Now my Z feels slow!!

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Your 240 existed during a time where "power" meant a clunky V8 that would destroy a cars balance unless the arse end weighed so much that it balanced it and then the car wasn't very "sporty" in the corners.

With the advent of advanced materials (fiber-glass, carbon-fiber, etc) and V8's that are lighter than V6's, (BMW's 4.4L, for example, weighs several kg less than their former 3.0 in the E46, respectively, and the LS7 only weighs @450# fully assembled), "light" sporty cars are expected to put down enough power to do double-duty from stoplights and at the drag-strip.

Technology has allowed for the advancement of the car outside of its original class, and that's where the rub is. The "muscle" and "pony" cars have been aimed at straightline performance for years, and now the sports-car is having to learn to be fast in a straight line. Similarly, the Camaro SS and pre '11 mustang GT's were laughed at for their "poor" handling and not pulling close to 1g on the skidpad. They are having to learn how to install a suspension on those cars. Over-all "performance" seems to be the new name of the game.

I too experienced the Z accelerating "deceptively" fast on a test-drive. It got me a fly-by with the lights on from an undercover car.
I think it has to do with the wonderfully FLAT! torque curve that Nissan has managed through some tuning magic. My LS1 had a similar (but not as flat) torque curve and it felt slower than my LT1, which hit more like a hammer. I raced a few LS1's (again, in my back-yard) with my LT1, and am here to say that is not the case. Even cammed LT1's couldn't hold a candle to the LS1 based on my experiences. There is a lot to be said for how far Nissan has stretched the life-span of the VQ. It makes me think of GM's 3800. For what it's worth, none of the 3 370's I was in or drove had any "high rpm harshness" that the magazines get after them for. I don't even know what that term means, really. They didn't like the exhaust note, I guess.