Outlet vrs inlet air ratio

[Tsabean]

Is there a published ratio for oulet air?
RV4, 0320, inlet area 52.5 square inches. Outlet at back of cowl 30 square inches.
Anybody?

[DanH]

Originally Posted by Tsabean

Is there a published ratio for outlet air?
RV4, 0320, inlet area 52.5 square inches. Outlet at back of cowl 30 square inches.
Anybody?

You can find authors who insist on some particular ratio, but it's nonsense.

Inlet/outlet claims are based on area. However, reality says an inlet may be a low velocity ratio type, or a high velocity ratio type. Here "velocity ratio" describes the velocity of flow through the inlet as compared to freestream velocity. For any given exit area, typical low Vi/Vo inlets are larger, while high Vi/Vo are smaller. As a result, the range of perfectly workable inlet/exit ratios is very large.

Your RV-4, as configured, is a low Vi/Vo system.

[Tsabean]

Still getting higher than hoped for CHTs. Have to come back to 25 squared in climb to keep them below 400. Engine only has 14 hours on it so still breaking in. Was considering enlarging the exit area slightly to see if that would help.

[DanH]

Originally Posted by Tsabean

Still getting higher than hoped for CHTs. Have to come back to 25 squared in climb to keep them below 400. Engine only has 14 hours on it so still breaking in. Was considering enlarging the exit area slightly to see if that would help.

Given typical engine baffling, the cowl exit is the system throttle. So yes, more exit area will allow more mass flow.

That said, there are many factors which can contribute to high CHT. You may want to look at some of them before cutting the cowl.

[scsmith]

Originally Posted by DanH

Given typical engine baffling, the cowl exit is the system throttle. So yes, more exit area will allow more mass flow.

That said, there are many factors which can contribute to high CHT. You may want to look at some of them before cutting the cowl.

Among those to check:

1) ignition timing. Retarding the ignition just a little will drop CHT, and hardly affect power at all.

2) leaky baffles. People swear up and down their baffles are tight, and for the most part they are, except there will be one glaring big leak. A common one is a flow path from the upper plenum to the lower plenum around the front of the engine where the inlets transition to the top of the cowl. Make sure the ends are sealed up.

3) not installed inter-cylinder baffles. Not likely, but easy to check.

4) Mixture leaner than you think, or one cylinder much leaner than others. Have you done a GAMI test? You want T.O. mixture to be 150F or more ROP.

probably other 'low hanging fruit', but those are the ones that come to mind.