We are talking about model airplane use here where there is a lot of air blowing past the carb. We are also referring to the stacks that are cut off at an angle.

You will pick up a couple hundred rpm but it is not due to any tuned effect of the intake charge. Sometimes, you can never get a gas engine to run right without one. They stabilize the flow of air into the carb. There are velocity stacks that do offer a tuned effect but they are rarely used on a model airplane because you normally are running all different throttle settings and the tuned effect would only be for wide open throttle.

As a previous post said, thet will stabilize the airflow into the carb and also prevent excess fuel from blowing down the side of your plane.

The majority of the time, the cut - off part angles towards the front but not always. You have to experment sometimes and turn it to different angles to see where the engine runs best.

For airplanes, they are available from Tower, Horizon, Carr Precision, etc.