4/8 SEQUENCE QUESTION

**w5blt** · 10-21-2004, 12:24 PM

I was just wondering, when 4 of the 8 cyllinders shuts down, which 4 are shut down? Or, is it variable? Anyone know exactly how that works?

Also, when they do shut down, what really happens? I know that the fuel is shut off, but, does this mean that the cylinder is just sucking air in and expelling it? Is the spark still firing?

There must be a web site somewhere tha explains the intermost workings in detail. If anyone knows, that would be real interesting.

**henry** · 10-21-2004, 12:57 PM

The MDS system is set up to trap an exhaust charge in the cylinder by keeping the exhaust valve closed (while keeping the intake valve closed as well) and then using that trapped exhaust charge as a gas spring for the piston to push against. This reduces pumping losses.

The intake and exhaust valves are kept closed by solenoids that shut down oil pressure to the eight appropriate intake and exhaust lifters. The lifters then collapse and the camshaft can't open the valves.

It will always shut down cylinders 1, 4, 6, 7.

It does not stop spark in the shut off cylinders. It will shut down fuel and air to those cylinders.

Henry

**w5blt** · 10-21-2004, 01:17 PM

Originally Posted by henry

The MDS system is set up to trap an exhaust charge in the cylinder by keeping the exhaust valve closed (while keeping the intake valve closed as well) and then using that trapped exhaust charge as a gas spring for the piston to push against. This reduces pumping losses.

The intake and exhaust valves are kept closed by solenoids that shut down oil pressure to the eight appropriate intake and exhaust lifters. The lifters then collapse and the camshaft can't open the valves.

It will always shut down cylinders 1, 4, 6, 7.

It does not stop spark in the shut off cylinders. It will shut down fuel and air to those cylinders.

Henry

Henry,

Excellent description. It's appreciated. I'm interested in the shutting down of the oil pressure to the exhaust lifters. I wonder if this would make any long term mechanical impact on the engine parts?

Also, it's interesting how they came up with 1-4-6-7 vs anything else. There must be a method to their madness.

I have noticed an intersting point. Someone recently asked me if I could tell when they "kicked out or back in". At the time, I said no, the transition was so smooth, it was hard to detect. The next time I was up on the freeway, I put it INTO cruse control and watched the tach. Sure enough, when it shuts down the cylinders, you'll see a slight increase in rpm.

**Rev.Hammer** · 10-21-2004, 01:28 PM

I think that is because that would cause it to function as a v4 engine with those cylinders firing 90 degrees apart, and on opposite sides of the crank, thusly reducing vibration harmonics.

**w5blt** · 10-21-2004, 01:37 PM

Originally Posted by HammerFoot

I think that is because that would cause it to function as a v4 engine with those cylinders firing 90 degrees apart, and on opposite sides of the crank, thusly reducing vibration harmonics.

In that case, they their efforts was a success. It's a totally smooth transition.

**kb9mzx** · 10-21-2004, 02:08 PM

Originally Posted by w5blt

Also, it's interesting how they came up with 1-4-6-7 vs anything else. There must be a method to their madness.

They shut down 1-4-6-7 because that's the odd-numbered cylinders in the ignition sequence. Since almost before dirt, Mopar v8's have fired 1-8-4-3-6-5-7-2, which balances front-to-back and side-to-side on the crankshaft, minimizing vibrations.

-Galen

**henry** · 10-21-2004, 02:30 PM

Originally Posted by w5blt

I have noticed an intersting point. Someone recently asked me if I could tell when they "kicked out or back in". At the time, I said no, the transition was so smooth, it was hard to detect. The next time I was up on the freeway, I put it INTO cruse control and watched the tach. Sure enough, when it shuts down the cylinders, you'll see a slight increase in rpm.

You should not see any increase in RPM unless the torque converter unlocks.

I can tell the difference - 4 cyl mode while it does not translate INTO the cabin as a different vibrational mode it does change the exhaust note somewhat. I'd describe it as a hollower slightly metallic sound rather than the deeper baritone of the full V8 exhaust.

Henry