Even a straight pipe (on motorcycles they are called drag pipes) has some pressure in the tube.

I am running a straight pipe on my Enya R1.20 in a biplane. But I pinched the end a little and drilled a number of holes in the end as well. The pressure fitting is about 1.5 inches from the exhaust pipe beginning at the engine.

Usually the 4 stroke engines prefer a little back pressure in the exhaust. I remember simply running a 1/4 inch bolt near the end of my drag pipes to get the requisite back pressure, for the engine to work good. Plus it kept a police officer from running his nightstick down the pipe and then giving me a ticket for running straight pipes.

You can see the straight pipe he on my plane. At the time I wasn't really interested in taking pics of the exhaust system.