ORIGINAL: Mike Connor
NACA testing int the 1920's showed an engine cowl alone could increase the speed by over 10%. You must consider that most of these were radial engines with multiple cylinders. The round cylinder is very bad for drag so it seems that even adding an extension off the rear of the cylinder, giving it an airfoil shape, would increase speed some.
Like you noted, the cowl study was for what? a cowl around 9 cylinders? Back in the 20s the cowl studies only dealt with one row also. But the engine still had a frontal area that would have been your delta's frontal area time what? seven?
They studied engines that completely blanketed the front of the airplane, and got 10% improvement? Sounds about right. Divide that by 7.
BTW, the numbers that seem to be in doubt here were actually worked out from NACA studies.
But yeah, the cowling on the Brewster Buffalo or the P-Shooter did wonders for the speed of those old flying barrels. But it really isn't sensible to expect 10% out of cowling just one cylinder that's already half shrouded.
In this case I'd say that you really can't compare rats to barrels.