There is no question about it if you want to run any sort of cooling devices under the cowl you need to have the cowl installed or you will hit the red line soon. My Jabiru uses cooling ducts to cool the heads. I tried to run it with the cowl off thinking the air from the prop would cool that engine. Ha Not a chance with in 5 minutes My head temps were close to red line. Installed the cowl and they stay nice and cool.
Now that you talk about that I do have a set of mc-4 laying somewhere Ill see if I can find them somewhere. And if they work. I bought them through Wentworth aircraft. If not ill let you know. Which are better Matco of Grove?