When it comes to powertrain philosophies, these two automakers are very simpatico. Both the XF and the M buck the trend toward engine downsizing. Infiniti still offers a V-6 (now 3.7 liters, in the M37), but it tossed out the old 4.5-liter V-8 in favor of a new, direct-injected 5.6-liter unit. Output has surged from 325 hp to 420 hp, and torque jumps by 81 lb-ft to 417 lb-ft. As you might expect, acceleration is strong, with 60 mph reached in 5.1 seconds and 100 mph in 11.6. You might not expect fuel economy to improve, but it has, to 16/25 mpg city/highway. At least part of the credit goes to a new seven-speed automatic that replaces the previous five-speed. The M56 has a drive-mode selector switch that offers a choice of snow, eco, normal, or sport; in anything but sport, the computer chips try to get you to feather-foot the throttle, and they are reluctant to let the transmission downshift. Naturally, we left it in sport mode.
Jaguar has also recently brought out a fresh, larger V-8. In the 2010 XF, the new 5.0-liter V-8 replaced the old 4.2-liter in all but the base car; for 2011, every XF will get the 5.0-liter. The Jag's bigger engine also ushers in a major power increase, with output going from 300 hp to 385 hp in normally aspirated form. With only six forward speeds, the Jaguar falls short of the Infiniti's highway fuel economy by 2 mpg but matches its city figure. With both gearboxes, we liked the transmissions' ability to match revs on downshifts and to hold a manually selected gear indefinitely. Like Infiniti, Jaguar offers a sportier transmission and throttle calibration, but even in the standard setting the XF's throttle tip-in is very aggressive.
Ultimately, though, the XF's response isn't quite as frenetic as the M56's. 60 mph arrives in 5.5 seconds and 100 mph in 12.6 -- although the Jag is half a second quicker from 30 to 70 mph. And the direct-injected engine emits a wonderfully throaty growl when you step on it. If you want more performance, Jaguar still has more to offer, in the form of the supercharged, 510-hp XFR. But that car -- in the grand tradition of BMW M5, Mercedes-Benz E63 AMG, and Audi RS6 models -- costs silly money (try $80,000), something that the Japanese automakers still don't go for.
Speaking of prices, our 2010 XF Premium ($57,000) started within a whisker of our M56 ($58,415), which is a 2011 model. Both cars stayed within $1500 of each other when optioned up -- the XF at $63,875 and $64,980 for the M56. Both were equipped with navigation, a rearview monitor, park assist, keyless ignition, Bluetooth, and heated and cooled seats, as well as the expected amenities, so the value proposition is a wash.