I believe pre-production is done to test and refine the manufacturing process ahead of regular salable production for the consumer begins and to also produce CTF cars for engineers and staff employees to drive and test after the car is revealed but before regular production begins. Basically its' just a test run of everything. Pre-producton cars are usually sent to the crusher after they have served their purpose since they are not legally salable cars.
Here's a post with a great explanation of GM's various development stages and timeline.
http://www.gminsidenews.com/forums/f...-stages-96009/
Quote:
GMI Goes Inside: GM Prototyping Stages
GMI goes through the phases of GM development.
www.gminsidenews.com
September 29, 2010
By: Nick Saporito
Recent spy shots of upcoming General Motors products have spurred a lot of debate here on GMI. From the recent shots of the “Cadillac ATS mule” to last week’s Camaro Convertible shots, confusion and debate sets in every time a spy shot is posted. Debate about what the vehicle is and how close to production said vehicle may be are the most prevalent conversations. GMI has put its sources together to generate a detailed account of each vehicle prototyping stage GM products go through before they can arrive at dealership lots.
After initial engineering work the first stage an upcoming GM vehicle goes through is the “mule” stage. During this stage they typically take a donor car and strip it of most of its components and then refit the car with components for the new car. In some cases they have to modify the body of the vehicle to fit the components of the new vehicle. Mules are always hand-built units intended for early pre-development and occur nearly a year in advance of more realistic prototypes. The recent Cadillac ATS shots were of it in the mule stage. The donor car in that case was obviously the Cadillac CTS. Mule testers are typically built at GM’s Pre-Production Operations (PPO) facility.
After the mule stage, future product programs enter the Integration Vehicle Engineer Release (IVER) phase. IVER testers are also built at the PPO on a modified assembly line. Typically each station on the PPO assembly line represents 5 to 10 manufacturing stations from the vehicle’s normal assembly plant. The purpose of the IVER stage is to test all electronic components of the vehicle and ensure that it is “running” correctly. These are the cars with the mis-matched bumpers, and non-production tail lamps.
Next up is the Production Process Validation (PPV) phase. PPV cars are built at the vehicle’s designated assembly plant with the intent on testing the manufacturing process. After the PPV cars are assembly, they are also tested to identify the current level of functionality.
The last non-sellable phase of development for a vehicle is the Manufacturing Validation Build (MVB) stage. The first stage is MVBns. During this process the vehicles are, again, built at the designated assembly plant and only last minute tweaking is done to the vehicle at this point.
After the round of non-sellable MVB units, the program enters the MVBs stage. These are test vehicles used to verify everything is ready for regular production. MVBs vehicles can be sold. The recent shots of truckloads of Camaro Convertibles, shot by a GMI member, were of MVB units.
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