The M35 should be fine for most drivers, as Infiniti tuned its V-6 this year for more high-revving horsepower — an impressive 303 hp, to be exact — but less torque, which is the low-end grunt you feel when pulling away from a stoplight. Even so, I found gobs of fluid power with no peaky bursts or sluggishness starting out, and under hard acceleration the high-pitched exhaust note should be music to any gearhead. The engine revs a bit more freely but doesn't feel noticeably stronger than last year's 275-hp M35, and dollar for dollar, it still provides a richer powerband than competitors like the BMW 528i or Audi A6 3.2. Even Hyundai's 3.8-liter Genesis, an impressive bargain alternative for this segment, feels less spirited.

This year's rear-wheel-drive M35 gets Infiniti's new seven-speed automatic; it moves gas mileage up a few ticks versus last year's version, which had a five-speed automatic. Other models, including the M35x I tested, carry over the five-speed. Though it's a gear or two short of the competition, that transmission remains one of my favorites. It holds lower gears tenaciously at lower speeds, refusing the mileage bait of an early upshift while you're still on the gas. Shifts are heard in engine pitch but rarely felt, and highway kickdown is quick and free of gear-hunting. Around town, lower gears arrive almost instinctively — accelerating out of a corner or pulling around a semi, for instance, you're rarely caught flat-footed in a higher gear. Jaguar's ZF automatic comes to mind in the same league, and I hope Infiniti preserved this sort of performance in the M35's seven-speed. Every M this year gets a Drive Sport mode that holds lower gears longer for more responsiveness, but it's hardly necessary.

If the M35 is confident, the M45 is authoritative. Don't let the mere 22-hp difference fool you — with an extra liter of displacement, the M45's V-8 boasts a torque peak that's 800 rpm lower, with 74 more pounds-feet of twist. It trades the M35's finesse for brute hang-the-tail-out power at pretty much any speed. In my brief time with the M45, years ago, I found the acceleration unmanageable at times. It would come in spurts, particularly through second and third gear, giving the car a restless feeling on city streets. If your commute involves roads that are open enough for the M45 to stretch its legs, though, it could be serious fun.

The M35x and M45x utilize an all-wheel-drive system that transfers power forward or backward electronically. It can split power from 50/50 front/rear all the way to 0/100, with all the power going to the rear wheels to optimize handling. Infiniti sums up the system's convoluted name with the equally convoluted acronym ATTESA E-TS — which, were it an actual word, would be a great way to dump all your T's in Scrabble.
Drivetrains: M35 vs. M45
M35 M45
Engine 3.5-liter V-6 4.5-liter V-8
Horsepower (@ rpm) 303 @ 6,800 325 @ 6,400
Torque (lbs.-ft. @ rpm) 262 @ 4,800 336 @ 4,000
Transmissions 7-speed auto (M35); 5-speed auto (M35x) 5-speed auto (M45 and M45x)
EPA gas mileage (city/hwy., mpg) 17/25 (M35); 16/22 (M35x) 16/21 (M45); 14/20 (M45x)
Fuel type Premium (recommended) Premium (required)
Source: Automaker and EPA data

Four-wheel-disc antilock brakes are standard, and the systems are identical on the M35 and M45. Last year's M35 had a somewhat mushy pedal for this class, but the M45 I tested in 2006 had more linear braking.