Pinch point debate

General engine tech -- Drag Racing to Circle Track

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Pinch point debate

Post by BrazilianZ28Camaro » Fri Mar 29, 2013 5:25 pm

Hello everybody

Lets take a hypothetical situation, say you have a set of good flowing heads on drag race oriented SBC but the velocity at the pinch point is higher than ideal for the Engine CI and RPM intended.

We can assume this engine is on a car with correct gears and converter for the job.

Without going specific on details, and ignoring any possible flow loss/gain, how would this engine respond to more pinch area and less velocity at that point in regards to the powercurve?

I mean, this is a generic question due lots of variables involved, but which are your experience when the pinch point is opened ? how the car responded after the pinch work, assuming good flowing heads from the starting point??
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Re: Pinch point debate

Post by rookie » Fri Mar 29, 2013 5:58 pm

I would like to here thoughts on this as well, In discussions with David Vizard about the PRP he said they tried a test years ago on a head that they thought was maxed out for the ci and rpm of the combination, so they welded it up and basically eliminated the PRP and although the head gained cfm it lost swirl and the engine lost power across the rev range.
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Re: Pinch point debate

Post by Amilcar » Fri Mar 29, 2013 6:24 pm

I think one very important thing to consider if it may help or not is,
besides how well the entire induction can flow, how much the engine is using/beeing efficient with it.

Another one, how fast it´s actually, at the pinch I think it`s a bit more forgiving than on the short turn for exemple.
I`ve not done any ABA tests focusing this but, thouse factors would be of great influence

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Re: Pinch point debate

Post by cspeier » Fri Mar 29, 2013 6:49 pm

If you open up the PRP and don't make it move more air, your going the wrong way in most cases. The PRP is all realative to how much air it's moving. To say a head had 2.50inches and it needs 2.80inches is wrong because velocity is king. For me it's all about the velocity vs air being moved vs volume.

I have a 2.50in head that peaks at 7300 on a 427 with 260/268 camshaft because the profile is correct vs the air it's moving..

All this is mute with cylinder heads that aren't velocity limited like a 23 degree.

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Re: Pinch point debate

Post by T-flow » Fri Mar 29, 2013 6:58 pm

For most cases it's going to be based on what's needed in CSA to get to your goals. The whole intake track need's to be figured in to the equation, not just the pinch.
MCSA to meet the requirments and where it's at...

We have done a bunch of them(weld the pinch), Im getting ready to test one real soon here with that being the only change.
They had a pinch of 2.26qsin at the pinch and is now 2.73sqin at the pinch. The pinch was his choke point. This is a 421ci and I feel it will respond well to the change!

Another note;
When looking at doing this mod, you must have your duck's in a row. You don't want to kill the port just by making the pinch bigger.
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Re: Pinch point debate

Post by BrazilianZ28Camaro » Fri Mar 29, 2013 7:02 pm

cspeier wrote:If you open up the PRP and don't make it move more air, your going the wrong way in most cases. The PRP is all realative to how much air it's moving. To say a head had 2.50inches and it needs 2.80inches is wrong because velocity is king. For me it's all about the velocity vs air being moved vs volume.
Yep I do agree velocity is king.

Then if you have say 400ft velocity at that 2.5" PRP (which would be the fastest area of the port) , and open it up to lower the velocity to say 330 ft, the port doesn't flow more air ?

Why the pinch shouldn't respond well to more area if the velocity is too high at that point? The rest of the port may choke the pinch somehow?
Last edited by BrazilianZ28Camaro on Fri Mar 29, 2013 7:07 pm, edited 1 time in total.
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Pump gas -T-ram - All motor SBC 427"
3308 lbs- leaf spring/Caltrac Car
29x10.5 Hoosiers
New best 1/8 mile ET
1.45 60' / 4.12 330' / 6.38@109mph :D

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Re: Pinch point debate

Post by BrazilianZ28Camaro » Fri Mar 29, 2013 7:06 pm

T-flow wrote:For most cases it's going to be based on what's needed in CSA to get to your goals. The whole intake track need's to be figured in to the equation, not just the pinch.
MCSA to meet the requirments and where it's at...

We have done a bunch of them(weld the pinch), Im getting ready to test one real soon here with that being the only change.
They had a pinch of 2.26qsin at the pinch and is now 2.73sqin at the pinch. The pinch was his choke point. This is a 421ci and I feel it will respond well to the change

Wow, thats a big area change on the PRP! Keep us informed how that 421" did.
Another note;
When looking at doing this mod, you must have your duck's in a row. You don't want to kill the port just by making the pinch bigger.
Yes, mainly due the fact the pinch point have a influence on flow distribution around the valve right? :roll:
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Pump gas -T-ram - All motor SBC 427"
3308 lbs- leaf spring/Caltrac Car
29x10.5 Hoosiers
New best 1/8 mile ET
1.45 60' / 4.12 330' / 6.38@109mph :D

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Re: Pinch point debate

Post by cspeier » Fri Mar 29, 2013 7:09 pm

BrazilianZ28Camaro wrote:
cspeier wrote:If you open up the PRP and don't make it move more air, your going the wrong way in most cases. The PRP is all realative to how much air it's moving. To say a head had 2.50inches and it needs 2.80inches is wrong because velocity is king. For me it's all about the velocity vs air being moved vs volume.
Yep I do agree velocity is king.

Then if you have say 400ft velocity at that 2.5" PRP (with would be the fastest area of the port) , and open it up to lower the velocity to say 330 ft, the port doesn't flow more air ?

Why the pinch shouldn't respond well to more area if the velocity is too high at that point? The rest of the port may choke the pinch somehow?

Depends, the valve is the restriction of the port...

You need to worry about the floor speed as well. You just can't open the pinch and not worry about the back of the port. Just like putting a 50 degree seat on a head that had a 45 degree will change the floor and pinch velocity.

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Re: Pinch point debate

Post by Lockwire » Fri Mar 29, 2013 7:16 pm

rookie wrote:I would like to here thoughts on this as well, In discussions with David Vizard about the PRP he said they tried a test years ago on a head that they thought was maxed out for the ci and rpm of the combination, so they welded it up and basically eliminated the PRP and although the head gained cfm it lost swirl and the engine lost power across the rev range.
Pinch is only some thing someone thought up. Check out the actual speed that fluid is moving along in the area of discussion. that is # 1. 2nd is the length of the distance that you decide to keep that speed moving at the speed of decision to gain momentum for benefit. The swirl is incorrect shape or form for Vizard people, not the same thing. Its really easy except allot of people think wrong about it. This IS ONLY referring to the area in question. You need to do the math, if you cannot count you are screwed . And it works well for the people I do stuff for as well as others I share with. JMO . ........... on the pinch , real heads like F1 or race heads do not have pinch, they have an area that speeds the fluid up ( air ) so that the intake system will work. hope I helped. .......?? :) Erland has some really accurate info so look up his posts. Stuart. :)

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Re: Pinch point debate

Post by T-flow » Fri Mar 29, 2013 7:48 pm

Why the pinch shouldn't respond well to more area if the velocity is too high at that point? The rest of the port may choke the pinch somehow?

As long as you know what the goal speed is at the short turn, so it can make the turn and fill the cylinder.



Yes, mainly due the fact the pinch point have a influence on flow distribution around the valve right?
T-flow wrote:For most cases it's going to be based on what's needed in CSA to get to your goals. The whole intake track need's to be figured in to the equation, not just the pinch.
MCSA to meet the requirments and where it's at...

We have done a bunch of them(weld the pinch), Im getting ready to test one real soon here with that being the only change.
They had a pinch of 2.26qsin at the pinch and is now 2.73sqin at the pinch. The pinch was his choke point. This is a 421ci and I feel it will respond well to the change

Wow, thats a big area change on the PRP! Keep us informed how that 421" did.
Another note;
When looking at doing this mod, you must have your duck's in a row. You don't want to kill the port just by making the pinch bigger.
Yes, mainly due the fact the pinch point have a influence on flow distribution around the valve right? :roll:

It speeds the air up. There is time when that is great and helps fill the cylinder. But there is also a time where that area will be the restriction and let it log jam up or make it plow into the back of the bowl. Not being able to make the turn. There is a reason why some of the best port's don't have any curves in the Intake track! Most have a better angle to the valve and won't have a dual tapper port.

Look at a sb Ford head VS. sb Chevy. The area or csa is different for XXX amount of cfm. Runner length and where the "pinch" is, this IMO seperates the two and show's the effect's.
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Re: Pinch point debate

Post by bigjoe1 » Fri Mar 29, 2013 8:13 pm

The few times I have reworked heads in this area, I have seen very little or no gain in Hp as a result. I am talking about one or two percent at the most on a high HP engine, For the amount of work involved, you can get better results with other changes.


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Re: Pinch point debate

Post by rookie » Fri Mar 29, 2013 8:37 pm

Lockwire wrote:
rookie wrote:I would like to here thoughts on this as well, In discussions with David Vizard about the PRP he said they tried a test years ago on a head that they thought was maxed out for the ci and rpm of the combination, so they welded it up and basically eliminated the PRP and although the head gained cfm it lost swirl and the engine lost power across the rev range.
Pinch is only some thing someone thought up. Check out the actual speed that fluid is moving along in the area of discussion. that is # 1. 2nd is the length of the distance that you decide to keep that speed moving at the speed of decision to gain momentum for benefit. The swirl is incorrect shape or form for Vizard people, not the same thing. Its really easy except allot of people think wrong about it. This IS ONLY referring to the area in question. You need to do the math, if you cannot count you are screwed . And it works well for the people I do stuff for as well as others I share with. JMO . ........... on the pinch , real heads like F1 or race heads do not have pinch, they have an area that speeds the fluid up ( air ) so that the intake system will work. hope I helped. .......?? :) Erland has some really accurate info so look up his posts. Stuart. :)
F1 vs 23 degree head...how do you take that leap?
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Re: Pinch point debate

Post by Lockwire » Fri Mar 29, 2013 9:11 pm

F1 vs 23 degree head...how do you take that leap?

You see 23 degrees, I see and have worked with most of the stuff thats out there. Intake design is just that,intake design it will not know the difference. I never or did not see the original poster say 23 degrees, if I missed it I'm sorry. That is what I am reffering to just like you said, people get hung up. JMO, I have been doing this a long time but it has never changed fluid management. Pinch is there only in 23 degree head that you see because it has a pushrod in the way. Fuel heads do too. sds. :)

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Re: Pinch point debate

Post by rookie » Fri Mar 29, 2013 9:24 pm

Lockwire wrote:F1 vs 23 degree head...how do you take that leap?

You see 23 degrees, I see and have worked with most of the stuff thats out there. Intake design is just that,intake design it will not know the difference. I never or did not see the original poster say 23 degrees, if I missed it I'm sorry. That is what I am reffering to just like you said, people get hung up. JMO, I have been doing this a long time but it has never changed fluid management. Pinch is there only in 23 degree head that you see because it has a pushrod in the way. Fuel heads do too. sds. :)
Lets take a hypothetical situation, say you have a set of good flowing heads on drag race oriented SBC but the velocity at the pinch point is higher than ideal for the Engine CI and RPM intended.
How many SBC's do you know that can compare to an F1?
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Re: Pinch point debate

Post by Kahuna » Sat Mar 30, 2013 12:41 pm

I know I'm a dumbass, but what is PRP and pinch?
Jim

Or, is there an area on this forum that would decribe these things?
Thanks

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