How Does Fresh Air Exhaust Work?
Fresh Air Exhaust injects the boat’s exhaust into the propwash to move it away from the boat. Once the boat is at about 8 mph the exhaust is pushed about 120′ behind the boat; and greatly diluted. This is similar to Outboard Engines that have the exhaust exit through the boat’s propeller. Since the “Downpipe” of the Fresh Air Exhaust is in the frothy prop-wash there is less drag than would occur if the Downpipe were in undisturbed water.
The picture shows that the flow of the water past the outlet of the Fresh Air Exhaust Downpipe creates a low pressure zone (blue zones); this lower pressure helps to pull the exhaust from the engine. I refer to this as a venturi effect but when I was working with the engineer to develop this Computational Flow Analysis he explained to me how it wasn’t technically a venturi. I have to admit, I really didn’t understand the explanation so I continue to call it a venturi effect. Suffice it to say that Fresh Air Exhaust literally Sucks the exhaust out of the engine; see the video.
Since the FAE Downpipe Sucks the exhaust out of the engine there is minimal back-pressure change until the boat is approaching Wide Open Throttle (WOT). Our back-pressure testing has shown FAE to not increase the back-pressure above 2 psi on some boats and up to 4 psi on some others, and this is at WOT. To put this in perspective, production boats with factory installed exhaust systems typically range between 2 to 5 psi back-pressure. For boating speeds up to about 3600 rpm the back-pressure with FAE is typically similar to the boat without FAE. By comparison, a stock Corvette (the high performance car) exhaust systems have about 3 psi back-pressure at WOT. Another perspective: 2 psi is a gentle puff to blow out a candle.
On a race car, the engine exhaust manifold, collectors and exhaust pipes are tuned (diameter, lengths, bends, etc) so that the Exhaust Pulse Wave is timed with the opening and closing of the exhaust valves. The Pulse Wave actually assists with sucking the exhaust out of the engine. On an inboard boat exhaust, water is injected into the exhaust gas just aft of the header; consequently there is no Exhaust Pulse Wave. Fresh Air Exhaust facilitates sucking the exhaust out of the engine similar to what the Pulse Wave does on a tuned exhaust system.
In general, inboard boat exhaust systems are not tuned. One of the best inboard boat exhaust systems is the Nautique Surf Pipe (just straight exhaust tube and the exhaust underwater; no muffler or flappers). This is the setup we’ve recommended to the boat manufacturers.
Our engine performance testing, and our thousands of customers tell us that there is little if any performance change with Fresh Air Exhaust installed!