Exhaust_heat hose routing

Hey guys,

I'm routing SCAT hose for exhaust and cabin heat. Do I need to route a hose off the right muffler to the back of the alternate air box or do I leave that open? My initial intent is to run both mufflers to feed the cabin heat box. Engine is an Mattituck IO-360. Appreciate your advice. Let me know if you can't view the pics.

Thanks,
Paul

Don't really understand your question (not unusual for me). Ideally, you want one or both mufflers to supply carb heat (or equivalent alternate air source) as a first priority. When not needed - and only then - the hot air should be dumped overboard or rerouted to heat the cabin via a selector box of some sort. Hot air availability for carb heat, regardless of source, should never depend on cabin heat selection or channeled through any cabin heat selector box. My 2c.

Nice job, though.

Since I'm fuel injected, I wasn't sure if I had to actually run a hose off one of the mufflers straight to the carb heat box to provide an alternate air source. I wasn't sure if the opening at the rear of the carb heat box was sufficient to provide alternate air just by itself. I'll try using the left muffler as an alternate air source and the right muffler for cabin heat. Appreciate all the feedback.

Your fuel injection system does not require any hot air source for the alternate air. The alternate air can just suck ambient air from the alternate opening in the air box without any feed source. The only purpose of the alternate air is to bypass a clogged air filter. You can pipe hot air from both exhaust heat muffs to the cabin.

The type of fuel injection system which you have does not make ice. Carburetors make ice due to the vaporization of the gasoline. Your fuel servo does not vaporize gasoline.

Some certified fuel injection installations like yours have a spring loaded flapper valve in the air box for alternate air, with no cockpit control what so ever. This flapper will open by suction if the air filter becomes restricted drawing air from under the engine.

While I agree that fuel injected engine induction icing is rare, it can happen - and has happened. Fuel evaporation in carbureted engines is responsible for about 70% of the temperature drop - as I recall. The rest is caused by pressure drop. The latter is a function of venturi effect created by the venturi itself of a carb, if any, and the position of the throttle butterfly. Anytime the manifold pressure is less than static pressure, there is a temperature drop in the induction system. Some engines compensate for this by transferring crankcase heat directly to the induction systems. Others might channel hot air from behind an exhaust wye. Skywagon8a is probably right in that the IO-360 belongs to the first category.

Another advantage of ducting air through the muffler is that you'll have ram pressure - from the pressurized side of the baffling - even with heat ON. Course you could get that by bypassing the muffler, too.

To clarify a bit, the method which I have described is that which was done, with my participation, on an airplane which was awarded a type certificate by the FAA in 1965. This is not to say that heat would not be beneficial, it would do no harm. It just is not required in this particular system.