[petri]

Sorry for bad English, I meant the air cooling cast stripes form outside of cylinders. 911 is a car which didn't need so much more of rear downforce over what you can achieve with rear spoiler, so I think factory didn't care of it too much. Understanding of underbody aero in 70ies was obviously by any means harsh. For early 911s the suspension adjustment to put front little bit lower than rear partly made the underbody aero effect in some point. But how far from understanding they were 917 development tells: aerodynamically designed longtail was worse than later developed short tail which probably by accident created some more downforce for the rear.
However I got interested of the idea to use underbody aero for the better front tyre grip as they have done in 956. Very sophisticated nice diffuser is necessary if there is a rule diminishing its maximum volume. But if there is no rule for underbody aero, it is obvious that most effective arrangement can be done with full length aero system. And in such setup the main task for he rear part is to have continuously growing sectional dimension. I think the canal arrangement between rear tyres and around the engine can be done and it doesn't have to be super finished by its form and surfaces. The most important part for the precise and fluent airflow is floor area under driver in front of engine and second important under front axe, and around transmission. Under and rear of the engine comes after these, so their importance for aerodynamical precision is not so important but the growth of dimension towards the rear.