Vent screen for cabin heat scat tube

I'm building a -7A with an O-360 (horizontal induction). I'm working on the heat muff assembly, and see that OP-54-4 step three says "For proper carb heat performance, a vent screen must be installed over the cabin heat tube at the location shown in Figure 3." Basically, you are supposed to put a screen at the outlet of the heat muff on the cabin heat side. Does anyone know why that's necessary "for proper carb heat performance?" I understand the idea of not including a screen on the carb heat side, just not sure why the cabin heat side has an effect on carb heat performance.

I am sure they are doing this to slow the air down. The 2" tubing allows too much airflow for the amount of heat generated by the muff. Reducing the airflow makes the muff more efficient in heating the cabin. I followed Robbins Wings advice and put a 1.25" reducer on the initial take off location on the baffling.

Larry

I ended up just putting an aluminum screen mesh over the aluminum firewall intake to the heater muff Then slide the scat tubing over it..
With the adjustable door and a double heat muff I have no problem with any heat plus it keeps the wasp and other insects from reaching the cabin... Also have the aluminum screen mesh on the NACA scoops to keep wasp from building nest in there... It would be really bad after takeoff to have a few wasp get mad and come in the cockpit
Jack

I put a screen over the inlet on the back baffle wall.

I'm not terribly interested in baking insects & such in the summer, and I have sufficient heat for flight down to about -10?F in the winter months.

The question posed in this thread is about a very specific heat muff installation that does not necessarily align with general practices used for other heat muffs. In fact if some of the advice given here is adopted for the OP-54 heat muff installation it could cause poor engine performance (or worse).
An example of the installation and details for proper installation of screens can be found HERE

Answer to original question -
The screen on the cabin heat outlet of the heat muff is intended to act as a flow resistor. This helps assure that there are no instances that the engine will draw air in the reverse direction from the cabin heat system when carb. heat is on, and it helps with carb. heat and cabin heat performance by slightly restricting the flow out to the cabin heat part of the system.