Pinch point debate

[BrazilianZ28Camaro]

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

PRP= Push rod pinch or pinch point means the curved area of the intake port about 1" from the head "mouth" for the push rod go through its slot.

[rookie]

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.

Chad, on an application like this I noticed that most heads of this size have a 1206 intake gasket (near 3" opening) it seems reducing this by filling the floor to just before the PRP to say a 2.7" opening would be better for more consistent velocity and on track acceleration...have you ever tested this?

[BrazilianZ28Camaro]

I'm wondering if the pinch point may act like a venturi to increase the charge density at the back of the valve when the piston is near BDC improving cylinder filling over a longer period

[rookie]

I'm wondering if the pinch point may act like a venturi to increase the charge density at the back of the valve when the piston is near BDC improving cylinder filling over a longer period

That is what DV has found in his testing. if you remove the short turn and bowl from a runner and flow just the port with the PRP it will easily flow 300+ cfm even on a small SBC head, the big issue I guess is when you reach a point where it is flowing at to high a speed...not sure where that is but some claim it is a problem.

[BrazilianZ28Camaro]

I'm wondering if the pinch point may act like a venturi to increase the charge density at the back of the valve when the piston is near BDC improving cylinder filling over a longer period

That is what DV has found in his testing. if you remove the short turn and bowl from a runner and flow just the port with the PRP it will easily flow 300+ cfm even on a small SBC head, the big issue I guess is when you reach a point where it is flowing at to high a speed...not sure where that is but some claim it is a problem.

Interesting.

Then probably a good port have this sorted out, I mean, set the pinch to a desired high velocity and the rest of the port to slow down the charge (creating turbulence on the boundary layer of the expanding pinch wall and port roof) after the pinch just enough to make the A/F flow around the valve eficiently, but not too slow, or the port will choke.

This turbulence may re-integrate the fuel droplets into the air stream just before it enter into the combustion chamber...


Lovely science ehh?

[Bob Hollinshead]

I'm wondering if the pinch point may act like a venturi to increase the charge density at the back of the valve when the piston is near BDC improving cylinder filling over a longer period

That's got to be the key to keeping the charge moving even when the piston isn't pulling on the port, the amount of taper upstream and the length of the intake tract is probably responsible for how much force is pushing.

[rookie]

This turbulence may re-integrate the fuel droplets into the air stream just before it enter into the combustion chamber...

This and swirl as DV is big on for fuel atomization...and before the DV bashers try to discredit swirl then start with this from Larry Meaux, Quote from this article http://hotrodenginetech.com/pipemax-cre ... d-porting/ "I’ve done some wet-flow and swirl tests many years ago and even combinations of wet-flow with swirl. I then purchased Quadrant Scientific’s Swirl Torque Meter, and it verified many of the test and swirl patterns I was seeing."

As a matter of fact most everything Larry talks about in this article is in direct line with what DV has been writing for years.

[user-9274568]

A venturi is designed to accelerate the air to a given velocity over a very short span and recover the energy at a given rate so as to not waste or use as little of the energy at hand to accomplish the task. Pressure recovery dynamics in the divergent region of the venturi are critical to the amount of air that can pass through it. That's why the bowl, throat, seat and chamber are so critical in a cylinder head. The bowl is the high pressure ( convergent ) region of the venturi and the throat is the venturi itself while the chamber acts like the ( divergent ) region to recover the energy.

[gmrocket]

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.

Chad, on an application like this I noticed that most heads of this size have a 1206 intake gasket (near 3" opening) it seems reducing this by filling the floor to just before the PRP to say a 2.7" opening would be better for more consistent velocity and on track acceleration...have you ever tested this?

its 3"(bigger area) because at the head/manifold face its not 90deg to the port centerline.

[rookie]

its 3"(bigger area) because at the head/manifold face its not 90deg to the port centerline.

Are you saying straight line?
Or right angle?

[SchmidtMotorWorks]

Swirl as measured on a flow bench has very little resemblance to what actually happens in the cylinder.
Air flows into an bottomless volume much differently than it follows a moving piston.


Pinch does not keep the velocity up between cycles, the flow stops and reverses with or without pinch.

[Pappy]

Could it be that in some cases the pushrod pinch has some effect on the tuned lengths? If so, which length? Pinch to valve seat or pinch to intake manifold runner entry? Would it enhance the tune of the overall length or fill the gaps to broaden the torque bandwidth? I don't think anyone starting with a clean sheet would design a port with a bulge to go around a pushrod.

[SchmidtMotorWorks]

I don't think anyone starting with a clean sheet would design a port with a bulge to go around a pushrod.

Exactly, push-rods and the rectangular shaped ports needed to go around them are one of the limitations of the American V8.

[rookie]

Could it be that in some cases the pushrod pinch has some effect on the tuned lengths? If so, which length? Pinch to valve seat or pinch to intake manifold runner entry? Would it enhance the tune of the overall length or fill the gaps to broaden the torque bandwidth? I don't think anyone starting with a clean sheet would design a port with a bulge to go around a pushrod.

I have seen several head porters on here say that the difference of where the pinch is on a SBF vs SBC dictates a difference in how you address the rest of the ports cross section...Maybe Chad will way in on this in more details.

Yeas the pinch was clearly a compromise from the OEM and is not a perfect port by any means.

[BrazilianZ28Camaro]

Pinch does not keep the velocity up between cycles, the flow stops and reverses with or without pinch.

Then why you can loose power and ET by increasing the pinch area too much, say up to 90% of the biggest CSA of the port?

I think I can explain my point better, it was, if the pinch act like a venturi it can increase the charge average velocity/density at the back of the valve just a few crank degrees before it closes, creating more momentum to the air charge pulse travel against the moving up piston.

This probably is a unique feature of the SBC 23° head.