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A question for experienced mechanics Answered

I  know someone who has a newer model of the Dodge Ram with a Hemi V8.  It has a special gas-saving feature which turns off 4 of the cylinders when it is at highway speeds.  I was wondering if this was a practical idea to do to an old 350 small block.  I know that you could just unplug the spark plugs to 4 cylinders.  Of course, you would have to do something to release the pressure also. (I was thinking of removing the entire spark plug.) I am not very experienced in auto mechanics so I was wondering if anyone here could figure this out.  PS : This option of shutting down cylinders was also available in older Cadillacs I beleive it was called the 4-6-8, which shut down either 2 or 4 cylinders.  This engine however, was known for it's problems, which is why I was wondering about doing this.


before reading anyone's answers on here i thought that it would be a matter of the ECM not sending a signal to the injectors on four of the cylinders, there for not using excess gas when you are coasting down the highway. I highly doubt there are kits like this for a SBC motor as people who usually use these motors are trying to get the HP and torque up and not caring to much about gas milage. You may be able to buy a crate engine (engine ready to be put into a vehical) that will do what you want.

You can disconnect the spark plugs but you'll only get less mileage. Those cylinders will still be sucking fuel/air mix and your emissions will skyrocket. Bad idea.

You can't just remove a spark plug and run the engine.  The piston would still be sucking air.  Some from the carb and some thru the sparkplug hole.  That air would contain dirt water etc.  bad for the engine.  It would also be expelling gasoline/air mix into the engine compartment where it might catch fire.  Very bad for motor and you.

I don't know how Dodge does it but I have a feeling that they open the exhaust valve on the four they don't want running.  That way they are not sucking gas/air mix.  You still would suck some of the exhaust mixture back and forth into the cylinder but at least it's been filtered once.  You still have the resistance of the pistons going up and down.

What you really need is a motor that is a v8 cut in half with a clutch between them so that you could turn off the front four cylinders and disconnect them from the back 4 that you are running.

I don't know of any way to implement turning off cylinders on motors with out re-designing the motor.

"In order to eliminate the pumping losses," says Meagher, "you need to disable both the intake and exhaust valve." This results in a completely sealed, deactivated cylinder, which is essentially an air spring being acted upon by a piston. Virtually all the work put into it during compression is returned to the crank during decompression, finally giving credence to the old joke about piston-return springs. (That's nothing. Wait 'til you hear about the muffler valve...)


Well, its not sealed, it's open to the exhaust and the intake. So you would have to open both valves AND have a diverter valve that closes those 4 intakes and shunts them to a filtered vent. Or something like that.

If you seal the cylinder you will incur a loss of power due to the compression stroke requiring more energy to accomplish than the pressure returns.


A modern engine is going to have a fuel injection system, not a carburetor, so the same source of air can go in because there isn't any fuel added to it.

You're right about the injector. you can just turn it off so fuel probably wouldn't be getting in the dead cylinder.

You should read the rest of the article. As far as I know their engine really does do what they say it's doing.

Multi-Displacement System
.  As has been pointed out, it really is a different creature than an old Chevy (or Ford, or Chrysler, or ...) SB (or BB). You can't turn one into the other.

I read Re's answer, and I am thinking that the Dodge is electronic fuel-injection. So "yes" in principle, but you'd need a similar fuel-control unit.