ORIGINAL: raidermike67

I believe<?>theyincrease the ethenol blend in the winter. Ethenol does not produce as hot of a spark as pure gas, this results in a decrease in power and mileage.

The whole point of this exercise was primarily to explain and demonstrate the 2005 Hemi's new Multiple Displacement System (MDS) in an enlightening manner more fun than your average Powerpoint presentation. We got to see the four solenoid valves--mounted directly to the engine block down in the valley of the V8--that disable cylinders 4 and 6 (the two inners on one bank) and cylinders 1 and 7 (the two outers on the other bank) under light engine loads. This switch-over is totally transparent to the driver, because the V4 operation utilizes even 180° firing intervals. The only thing the driver feels is joy when he calculates how much less fuel he's using on a long drive. Chrysler says the Hemi in the 300C sedan is rated for 25 mpg on the highway.

Here's how MDS works: There are four special 2-piece roller lifters for the cylinders that get disabled. The two pieces are concentric--there's an inner portion surrounded by an outer sleeve. The two parts are locked together by two spring-loaded pins when the engine is in 8-cylinder mode and all the pushrods are operating their valves normally. Under light loads, however, the engine management computer signals each solenoid valve to open and apply high oil pressure to its respective exhaust- and intake-valve lifters. This action forces the two pins in each disabling lifter to unlock the two concentric parts. (To help minimize oil contamination, there’s a magnet at the bottom of each solenoid unit to collect ferris debris.)Now, the outer sleeve continues to follow the camshaft lobe, but the inner portion of the lifter just floats. This results in no pushrod movement, so the valve springs keep the valves closed. Spark and fuel for the respective MDS cylinders are shut off, and now the engine is in 4-cylinder mode.

Yes, it seemed strange to us, too, that the valves are held shut while the piston continues pumping. Doesn’t that create more load on the other four cylinders? Actually, no. Ken explained that a combustion event takes place just before the valves slam shut (exhaust valve first, to trap the hot gases). So there’s nothing in the cylinder during the fourth stroke except spent exhaust. It’s as if the piston pumps “against a spring,� he said. Well, initially, that’s true. But during extended V4 operation, the gases in each deactivated cylinder escape past the rings, and the pistons just continue pumping air.

Naturally, Chrysler had to ensure robust MDS operation for proper performance, durability and customer satisfaction, among other criteria. So the new Hemi was tested with oil ranging from 10 to 50 weight at low and high temperatures and under a variety of pressures. Once satisfied, the engineers settled on 5W-20 grade, and the minimum and maximum oil pressure limits for cylinder deactivation and reactivation were established at 75 psi and 18 psi, respectively.

Switching from eight cylinders to four decreases the frequency but increases the amplitude of engine vibrations. So the techies had to come up with a solution. They found it in the form of a torsional torque converter damper acting on the turbine side, in combination with a moderate amount of torque converter slip--but not so much slip that it negates the fuel economy benefits of running on just four cylinders. Together these two strategies absorb the V4’s harsher firing pulses, making the cylinder-deactivation transition seamless.