DR I love your Jed Clampit diorama beautiful work. And yup Larry we are both in total agreement about the cooling arraignment of DR's Cub.

DR its going to be just about impossible to configure your airplane like the pic of the full scale airplane and here is why and I am quite familiar with J-3's having owned two of them at different times in the past. One a J-3c65 and the other a J-3c-75: If you look at your photo you can see both the upper and lower cowl halves taper from the firewall to a rather narrow wedge at the propeller to just wide enough for the crank case. Above the cylinders are eyebrow baffles which are intended to deflect high energy air down between the cylinders. The were also intercylinder baffles but for our purposes those are out of sight and mind.

Now take a close look the cowl of your cub. Almost no taper and in fact its as if the designer was actually trying to make the cowl like a PA-11, PA-18 Super cub and of course these full scale cubs did have conventional fully enclosed pressure cowl. DR you will have a bake oven there unless you open some high energy air from the front I now cheek openings will not be scale. Well I have seen far to many beautiful scale airplane live very short lives simply because their builder/owners would not cut their pretty cowls for anything even cooling. Yes of course the tunnel will let out some air but no where enough.. In most cases were the tunnel were being used the cowl would be cut away a considerable amount for the muffler. However no matter how much outlet you provide it all comes to naught if there is no high energy inlet(s) .

My recommendation for that airplane and that cowl would be cut away a lot more outlet at the bottom and provide reasonable sized cheek inlelts up front scale fidelity can go be hanged cooling is far, far more important if the airplane is actually intended to be flown.

John