Robert,

There is a minimum length requirement otherwise you’ll end up with warped exhaust valves from cyclic cooling. For estimating drag, a rule of thumb is to imagine each pipe extended 12-15 inches longer than it’s physical length and get out your drag tables and graphs for a round pipe of that diameter and length.

In an O-200 installation, two of the pipes are aerodynamically “shielded” behind others so you’d only need to account for the addition of two bucket-loads of drag instead of four; keep in mind that the drag of a round cylinder is about 300 times more than the drag of a faired airfoil-shaped cylinder.

If you just wanna go, straight pipes will “go”; if you wanna go fast, point them aft. In all cases (even straight pipes) a secondary structural attachment is recommended.

Just my $.02