Pressure recovery...am I understanding it correctly?
Posted: Mon Dec 02, 2019 4:30 pm
Comparing 2 heads exhaust port flow numbers...my sbf Promaxx 9175’s vs my barely ported 601 305 heads. The Promaxx heads have a 1.60 undercut stem valve, a short perfectly straight port with a “textbook” short turn, a short length, and a square exit that is obviously bigger than the 601’s. The 601’s have an undercut 1.50 valve, more length, a slight bend in the port (exit isn’t right inline with the valve), and a smaller more rounded exit.
Flow bench results between the 2 heads show that with or without a pipe, the smaller more round exhaust port of the iron heads flows better at all lift points EXCEPT at .300”, where the Promaxx is 11 cfm better. The Promaxx also flattens our above .500” lift, only gaining 2-3 cfm at each point above that, where the 601’s are still gaining 9-12 cfm at each point above .500”.
I’m trying to wrap my brain around what is going on here. Pressure recovery has been mentioned before on similar questions, so I am trying to make sense of that. Here is what I am thinking...the smaller CSA of the exhaust is creating a higher velocity “slug” of air that can more easily penetrate the atmosphere outside of the port, where as the larger CSA, slower moving air from the aftermarket heads sees atmosphere as more of a wall, so to speak. Similar to a pointed nose bullet vs a flat nosed bullet hitting and penetrating the same material.
Am I visualizing this correctly?
In general, if you had the same intake port, and could use either of these 2 exhaust ports...would the smaller faster exhaust make more average power?
Flow bench results between the 2 heads show that with or without a pipe, the smaller more round exhaust port of the iron heads flows better at all lift points EXCEPT at .300”, where the Promaxx is 11 cfm better. The Promaxx also flattens our above .500” lift, only gaining 2-3 cfm at each point above that, where the 601’s are still gaining 9-12 cfm at each point above .500”.
I’m trying to wrap my brain around what is going on here. Pressure recovery has been mentioned before on similar questions, so I am trying to make sense of that. Here is what I am thinking...the smaller CSA of the exhaust is creating a higher velocity “slug” of air that can more easily penetrate the atmosphere outside of the port, where as the larger CSA, slower moving air from the aftermarket heads sees atmosphere as more of a wall, so to speak. Similar to a pointed nose bullet vs a flat nosed bullet hitting and penetrating the same material.
Am I visualizing this correctly?
In general, if you had the same intake port, and could use either of these 2 exhaust ports...would the smaller faster exhaust make more average power?