When did Porsche stop producing air-cooled engines in the 911?
Below is an instruction that describes a task, paired with an input that provides further context. Write a response that appropriately completes the request.

### Instruction:
When did Porsche stop producing air-cooled engines in the 911?

### Input:
The Porsche 911 (pronounced Nine Eleven or in German: Neunelfer) is a two-door 2+2 high performance rear-engined sports car introduced in September 1964 by Porsche AG of Stuttgart, Germany. It has a rear-mounted flat-six engine and originally a torsion bar suspension. The car has been continuously enhanced through the years but the basic concept has remained unchanged.[1] The engines were air-cooled until the introduction of the 996 series in 1998.

### Response:

The Porsche 911 (pronounced Nine Eleven or in German: Neunelfer) is a two-door 2+2 high performance rear-engined sports car introduced in September 1964 by Porsche AG of Stuttgart, Germany. It has a rear-mounted flat-six engine and originally a torsion bar suspension. The car has been continuously enhanced through the years but the basic concept has remained unchanged.[1] The engines were air-cooled until the introduction of the 996 series in 1998.
Porsche stopped producing air-cooled engines in the 911 in 1998 with the 996 series.