Both you and Dick are right. However, to say it is negative pressure pulling the air though is not seeing the whole picture, there is both a positive and negative pressure so the air is both pushed and pulled through. I used prop blast because that is what the air flow from the prop is called, I did not mean to say that it blasts through. In fact this area of the prop does not produce a huge amount of air flow. Still, many early radials did not have baffling, either way a well designed cowl allows air to go both through and around. Take a look at the Gee Bee racer, the cowl is almost as big as the prop, but if you look closely you see that the opening is actually much smaller and most of the air is flowing aound the cowl.