Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| Quattroporte IV | Units Produced | Production Period | Engine Capacity | Power | Max Speed |
| --- | --- | --- | --- | --- | --- |
| 2.0i V6 24v | 587.0 | 1994–1998 | 1,996 cc | 287PS (211kW) @ 6500rpm | 255km/h (158mph) |
| 2.8i V6 24v | 668.0 | 1994–1998 | 2,790 cc | 284PS (209kW) @ 6000rpm | 255km/h (158mph) |
| 3.2i V8 32v | 415.0 | 1996–1998 | 3,217 cc | 335PS (246kW) @ 6400rpm | 270km/h (168mph) |
| 2.0i V6 Evoluzione | 200.0 | 1998–2001 | 1,996 cc | 287PS (211kW) @ 6500rpm | 255km/h (158mph) |
| 2.8i V6 Evoluzione | 190.0 | 1998–2001 | 2,790 cc | 284PS (209kW) @ 6000rpm | 255km/h (158mph) |
| 3.2i V8 Evoluzione | 340.0 | 1998–2001 | 3,217 cc | 335PS (246kW) @ 6400rpm | 270km/h (168mph) |
Question:
When were 668 units produced?
Answer:
1994–1998