Speaking of choices, we have a little bit of history happening in the
automotive industry as well and it’s called Displacement on Demand,
Active Fuel Management, Multi-Displacement system, Variable Cylinder
Management and the list goes on. The bottom line is, technology that
changes the number of cylinders a vehicle runs on can use just 25-30
percent of the available horsepower of your engine to keep your vehicle
moving.

I remember something from Sir Isaac Newton who said “a body in
motion stays motion” or something like that. Anyway once you are done
smoking the tires and are up to speed it takes very little horsepower
to stay there. So if you could take out four of the eight cylinders of your engine it
forces more load on the running cylinders that opens the throttle more
fully and allows the engine to breathe.

Better airflow reduces the drag on the pistons and associated pumping
losses. The result is improved combustion pressure, more efficient
power unleashed on the pistons (those that are working) as well as
improved highway and cruising mileage. You get the double bonus of
spending less money on gas but still have the horses when you need
them.

Since GM was the ultimate pioneer of this technology in 1981 with
the V8-6-4 Cadillac engine (that only lasted for a year except in limos
until 1984), I consider it deserving of first bragging rights. So it’s
only fair to talk about GM engines in this article.

But we can’t talk Jack unless we know how it works so I will give you
my best attempt at clearing up the confusion. For those of you who
never read a book, only Cliff’s Notes, here’s a quick summary of
Displacement On Demand (DOD). In a pushrod engine the hydraulic lifters
are collapsed by using solenoids to alter the oil pressure delivered to
the lifters. In their collapsed state, the lifters are unable to
actuate their pushrods resulting in valves that cannot be actuated and
remain closed.

For you techno guys: The exhaust valve is prevented from opening after
the power stroke and the exhaust gas charge remains in the cylinder.
Following the exhaust stroke, the intake valve is kept closed as well.
The exhaust gases trapped in the cylinder are compressed over and over
again and act like a gas spring. By using the running mates of these
cylinders the compression of the exhaust gas in one cylinder is counter
acted by the decompression of the retained gases in another.

For all of this to work (in a V8) you need 4 solenoids and an activation manifold (Figure 1) along with special lifters (Figure 2).
Typically the roller rides on the cam and pushes up the pushrod, rocker
and opens the valve. Eaton designed a lifter for GM that hydraulically
lashes the inner and outer portion of the lifter, so there is an
internal and external body that can be engaged and disengaged by oil
pressure via a locking pin.

When there is hydraulic (oil) pressure on the lash pin it detaches the
outer body – this is then an activated lifter. So the lifter rides up
and down like a telescoping device as opposed to moving the pushrod up
and down (Figure 3). When there is no hydraulic oil pressure the lifter is deactivated and it operates as a normal lifter would.

The most popular engine presently is the GM 5.3L Gen IV engine with the deactivation ports built directly into the valley (Figure 4)
over which either the manifold with solenoids (Figure 1) or a plain
cover can be installed so that the engine may be used with or without
DOD. Remember, if you use a DOD manifold on a plain lifter engine
nothing will happen, and if you use a plain cover on a DOD engine it
will act just as if it is a non-DOD.

You’ll notice in Figure 1 an inset photo of a small screened filter.
You will have to check this filter carefully for contamination during
the reman process.

According to GM if you replace an engine you need to replace the entire
DOD manifold and solenoid assembly. It comes with a gasket since it is
located under the solenoids that are riveted to the cover. However if
the cover gasket should leak or need to be replaced for a DOD engine
the recommendation is to cut the tabs on the perimeter of the solenoids
and use the plain non-DOD cover gasket. I leave it up to you but the
manifold assembly is quite expensive. When replacing the DOD lifters
make sure your credit is good, I received quotes of as high as $80
each, but at least you only need 8.

So that is the quick overview of the DOD-Active Fuel Management system
from GM, be aware they are out there in big numbers and you will start
seeing them soon. Be prepared to make some history of your own.

Roy Berndt has decades of machine shop experience. He is the Program Manager for PROFormance Powertrain
Products, a PER in Springfield, MO. [email protected]