Roger, I checked your question with a couple gearheads I know; here’s what I found out.
We aren’t aware of any conversion kits. With the right engine, the idea is possible, as we described back in 1979 in V8 to a V4 Conversion: Get 40 Percent Better Gas Mileage.
But, unless you’re a savvy mechanic and you have a really old truck, the conversion will be more trouble than it would be worth. First off, for any recent car or truck with a V8 engine, there are going to be complicated computer controls on the fuel-injection system — you’d have to be a professional mechanic and/or have access to the manufacturer’s programming codes.
Second, even if you could crack those codes, the end result would be sluggish.
“Power would probably stink, and you’re likely better off to buy a small car with a 4-cylinder engine,” says Richard Backus, editor in chief of Gas Engine and Motorcycle Classics magazines. “It can be done, absolutely, but I seriously doubt it’s worth the effort.”
Another problem: In using less power to propel such a large vehicle that originally warranted a V8, at times you won’t really save gas.
“Running a V4 at full throttle under load will suck more gas than a V8 at less than half,” says GRIT editor Hank Will.
The idea isn’t without warrant, though. General Motors is using what it calls Active Fuel Management in its vehicles with V8 engines, which shuts down some or even half of the engine’s cylinders when the vehicle doesn’t need all the oomph. Basically, it creates real-time transitions between 8-, 6- and 4-cylinder operation.
“You get the thing rolling on all eight cylinders and when conditions are right, some cylinders start shutting down,” Will says.
It’s estimated that most trucks only need their full power 25 percent of the time they run. That means significant improvements in gas mileage (about 5 to 7 percent) can be had by matching the engine’s output to the power the truck actually needs in the moment.
You can read more about this technology in Variable Displacement for Better MPG and Save Gas: Cylinders Get Smart.
So, while the DIY conversion idea sounds intriguing, if your goal is to improve gas mileage and save money, you’ll get there faster by simply buying a more fuel-efficient ride.
