The drivers rev their engines.

[Topstudent]

Two cars roll up to the starting line. Just then, a countdown starts. The drivers rev their engines.

Would this sound odd with 'their' since it's their cars' engines and not theirs or is it natural?

 

[emsr2d2]

It's perfectly natural. When we speak directly to someone in this way, we word it as if the engine belongs to the driver (which it does, indirectly).

We don't say "Drivers - start your cars' engines!" We say "Drivers - start your engines!"

 

[Tdol]

I'd use Then rather than Just then.