(MONEY Magazine) – Q. I've been seeing TV ads where a couple of guys ask the Dodge pickup driver, "That thing got a Hemi in it?" So what exactly is a Hemi? And do I really want one?

A. Sure, if you want to dust the neighbor's Honda. A Hemi is an engine with hemispherical combustion chambers in its cylinders. Chrysler's unique, power-boosting design achieved gearhead legend status in the '60s, when Hemi V-8s dominated the street, drag strips and even NASCAR. But when '70s pollution controls dealt a deathblow to the muscle-car era, the Hemi was benched. Now it's back, cleaned up but as strong as ever. The Hemi's dome-shaped combustion chambers burn fuel more efficiently than the flatter traditional cylinder, and this roomy design allows for nice, big intake valves to suck in lots of fuel and air. Today's version gets two spark plugs per cylinder for even more complete combustion.

First introduced in the latest Dodge Ram pickup, the 5.7-liter Hemi V-8 makes a macho noise and has mucho power, ranging from 330 to 345 horsepower, about 40% more than the larger engine it replaced. Of course, there's no free punch: The Hemi still burns more fuel than a NASA mission. Our Dodge Ram managed just 14 miles per gallon overall. A newer version shuts down half the cylinders to conserve fuel while you're cruising. Impressive technology, so-so result: We saw 16 mpg in the 2005 Jeep Grand Cherokee, a mere 17 in the Chrysler 300C sedan. Maybe the engineers should get the Hemi to run on one cylinder; it might have a shot at 20 mpg.

Q. My husband and I would like to know if it is a good or a bad idea to use our home-equity line of credit (HELOC) to buy a new car.

A. Pawning your house to buy a Ferrari, that I can understand. But in most cases, there are few compelling reasons to borrow against your home to buy a fast-depreciating asset like an automobile. Even if your rate today is lower than you could get on a car loan, HELOCs are almost always variable rate. A spike in interest rates could make that dealership financing look attractive in hindsight. Home-equity lines are typically set up for you to make interest-only payments, so unless you and your husband have the discipline to pay off the principal over the life of the car, you could end up with two unsightly heaps in a few years—one of metal, one of debt. All in all, you might want to reserve your HELOC for a wiser purchase, like a his-and-hers garage for your beloved cars.