Pinch point debate

[dezza]

300 fps would be setting the avg air speed of a port at 25 inches on the flow bench

[user-9274568]

I would be curious as well to what is considered the Ideal port speed, I could be doing the math wrong but on a 4.030 bore 3.50 stroke with a PRP of 2.4 at 7000 rpm I am coming up with 585ft/s

That would be .50 mach or thereabout (depending on what you consider SOS) and on the slow side, IMO.

[rookie]

I would be curious as well to what is considered the Ideal port speed, I could be doing the math wrong but on a 4.030 bore 3.50 stroke with a PRP of 2.4 at 7000 rpm I am coming up with 585ft/s

That would be .50 mach or thereabout (depending on what you consider SOS) and on the slow side, IMO.

But is my math correct for that area, I know average through out the port is what is important, I am just trying to make sure I have the equation correct.

[BrazilianZ28Camaro]

I'll let you figure all this out, I did. But your wrong in most of what your saying..

Thanks for the insight.

I'll start studying the PS engines ....afterall they haven't pinch point on their heads and haven't heat also running the engine Ice cold .

[user-9274568]

I'll let you figure all this out, I did. But your wrong in most of what your saying..

Thanks for the insight.

I'll start studying the PS engines ....afterall they haven't pinch point on their heads and haven't heat also running the engine Ice cold .

They do have a secondary choke.

This subject has many, many people confused. I'm going to leave it that way.

Pressure, Temperature, velocity, and area are all proportional or inversely proportional to one another BUT energy always stays the same. None is lost or wasted it just changes from one form to another.

[SchmidtMotorWorks]

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.

Nope, that would be perpetual motion, you can't add density to flow with an upstream restriction.

BrazilianZ28Camaro is correct.
Restrictions are put into cylinder ports all the time in order to create more kinetic energy. That energy gets turned into pressure and density at IVC. The difference between kinetic & potential energy is just time.

Can you or anyone point to a non-push-rod engine of modern design that achieves competitive power that has a restriction built into the port more than 3" from the valve?
Maybe there is one but I can't think of one now.

[MrBo]

How about George Bryce’s Pro Stock motorcycle heads?

Re: bore size vs valve size?
________________________________________
"If you are going to race it..put in the biggest valve that will fit and the biggest cam that will fit.
To get the RPM you want..you need to do three things in this order in MHO.
1. Get the min. cross to get air speed to support WHERE you want peak power.
2. Close the intake valve at the point to match the min. cross section to suite.
3. Make the total intake tract lenght to match the port size and IVC for proper tuned length."


See where all I talk about is the min cross and airspeed above?
Number 1 is number 1 of importance...as long as the Valve is "too" big

Scott, I believe and use the idea that most engines are valve limited.
We built a HUGE bore engine and filled it full of valve to see what was up.
The valve was so big and the throat at 90%....we control the velocity.
With the min. cross section in the port before the turn.For instance, want peak at 7000 we would run 4.00 mcsa.
If we need peak at 9000, then we would redo the port with the min cross
in the same place, but say 4.60 mcsa.....works awesome.
All of this is with a 2.80 valve in it and made the minn cross 5.00
Now it flows 700 cfm @28 and peaks at 9800
As we need to go faster we may open the throat to say 94%
And make sure the throat is NOW the min. cross section area. Does that make sense?

This is not new at all..but it is not very used. The big advantage is when a
BIG valve opens say .200" tons of curtain area is exposed..4 valve style.
This produces big psi drop and early rush into the cylinder..airspeed!!!! LOTS

http://www.yellowbullet.com/forum/showt ... 768&page=2

[SchmidtMotorWorks]

How about George Bryce’s Pro Stock motorcycle heads?

I read about 6 posts by George in the thread, it doesn't sound the same.

Post edit,,,now I read all of the thread, George measures CSA before the turn, that doesn't mean intentionaly restricting at PRP, in fact he argues against it late in the thread.

[MadBill]

Stepping back briefly to the nominal Mach number of a port, two significant factors are 1. overall port design, e.g. straight, high port 'Superbike style' or low ports resulting in much of the flow being across the back of the valve, and 2. the lack or presence of sectional discontinuities causing local turbulence-induced restrictions and local high mach numbers. Either of the latter situations results in a lower optimum port velocity.

[MrBo]

There are other heads/manifolds with ports that are smaller in the middle than the entry or exit.

I am pretty sure there are NASCAR Cup engines that they purposely put a secondary choke in the port for certain tracks.
If the area of the port gradually gets smaller than gradually gets bigger, you can efficiently increase the average airspeed in the port.
Average airspeed is the 800 lb gorilla.
Who says all the pressure recovery has to happen in the chamber?

[SchmidtMotorWorks]

I am pretty sure there are NASCAR Cup engines that they purposely put a secondary choke in the port for certain tracks.

I have never heard of that, where do you hear about it?

If the area of the port gradually gets smaller than gradually gets bigger, you can efficiently increase the average airspeed in the port.

Wouldn't that would be a purpose of the inlet radius/bellmouth?

Who says all the pressure recovery has to happen in the chamber?

The fact that the valve is the flow restriction?

[user-9274568]

I'll add that I agree some of the cup guys about 5 years ago thought that pressure recovery didn't need to be in the chamber and the heads were jacked up. Look at a current cup head, it is light years ahead of a 5 year old design. It's a Bernoulli port. Which at the end of the day is proper in my eyes.

[MrBo]

If the area of the port gradually gets smaller than gradually gets bigger, you can efficiently increase the average airspeed in the port.

Wouldn't that would be a purpose of the inlet radius/bellmouth?

Entry losses....Ce instead of Cd

Who says all the pressure recovery has to happen in the chamber?

The fact that the valve is the flow restriction?

At what lift? What is the purpose of the bowl then?

The whole idea of making a port the “correct size” is in fact restricting the inlet stroke. If the port could deliver exactly what the cylinder demanded in real time, the engine would not even get 100% VE.

Hey Chad: Could you at least tell us where the secondary choke( from your post above) is on some Pro Stock engines?

[user-9274568]

If the area of the port gradually gets smaller than gradually gets bigger, you can efficiently increase the average airspeed in the port.

Wouldn't that would be a purpose of the inlet radius/bellmouth?

Entry losses....Ce instead of Cd

Who says all the pressure recovery has to happen in the chamber?

The fact that the valve is the flow restriction?

At what lift? What is the purpose of the bowl then?

The whole idea of making a port the “correct size” is in fact restricting the inlet stroke. If the port could deliver exactly what the cylinder demanded in real time, the engine would not even get 100% VE.

Hey Chad: Could you at least tell us where the secondary choke( from your post above) is on some Pro Stock engines?

Same place in a Comp head... Well actually that isn't entirely true. Some really fast Comp heads have the MIN at the PR area. But, that is a compromise from a direct result of a balanced port.

ALL properly prepared heads have the same average airspeed, or real close to it. Doesn't matter if it's on a 300cid or a 632cid...

[user-9274568]

If your building a cylinder head for an engine that is going to take a 2.180 valve for the required cfm demand. Let's say it's a head that can get by with a faster average velocity. You use your pitot and decide you need 3.00in² MIN. But the throat is set at 91.5% which is 3.08in²..

Would you make the front half of the port bigger than the throat?