What happens at the “choke” point?

General engine tech -- Drag Racing to Circle Track

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What happens at the “choke” point?

Post by travis » Sat Jun 23, 2018 8:49 pm

I’ve been playing with some of the calculators on the Wallace website, as always trying to learn more.

What happens to airflow, and power once you reach the smallest choke point of the intake tract? Air goes turbulent? No increase in flow? Power peaks and then starts dropping?

For example...per AFR’s website, their 165 sbf Head has a mcsa of 1.935”. Per the calculator, a .040” over 351w with this head will choke at approximately 5600 rpms. Does this mean that with appropriate supporting hardware that 5600-ish should be the power peak? What happens when you put bigger supporting parts like a tunnel ram, giant cam, etc? Power still peaks at 5600-ish but just drops off slower?

The AFR 165 is advertised as 250cfm@.550”, which per another calculator this will support up to 475hp. Engines this size don’t make that kind of power at 5600 rpms without nitrous or some sort of forced induction. So what’s the deal here? I’m assuming part of this discrepancy is because engines don’t pull a steady 28” of depression?

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Re: What happens at the “choke” point?

Post by modok » Sat Jun 23, 2018 8:57 pm

VE starts dropping, power levels off.
Power may begin to drop soon after, or hold-on for quite a ways up. Depends on the cam and manifolds.
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Re: What happens at the “choke” point?

Post by hoffman900 » Sat Jun 23, 2018 9:09 pm

No more mass can get through the orfice despite greater depression on the cylinder side. Eventually pumping losses catch up and power starts to nose over.

However, plenty of race engine with sonic restrictors for rules (or be it some sort of porting limitation, choke limitation, etc.) all make power well above where the restrictor goes sonic, it's about managing the drop. Run as much compression as you can, careful selection of cam timing, and the header design is a huge player.
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Re: What happens at the “choke” point?

Post by swampbuggy » Sat Jun 23, 2018 9:17 pm

Bob do the NASCAR guys run smaller header tubes with the restrictor plate. Mark H

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Re: What happens at the “choke” point?

Post by hoffman900 » Sat Jun 23, 2018 9:47 pm

swampbuggy wrote:
Sat Jun 23, 2018 9:17 pm
Bob do the NASCAR guys run smaller header tubes with the restrictor plate. Mark H
I don't think so, that's a question for Calvin, Warp, Pro-Header (though I haven't seen him on here in a long time).

Not that I'm in the market, but I do frequently search and see 1 3/4" primary NASCAR take-offs popping up on Ebay from time to time (typical is 1 7/8"). Not sure if these are test pieces or restrictor plate pieces.
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Re: What happens at the “choke” point?

Post by travis » Sat Jun 23, 2018 9:53 pm

hoffman900 wrote:
Sat Jun 23, 2018 9:09 pm
No more mass can get through the orfice despite greater depression on the cylinder side. Eventually pumping losses catch up and power starts to nose over.

However, plenty of race engine with sonic restrictors for rules (or be it some sort of porting limitation, choke limitation, etc.) all make power well above where the restrictor goes sonic, it's about managing the drop. Run as much compression as you can, careful selection of cam timing, and the header design is a huge player.
So it sounds like the choke point may not necessarily be the power peak, but probably real close on a street type engine because the pumping losses will start climbing rapidly at that point?

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Re: What happens at the “choke” point?

Post by groberts101 » Sat Jun 23, 2018 11:18 pm

travis wrote:
Sat Jun 23, 2018 9:53 pm
hoffman900 wrote:
Sat Jun 23, 2018 9:09 pm
No more mass can get through the orfice despite greater depression on the cylinder side. Eventually pumping losses catch up and power starts to nose over.

However, plenty of race engine with sonic restrictors for rules (or be it some sort of porting limitation, choke limitation, etc.) all make power well above where the restrictor goes sonic, it's about managing the drop. Run as much compression as you can, careful selection of cam timing, and the header design is a huge player.
So it sounds like the choke point may not necessarily be the power peak, but probably real close on a street type engine because the pumping losses will start climbing rapidly at that point?
It's often much more about WHERE the choke point is located. An mCSA located out by the PR pinch can easily be mitigated with wider SSR floor width, thinned/shortened guides and blown out bowl sizes. Whereas an mCSA shortly BEFORE, AT, or OVER the SSR will kill peak and drop over the proverbial cliff much more quickly.

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Re: What happens at the “choke” point?

Post by GARY C » Sat Jun 23, 2018 11:56 pm

What happens at the “choke” point?... It Chokes!
Sorry I couldn't resist! :D
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THE ABOVE POST IN NO WAY REFLECTS THE VIEWS OF SPEED TALK OR IT'S MEMBER AND SHOULD BE VIEWED AS ENTERTAINMENT ONLY...Thanks, The Management!

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Re: What happens at the “choke” point?

Post by travis » Sun Jun 24, 2018 12:04 am

GARY C wrote:
Sat Jun 23, 2018 11:56 pm
What happens at the “choke” point?... It Chokes!
Sorry I couldn't resist! :D
So your saying I’m making this more complicated than it needs to be? Well...I’ve been known to do that :mrgreen:

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Re: What happens at the “choke” point?

Post by GARY C » Sun Jun 24, 2018 12:10 am

travis wrote:
Sun Jun 24, 2018 12:04 am
GARY C wrote:
Sat Jun 23, 2018 11:56 pm
What happens at the “choke” point?... It Chokes!
Sorry I couldn't resist! :D
So your saying I’m making this more complicated than it needs to be? Well...I’ve been known to do that :mrgreen:
Possibly but I think the answers that come from it will give people areas to explore.
It's just that when I see a headline I have an instant smart @ss comment pop into my head in response and this time I failed to restrain my self.
Please Note!
THE ABOVE POST IN NO WAY REFLECTS THE VIEWS OF SPEED TALK OR IT'S MEMBER AND SHOULD BE VIEWED AS ENTERTAINMENT ONLY...Thanks, The Management!

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Re: What happens at the “choke” point?

Post by andyf » Sun Jun 24, 2018 12:25 am

Well first off there probably isn't a well defined choke point in most cylinder heads. In a lab you might be able to create a choke point with a sharp edged orifice or some sort of lab grade type of device, but cylinder heads are fairly complex shapes.

The simple calculators like you'll find on the Wallace page just give you an approximation, they don't really define how the engine is going to work. You can prove that by comparing a 23 degree cast iron Chevy head that has the same CSA as a high port 15 or 18 degree head. The high port head will make a lot more power even though the CSA is the same. The Wallace calculator doesn't take that into account. Shape is really important when talking about choke point.

Think of it in aero terms. Coefficient of drag is a different factor than frontal area. You can have a small car that has high drag numbers or you can have a big car with good drag numbers. The CSA is just the frontal area term. You still need to know shape before you know how the port is going to work.

Short summary is don't pay too much attention to CSA and choke point unless you have a well refined head program. If you are a NASCAR shop and all of your heads have the same shape but the CSA varies from head to head then you probably can figure out what the choke point will be. If you're just picking random heads out of the Summit catalog for a bracket race car then the calculated choke point isn't going to be all that accurate of a number.
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Re: What happens at the “choke” point?

Post by digger » Sun Jun 24, 2018 2:43 am

it shouldn't be called "choke" because what you are referring to is not "choked flow" (in most cases) which has specific meaning in fluid flow and in most cases people get confused

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Re: What happens at the “choke” point?

Post by modok » Sun Jun 24, 2018 3:29 am

People getting confused is their own problem. if "math says" you can't pass .5 mach it's because your math is not realizing the density drops. that is the speed going IN, not the speed coming out.
Cars are going into the traffic jam at 10 mph, leaving at 60. As soon as they see open road they hit the gas, and thus they become farther apart. The faster they go the farther they are apart, but the through-put, in cars per minute, or in molecules per minute, is the same. throughout the road. I know people don't understand it because if they did they would drive less stupid. If a three lane road goes down to two lanes, you gotta speed up! That's the only way your gonna fit this many cars THROUGH a smaller road. And if the road goes from three lanes to two, you can slow down. Instead, they do the opposite, and the result is any day now my tiny car is going to be sandwiched under a truck.

There has to be a smallest point, and it should be placed wisely. The cars come out of the traffic jam, accelerate, and then a sharp turn? fork in the road? can't be good :shock: The tail of a venturi is usually 5-7 degrees, can't diffuse it much faster than that. They can't change lanes any faster than a 1/4 inch per inch. Each piece is not terribly hard to understand, but the whole of them is complex indeed.
Last edited by modok on Sun Jun 24, 2018 3:33 am, edited 1 time in total.
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Re: What happens at the “choke” point?

Post by digger » Sun Jun 24, 2018 3:31 am

the math doesnt say anything about 0.5 mach

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Re: What happens at the “choke” point?

Post by modok » Sun Jun 24, 2018 3:34 am

Ok, then what is this math and what does it say?
Looks like 0.6
I have heard assumptions from .5 to as high as .7

This is probably an adjustment to calibrate the results to reality, but as basic theory, .5 makes good sense.
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