The Humvee fell out of favor in Iraq and Afghanistan as homemade bombs, the biggest killer of American troops, ripped through its light armor and turned it into a death trap.

But recent blast tests show that Humvees built with the new chimney could provide as much protection as some of the heavier, and more costly, mine-resistant vehicles that have replaced them in many uses.

And if the final tests go well, the invention could save billions in new vehicle costs and restore much of the maneuverability that the Army and the Marines have lacked in the rugged terrain in Afghanistan, military officials say. Engineers say the chimney, which rises through the passenger cabin, releases some of the explosive gases — traveling at twice the speed of a fighter jet — that have mangled and flipped many of the vehicles.

Pentagon officials have said little about the 11 blast tests so far, in which the prototype vehicles are engulfed by a cloud of smoke, dust and fire, but the passenger cabin remains intact.

Dr. Leo Christodoulou, who has overseen the tests for the Defense Advanced Research Projects Agency, said in a written statement that the changes represented a “significant improvement” over the classic Humvees.

He said the new design also provided safety levels comparable to the smallest mine-resistant vehicles, which can weigh twice as much as the Humvees, and might be useful in protecting other military vehicles.

John M. McHugh, the Army secretary, recently told a Senate committee that the new approach held “a great deal of promise, and it’s exciting.” He said commanders had been reluctant to send Humvees off bases in Afghanistan “because of the problems with survivability.”

The chimney was designed by a small Maryland firm, Hardwire L.L.C., which is working with AM General, an Indiana company that has built 270,000 Humvees since the mid-1980s. Hardwire is run by a colorful group of aeronautical engineers who say they took a fresh approach to evaluating how to make the vehicles safer.

George Tunis, the company’s chief executive, likened the chimney to an exhaust vent on a rocket.

He said that rather than just piling on more armor to absorb the blasts, as has been typical in the past, the idea was to disperse as much of the explosive energy as possible.

Tests show that the explosive gas from a roadside bomb can accelerate to speeds as high as Mach 4 in less than a millisecond, Mr. Tunis said, or far less time than it takes to blink an eye.

Mr. Tunis said he was inspired to work on the vehicle’s safety after a chance meeting with Octavio Sanchez, a Marine staff sergeant who lost a hand and was badly burned when his Humvee blew up in Iraq in 2005.

Sergeant Sanchez said Friday that he told Mr. Tunis that small safety improvements might have saved his hand, “and I think that turned a light bulb on for him.”

Mr. Tunis said the chimney, which is hidden next to a gunner’s turret atop the Humvee, is the biggest change. But like the mine-resistant vehicles, the Humvee prototypes have V-shaped steel bottoms to deflect parts of the blasts.

Mr. Tunis said his engineers were inspired by sports gear in making other changes.

He said that Dyneema, a thin fiber that links surfboard riders to kite sails, is so strong that it is used in bulletproof vests, and that his team sandwiched plates of it between metal panels throughout the vehicles. It also adapted a rock-climbing device to drop the gunner into the vehicle when a blast occurs.

The Pentagon will conduct five more blast tests, and the Army could request bids this fall for a new version of the Humvee.

Several companies, including Oshkosh, BAE Systems and Textron, are expected to bid. Charles Hall, AM General’s chief executive, said his company had also been working with Plasan, an Israeli armor manufacturer, on another prototype.

But he said in an interview that the blast tests “demonstrate very clearly” that the chimney could offer protection well beyond what the Army was expected to seek.