The new engine is believed to be a twin-turbochared 4.0-litre V8, and could well see service in the C190 when it's launched at the end of next year. That model was recently caught testing in Europe.
This test mule was also spotted alongside a coupé version of the SLS.
The prototypes are both wearing camouflage around their front and rear bumpers, as well as on the bonnet of the Roadster. This is believed to be part of cooling tests for the new engine.
The C190 will be the successor to the SLS AMG and is mooted to have a price tag of between £80-£120,000.
Despite the new engine being tested in an SLS, it's expected that Mercedes won't replace the 591bhp 6.2-litre V8 engine which currently powers the coupé and roadster models of that car.
It is believed that following the C190's launch, Mercedes will gradually phase out the current SLS.