Pinch point debate

[Lockwire]

Rookie, Build Horsepower..........page 65 under cross sectional area. I looked it up. Stuart.

[rookie]

Rookie, Build Horsepower..........page 65 under cross sectional area. I looked it up. Stuart.

Ok I got that one, he has that info in his porting book on page 103 as well.

I'm just not familiar with some of the science terminology used at times.

[MrBo]

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.

[user-9274568]

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.

The heat energy was used to accelerate the air! Once the air slows down and the pressure rises the heat is absorbed giving back the working fluid its pressure. The cause of all this change is, AREA.

And many believe, at the .55 mach the pumping losses reach a point where the energy used is more than the energy gained from the inertia ram effect. In other words, the engines converting pressure drop into velocity and keeps doing so in an increasing manner until 690ft/sec. At that point the conversion of energy into air speed and inertia is more than the return one gets from the inertia. This does not mean the flow stops. It means the power increases stop and both TQ and HP drop like a rock. All your doing past 690ft/sec is stretching air molecules out and getting nothing in return (inertia ram effect) as far as cylinder fill.

[rookie]

At what point do you reach 690ft/sec on a SBC head?

[user-9274568]

To calculate the limiting port velocity: LPV=(.00353*RPM*S*B2)/CA Where:

S = stroke (in)
B = bore (in)
CA = minimum port cross sectional area in sq./in.’s
RPM = peak power rpm


You CAN design ports slower/faster than .55 mach.

[rookie]

To calculate the limiting port velocity: LPV=(.00353*RPM*S*B2)/CA Where:

S = stroke (in)
B = bore (in)
CA = minimum port cross sectional area in sq./in.’s
RPM = peak power rpm


You CAN design ports slower/faster than .55 mach.

Is that bore twice or bore squared?

[user-9274568]

squared

[rookie]

squared

Thanks

[BrazilianZ28Camaro]

The heat energy was used to accelerate the air! Once the air slows down and the pressure rises the heat is absorbed giving back the working fluid its pressure. The cause of all this change is, AREA.

Chad I believe your talking about the moment after the intake valve start to close?

AFAIK, heat the charge make its displace more volume, but less density. Less density coming into the cylinder makes less power.

The heat the charge absorbs will expand it but not add momentum (ram filling) to the charge because it will expand in both directions in relation to air flow. A hot intake valve do the same like "thermal choke". Probably this is the reason they like to place injectors aimed to cool the intake valves, coating on the valves ,etc.

And many believe, at the .55 mach the pumping losses reach a point where the energy used is more than the energy gained from the inertia ram effect. In other words, the engines converting pressure drop into velocity and keeps doing so in an increasing manner until 690ft/sec. At that point the conversion of energy into air speed and inertia is more than the return one gets from the inertia. This does not mean the flow stops. It means the power increases stop and both TQ and HP drop like a rock. All your doing past 690ft/sec is stretching air molecules out and getting nothing in return (inertia ram effect) as far as cylinder fill.

Agreed.

But I aways heard the ideal maximum port velocity would be around 300-310 ft/s @ pk HP. The number you suggested is way higher.
What I'm missing?

Thanks for reply everybody, keep the thoughts coming!

[Amilcar]

To calculate the limiting port velocity: LPV=(.00353*RPM*S*B2)/CA Where:

S = stroke (in)
B = bore (in)
CA = minimum port cross sectional area in sq./in.’s
RPM = peak power rpm


You CAN design ports slower/faster than .55 mach.

Chad,
Is that math related to 100% V.E ?

[user-9274568]

The heat energy was used to accelerate the air! Once the air slows down and the pressure rises the heat is absorbed giving back the working fluid its pressure. The cause of all this change is, AREA.

Chad I believe your talking about the moment after the intake valve start to close?

AFAIK, heat the charge make its displace more volume, but less density. Less density coming into the cylinder makes less power.

The heat the charge absorbs will expand it but not add momentum (ram filling) to the charge because it will expand in both directions in relation to air flow. A hot intake valve do the same like "thermal choke". Probably this is the reason they like to place injectors aimed to cool the intake valves, coating on the valves ,etc.

And many believe, at the .55 mach the pumping losses reach a point where the energy used is more than the energy gained from the inertia ram effect. In other words, the engines converting pressure drop into velocity and keeps doing so in an increasing manner until 690ft/sec. At that point the conversion of energy into air speed and inertia is more than the return one gets from the inertia. This does not mean the flow stops. It means the power increases stop and both TQ and HP drop like a rock. All your doing past 690ft/sec is stretching air molecules out and getting nothing in return (inertia ram effect) as far as cylinder fill.

Agreed.

But I aways heard the ideal maximum port velocity would be around 300-310 ft/s @ pk HP. The number you suggested is way higher.
What I'm missing?

Thanks for reply everybody, keep the thoughts coming!

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

[user-9274568]

To calculate the limiting port velocity: LPV=(.00353*RPM*S*B2)/CA Where:

S = stroke (in)
B = bore (in)
CA = minimum port cross sectional area in sq./in.’s
RPM = peak power rpm


You CAN design ports slower/faster than .55 mach.

Chad,
Is that math related to 100% V.E ?

no, based off mach

[user-9274568]

Again, it's ALL ABOUT AVERAGE VELOCITY!

[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