What is cylinder head choke

[peejay]

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.

You touched on something I feel to be true with my limited experience, and that is the pushrod bump can screw up the column of air and cause the flow to detach from the short side. On an uported 906/062 Vortec iron head, I feel Chevy engineers put deliberate shapes at the pushrod pinch to force air to flow down the straight side for part throttle velocity. I feel the whole port is optimized for part throttle efficiency and swirl, which makes sense given 906/062 were on a truck motor intended for 4-speed auto potentially towing 8,000 lbs at 2,000rpm

Practically all engines are optimized for part throttle efficiency, since they live 99% of their time between idle and 20-40hp.

Oddly a truck engine should be better at higher loads compared to, say, a passenger car that only needs 20hp to tool down the road at 60mph.

[David Redszus]

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.

Consider the flow characteristics of an inlet with forced induction where the pressure ratio can easily exceed the critical pressure ratio.
While velocity becomes choked and does not increase with increased pressure ratio, air density does increase with increased pressure ratio. And so does power.

Flow separation need not occur only at high velocities. A change in area or direction can produce flow separation which is a change in boundary layer. The result is a change in flow co-efficient (Cd) which can be significant. Friction factors will also affect Cd.

Here's the kicker. Flow behavior is also a function of fuel vapor content, temperature and water vapor.

[travis]

So it sounds like “choke” would be the same thing as (term wise) running out of port, valve, curtain, etc.

On a flow bench, would this “choke” point be were flow stops increasing even with more and more lift, at a constant depression?

Flow separation, or “going turbulent”, is easily seen/heard on a flow bench, and in my limited experience comes with a pretty significant drop in flow and some interesting sound effects

[David Redszus]

So it sounds like “choke” would be the same thing as (term wise) running out of port, valve, curtain, etc.

On a flow bench, would this “choke” point be were flow stops increasing even with more and more lift, at a constant depression?

Flow separation, or “going turbulent”, is easily seen/heard on a flow bench, and in my limited experience comes with a pretty significant drop in flow and some interesting sound effects

All cylinder air flows are turbulent flow based on Reynolds Number.

A graph of lift vs flow for several different induction systems would indicate where they are the same and where they differ.