Several engine upgrades were completed during the lifetime of these aircraft.

The gear ratios were also revised to match the engine upgrade.

Most of the deal's cost will cover structural modifications, engine upgrades, support equipment and training, officials said.

The engine upgrades of the cars range from $800 to at least $2,000,000.

The most notable difference from the previous generation is the engine upgrade.

By the time you reach the power peak at 6,500rpm the engine upgrade really makes itself felt over the standard unit.

The original 55-57 Thunderbirds came with several engine upgrades and were considered high performance cars of the day.

"The final engine upgrades have been installed and all systems reset."

Originally designed to carry eight passengers, an engine upgrade on later examples allowed the addition of two more seats.

The change in model designation reflected an engine upgrade.