the XRS does feature a more rigid steering column, designed to improve response. A newer power steering rack was specifially desigened for the XRS to allow better steering feedback.
Overall, the XRS is much improved over the traditional Corolla. Stiff suspension and much cheaper than all the rest does indeed make this an attractive vehicle, but the only advantage I can see with the XRS is the weight ratio. THat's it. Everything else, the SI pretty much owns it. I don't know if that weight difference alone will make the XRS that much faster to the SI, but displacement owns all and with the SI's current setup -- it impresses me very much. If you're willing to shell out $25,000 with all the options