Don Terrill’s Speed-Talk

Little Mouse wrote: ↑Mon Jan 13, 2020 5:27 pm What is happening when say a sbc cylinder head chokes at the pushrod pinch. I assume by choke it's what it says the head cant flow anymore air regardless how big the cam is.

mag2555 wrote: ↑Tue Jan 14, 2020 6:30 am With a 23 degree SBC head in terms of aftermarket ones you need to to be in port volume range of 195 to 215 CCs and air flow numbers of greater then 270 cfm@28" before the port area at the push rod buldge becomes a choke point, and even then the flow loss seen is more due to the air stream being forced to compress and then realign itself once passed that pinch point.

Ken_Parkman wrote: ↑Wed Jan 15, 2020 12:13 pm Choke is an engineering term to signify sonic flow has been achieved, and at that point any increase in pressure ratio cannot increase velocity. This is not what happens in an intake port.

I believe it is used commonly and incorrectly when a different aerodynamic phenomena occurs - flow separation. This is when the air can no longer follow a wall of the port, and pulls off the surface or "separates". The result of this is a portion of the port stops working, your effective flow cross section gets smaller, and the flow either stops increasing or actually reduces with increasing lift. A real problem on bends or expansions. Worse still is both, like when an oversize valve is put in a stock type casting.

Another time I've seen it used is when to port limiting velocity is reached. This is the point where the further increases velocity in velocity don't increase power any further. Still far less than actual sonic and choke in the proper definition.

Ken_Parkman wrote: ↑Wed Jan 15, 2020 12:13 pm Choke is an engineering term to signify sonic flow has been achieved, and at that point any increase in pressure ratio cannot increase velocity. This is not what happens in an intake port.

I believe it is used commonly and incorrectly when a different aerodynamic phenomena occurs - flow separation. This is when the air can no longer follow a wall of the port, and pulls off the surface or "separates". The result of this is a portion of the port stops working, your effective flow cross section gets smaller, and the flow either stops increasing or actually reduces with increasing lift. A real problem on bends or expansions. Worse still is both, like when an oversize valve is put in a stock type casting.

Another time I've seen it used is when to port limiting velocity is reached. This is the point where the further increases velocity in velocity don't increase power any further. Still far less than actual sonic and choke in the proper definition.