Muscle Head; Infiniti's M45 delivers pumped-up thrills in a luxury package.(Infiniti Q45 automobile information)

If we think of Infiniti's flagship Q45 as fictional weightlifter Franz, then the M45 must be Hans: a little smaller and lighter, with the same steroid-affected heart. Judging by responses to our AutoFile survey, it certainly possesses enough juice to pump you up.

``The acceleration is absolutely mind numbing,'' said one owner. A quick glance under the hood explains why. Infiniti took the 4.5-liter 32-valve V8 from the Q45 and installed it in the M45's engine bay. The result: an inconspicuous, luxury-dragster with enough cajones to light up its rear tires at will.

We learned this the hard (and fun) way at the track; the best launch technique is to quickly sidestep from brake to throttle. With 340 hp and 333 lb-ft of torque on tap from the all-aluminum engine, brake-torque starts easily smoke the tires. Get the sticky Bridgestone Turanzas to hook up, however, and the M45 delivers on its muscle-car promise: The speedo indicates 60 mph in 6.12 seconds, and the quarter-mile rips by in just 14.68 seconds at 95.7 mph.

For comparison, we piloted the 2002 Q45 (AW, March 11, 2002) to 60 mph in 6.87 seconds and covered the quarter-mile in 15.22 seconds at 93.5 mph. Our Mercury Marauder test (AW, Nov. 25, 2002) yielded significantly inferior numbers: 8.01 seconds to 60 mph and 15.99 seconds at 91.1 mph over the quarter-mile. Yes, the M45 commands an $8,300 premium over the Mercury, but this is the straight-line performance we demand of a factory hot rod. The only complaint about the drivetrain is the transmission tends to lurch and jerk when you accelerate slowly. What's more, the gearbox often refuses to hold a gear as long as we'd like, even in manual mode. Not uncommon, but questionable for a car in this price range.

The $44,855 sticker price is a source of unrest. ``Infiniti is touting this as a less-expensive alternative to the Q45, but still, $45,000 isn't exactly cheap,'' said one tester. In a market where image plays a role, does the M45's understated yet elegant styling carry enough cache?

``This is certainly not the most handsome car on the block,'' another tester said. ``But the slab sides ...