I'm just in the process of building a teardrop subcowl for under the plane. My oil & coolant rad outlets will be dumped into the subcowl, so they can be streamlined and spit out the back. Along with it, the exhaust will flow through there. My idea is to add louvers when the exhaust pipe enters the subcowl, so it pulls the air in the subcowl (oil & coolant, plus any air that leaks into the main engine cowl) and thrusts the exhaust. I found a company that just sells the perforated louvers without a muffler shell around it. Of course the subcowl and bottom of plane will be protected with Fiberfrax and SS plates. Can the exhaust pipe (2.5") end inside the subcowl, or should I extend a pipe (3") outside the airplane for a few feet ?