[freebeard]

AutoSpeed - Ballistic Bellmouths

Testing Shapes

Three intake designs were tested on the flow-bench. First up was a straight piece of 3-inch tube, cut off square at the end. The actual ID of the tube was 73mm, giving a cross-sectional area of 41.8 square cm. At a pressure differential of 3 inches of water, the 215mm-long tube flowed 255 cfm.

Next up was a 3-inch pipe running a straight flare. Again the tube was 215mm long, with the length of the 11-degree flare being 75mm. The mouth opening was 115mm in diameter, giving an intake area of about 104 square cm. The airflow with this design was noticeably improved, with a measured flow of 318 cfm recorded.

This represents a 25 per cent increase over the straight pipe!

Finally, a custom-made bellmouth was trialed. Again 215mm long, the bellmouthed intake used the same flare angle as the previous pipe. This time, though, a 22mm tube was rolled around the top, being brazed to the edge of the flare. Plastic body filler was used to fill any imperfections, giving a very smooth radius entry into the tube.

And the result? 322 cfm was the measured, up over the straight flare by 1.2 per cent.

A megaphone made more difference than the bellmouth. His $1.84 stainless steel egg-cup has an internal diameter of 2.3". So a 1 to 2.3 inch megaphone would fit the 1—1 1/8" collector.