NEW YORK (CNN/Money) - Two minivans with standard side airbags earned "Best pick" ratings in the Insurance Institute for Highway Safety's first side-impact crash tests on minivans. Two other minivans without side airbags were rated "Poor."The Toyota Sienna and Nissan Quest earned IIHS's highest rating of "Good" for side impact protection and the added designation of "Best Pick," indicating that they performed particularly well. The Honda Odyssey also earned a rating of "Good" in the test.The Ford Freestar and Mazda MPV, when tested without their optional side airbags, were rated "Poor," the Institute's worst score.When the Freestar was tested with side airbags installed, it earned a rating of "Acceptable," the second-best of four possible ratings.The minivans that earned "Good" ratings in the test were equipped with curtain-style side airbags designed to protect the heads of occupants in all three rows of seats.The Institutes' side impact test uses a 3,300 pound barrier pushed into the side of the vehicle at 31 miles per hour. The barrier is shaped like the front end of a typical pick-up truck or SUV. Crash-test dummies, about the size of small women, were seated in the driver's seat and the seat behind the driver.Crash tests performed by the U.S. government use a barrier that is lower to the ground, simulating an impact from a car. An impact from an SUV or truck, like that simulated by the IIHS test, poses a much greater risk of head injuries."In each of the top-performing minivans, the curtain airbags did a good job of keeping the dummies' heads from being struck by the intruding barrier," said Institute chief operating officer Adrian Lund. "This is important because head injuries are factors in many serious side impact crashes, especially when the striking vehicle is a pickup or SUV with a tall front end."The minivans that performed well on the test also had structures that resisted crushing into the passenger compartment.The head of the dummy in the driver's seat of the Mazda MPV was struck by the barrier as it crashed into the minivan. While the impact was not hard, the Institute said, any impact should be prevented."Plus the MPV's structure was marginal, so there was more intrusion that contributed to high forces on other parts of the dummies," said Lund. "In a crash of similar severity in the real world, it's likely that a driver would suffer internal organ injuries as well as rib fractures. The rear passenger possibly would sustain a fractured pelvis."The MPV is available with optional side airbags. In cases where a safety device such as side airbags is offered as an option, the Institute tests the vehicle without the device but will re-test the vehicle with the device installed if the manufacturer requests it and pays for the additional vehicle.In a statement released by Mazda, the company said it stands behind the safety of its minivan."In contrast to IIHS' findings, the U.S. government's National Highway Traffic Safety Administration (NHTSA) awarded the Mazda MPV a rating of five-stars, the highest possible score, in both frontal and side impact tests," Robert Davis, senior vice president for research, development and quality for Mazda North American Operations, said in the statement. "And, of course, the MPV meets or exceeds every NHTSA-required safety standard."The Chevrolet Uplander and Dodge Caravan/Chrysler Town & Country minivans were not tested because they are undergoing design changes to improve their side impact safety. They will be tested early next year, the Institute said, along with the Kia Sedona minivan.Vehicle photos and details:Toyota SiennaNissan QuestHonda OdysseyFord FreestarMazda MPVFeedback