Let's look at a horsepower-per-cubic-inch comparison of the engines. The 5.7 Hemi is 345 ci and the '05 version makes 345 hp (as rated in the Ram 1500), or exactly one horsepower-per-cubic-inch. In comparison, the 425hp, 426ci Hemi makes .997 hp-per-cubic-inch-making the two engines nearly equal in power output per-cubic-inch. The 6.1L (372ci) Hemi available in SRT models is not as tame as its little brother, making an amazing 425 hp, a whopping 1.14 hp-per-cubic-inch. The 6.1 Hemi is the hands-down winner. When drivability is considered, the new Hemi is also a winner. The 426 Hemi, while known for high-end horsepower, was notoriously low on bottom end torque, not so with the new version. The new 5.7 Hemi not only makes 390 lb-ft of peak torque, it also makes over 300 lb-ft from 1,200-rpm up-giving it the best of both worlds. Driving a 5.7 Hemi-powered car or truck is like launching a powerful 440 torque-monster (well, almost) with a smooth transition to the top-end Hemi power. The net result is a vehicle that marries great power with excellent drivability. We know what you're thinking, the new Hemi engine sounds powerful, but can I afford to feed it? Thanks to some pretty incredible engineering, the answer is yes.
The camshaft is a hydraulic roller providing additional power through reduced rotating friction. The cam is driven by a conventional timing chain, and the crank gear not only drives the cam, it drives the oil pump as well. | 
The trick to MDS is this electronically operated valve that allows oil pressure to pump up the exhaust lifter, allowing cylinder pressure to bleed off when the cylinder is not being used. The computer deactivates the cylinder's coil pack and injector during MDS operation, which results in a twenty-percent increase in fuel economy. The entire process happens in milliseconds and is seamless. |
One of the more impressive aspects of the new Hemi engine is its oiling system. Engine builders have long known that horsepower is available by keeping crankcase windage to a minimum. Oil from the top end of the new Hemi is directed through seven passages in the block (shown here), sending the return oil directly to the pan and keeping it away from the crankshaft. | |
We had a chance to drive an '06 Charger R/T with the 5.7 Hemi recently and were somewhat surprised by the economy advertised by DaimlerChrysler. How could a 340hp V-8 in a 4,300-pound car get 25 mpg? The answer is technology that Chrysler calls a Multi-Displacement System or MDS. The theory behind MDS is when the on-board computer senses that all the engine's power is not needed to sustain vehicle speed, four of the cylinders are dropped, and the engine runs on the remaining four, netting a 20-percent increase in economy. While this sounds good in theory, we had visions of a rough-running V-8 with four plug wires off-not a nice ride. We were amazed the MDS works so well-the transition is seamless. Not only does the engine continue to run smoothly on four cylinders, it responds well and picks the other four back up as soon as the throttle is depressed. The driver is unaware the system is working; other than noting improved economy.
The Multi-Displacement System works by electronically dropping the coil pack and the injector from the selected cylinders and utilizes special oil restricting valves in the lifter galley to force oil to the exhaust lifter of each dropped cylinder, pumping the lifter up to keep the valve off its seat so that cylinder pressure is bled off with no vibration as the dropped cylinders cycle through their compression stroke. The remaining four cylinders now work a little harder as they are both propelling the vehicle and spinning four extra pistons and connecting rods, and the net result is a 20-percent gain in economy, giving the new Hemi-equipped vehicle V-6 or even four-cylinder economy, while retaining its horsepower and torque numbers. Does it work? While we didn't see 25 mpg as the window sticker on our Charger advertised, we did find it easy to average 20 mpg during a combination of highway and city driving. We're sure that 25 mpg is attainable, just not with our lead feet. Adding to the overall economy of the package is the fact that as a function of the detonation-resistant Hemi head, the engine runs great on the recommended mid-grade fuel and does not require premium. We'll use the 20 or so extra pennies per gallon to save up for a cold-air kit and maybe an exhaust upgrade.