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What is cylinder head choke

General engine tech -- Drag Racing to Circle Track

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Little Mouse
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What is cylinder head choke

Post by Little Mouse » 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.

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Re: What is cylinder head choke

Post by Walter R. Malik » Mon Jan 13, 2020 5:37 pm

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.

"Air Flow Choke " ... the way I have always understood it,
is when the window of lift area above the valve seat is not large enough to increase airflow more than what the port is able to flow.

Maybe there are other meanings as I presume that air flow can be "choked" in other areas.
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Re: What is cylinder head choke

Post by Little Mouse » Mon Jan 13, 2020 5:57 pm

That sounds right all I can read about it seems sbc have problems at the pushrod pinch and bbc have a problem with restriction at the valve area to how many cubes they have compaired to valve size.

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Re: What is cylinder head choke

Post by digger » Mon Jan 13, 2020 9:45 pm

its is fairly well know that increasing airspeed in the engine provides better "ramming" at the 2nd half of the lift curve (i.e peak lift to IVC) assuming things are setup fairly well with regard to intake manifold geometry, camshaft etc

choke in the inlet port is when adding more velocity means the gains in VE due to better "ramming" are offset (lost) due to higher pumping losses as the engine has to work harder to pull the piston down and ingest air and therefore you get no nett power gain.

choke is a poor description as it's usually happens well below "choked flow" as in Mach 1 air speed.

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Re: What is cylinder head choke

Post by mag2555 » 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.

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Re: What is cylinder head choke

Post by E.Roy » Tue Jan 14, 2020 7:28 pm

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.
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
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Re: What is cylinder head choke

Post by Little Mouse » Wed Jan 15, 2020 1:10 am

Thanks for all the replies, looking at that picture sure makes me wish there was no such thing as the pushrod pinch.

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Re: What is cylinder head choke

Post by steve cowan » Wed Jan 15, 2020 5:30 am

i would not be dejected by the the PRP
its part of the deal on a 23 deg chev head
i have a dart 180cc head with a pinch of 2.3''
and avg CSA through the port at 205 cc will work on a 383 sbc at around 6500- 6800rpm.
if you try to make the throat the MCSA the port will be to big or will push peak hp rpm higher.
eg- 2.02 valve @ 89% is 2.44'' csa
on a 383 peak will be around 7000rpm with a average csa of 2.45'' and a welded pinch and offset rockers etc,depending on the brand of cylinder head.a small pinch will work on a engine that see 5500-6000 rpm because i doubt it would go sonic choke at that rpm.
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Re: What is cylinder head choke

Post by SpeierRacingHeads » Wed Jan 15, 2020 10:28 am

PR pinch in a 23º isn't near as important as the floor speed over the apex.

Choke/Sonic is nothing more that stretching molecules and getting nothing in return. It can happen from anywhere in the port.

So they say cfm isn't important.. OK. Got it.. I put it a little higher than last on the list.
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Re: What is cylinder head choke

Post by David Redszus » Wed Jan 15, 2020 11:13 am

Choked air flow is always a function of pressure ratio.

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Re: What is cylinder head choke

Post by Ken_Parkman » 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.

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Re: What is cylinder head choke

Post by Mikej26 » Wed Jan 15, 2020 1:05 pm

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.

Excellent explanation

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Re: What is cylinder head choke

Post by SpeierRacingHeads » Wed Jan 15, 2020 1:07 pm

Don't confuse "choke" with "separation", two different things.
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Re: What is cylinder head choke

Post by digger » Wed Jan 15, 2020 3:37 pm

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.
Agree that choke is not the correct term for what people actually mean
IMO choke on an inlet port is not separation, consider a straight short 4V with effectively no short turn. it can simply be too fast (too small port for the engine size and rpm) even though it may be quite a bit less than Mach 1 and be not separating from the turn.

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Re: What is cylinder head choke

Post by SpeierRacingHeads » Wed Jan 15, 2020 3:44 pm

You can witness "choke" in a Pro Mod. Just watch the exhaust pipes. Exhaust is in choke conditions. And I will say it's the coolest thing you'll see, when you see one that does it.
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