Our spy photographers continue to capture test mules of the upcoming, next generation Porsche 911, which is internally referred to as the 991, ahead of the sports car world premiere at the 2011 Frankfurt Motor Show in September.

We have already shown you the regular 911 Carrera models in both coupe and convertible format as well as the 911 Turbo and what we believe is the 911 Turbo S coupe, but now we have some fresh spy shots of the 911 Turbo Convertible with its top down, caught during hot weather testing.

If we’re right about the Turbo S model featuring quad exhaust tail pipes instead of the regular Turbo’s two large single units on either side of the rear bumper, then this prototype could be wearing the more powerful version of Porsche’s force-fed flat-six engine delivering an estimated output of around 550-horsepower.

While we already know that Porsche will show the regular 991s in Frankfurt, it is believed that the Turbo models may make their world premiere later on, possibly at a North American show.

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