How much CSA will be ideal

[swampbuggy]

Ok. 1 question at a time
#1. If an intake port is to small the air speed will be faster than if it was bigger? Mark

[Scotthatch]

Ok. 1 question at a time
#1. If an intake port is to small the air speed will be faster than if it was bigger? Mark

We talking cc now? To small as compared to valve flow limit? And airspeed on the bench or car?

[groberts101]

Ok. 1 question at a time
#1. If an intake port is to small the air speed will be faster than if it was bigger? Mark

Pretty obvious YES to such a generalized question, Mark. But more specifically towards the final result of what actually makes its way into the cylinder.. it matters WHERE it is too small!

IOW, you could have a horrible PR pinch point mCSA dragging the number down but a hellaciously large floor width to try and tame things enough to help the short turn out a little more. Conversely, you could blow the PR pinch out and have a horribly narrow apex width with no streamlined guide work and the motor will likely fall over faster than the previous port. Which is of course why pitot work is so important to stabilize and average out localized velocities in the area's where it matters most.

[Rick360]

Ok. 1 question at a time
#1. If an intake port is to small the air speed will be faster than if it was bigger? Mark

On the same engine and the same rpm the airspeed in a smaller port will be faster.

On a flowbench ... Nope. It might be faster or slower.

Rick

[Scotthatch]

Ok. 1 question at a time
#1. If an intake port is to small the air speed will be faster than if it was bigger? Mark

Pretty obvious YES to such a generalized question, Mark. But more specifically towards the final result of what actually makes its way into the cylinder.. it matters WHERE it is too small!

IOW, you could have a horrible PR pinch point mCSA dragging the number down but a hellaciously large floor width to try and tame things enough to help the short turn out a little more. Conversely, you could blow the PR pinch out and have a horribly narrow apex width with no streamlined guide work and the motor will likely fall over faster than the previous port. Which is of course why pitot work is so important to stabilize and average out localized velocities in the area's where it matters most.

Well put

[Scotthatch]

Remember everything you do is in support of the valve flow .... the goal is to have the valve as the restriction ... if the pinch point is the you will drive up the airspeed at that point and on the running engine this will cause it to nose over a little early as the air will be screaming into the bowl at to high a speed ... you are not looking for an exact number but the relationship to what the rest of the port is doing ....

A port that is to big will take forever to get the airspeed up on a running engine ... loss of low end torque .. fuel mix problem at low speed ....

As for best cross section ... I start with the area of the valve at target lift .. then take some measurements on the port to see where I am at in relationship to that .. some spots become quickly apparent as to needing work .. then you can sort out speed and how much of the port is being used if and if the shape is right .... the biggest problem is most of the time where you need to cut does not have material to cut .... and taking out material at slow spots does no good...

[swampbuggy]

Groberts, this port does not have any problems that i am aware of.....just wondering what CSA would be ideal for my application.
Rick, i thought IF the valve is NOT a restriction , and you increase the CSA. Or. MCSA the airspeed would slow down ?? Mark

[user-30257]

Groberts, this port does not have any problems that i am aware of.....just wondering what CSA would be ideal for my application.
Rick, i thought IF the valve is NOT a restriction , and you increase the CSA. Or. MCSA the airspeed would slow down ?? Mark

Your initial question has been answered a few times.. start there

[Rick360]

Rick, i thought IF the valve is NOT a restriction , and you increase the CSA. Or. MCSA the airspeed would slow down ?? Mark

If the valve is NOT the restriction and you make the port bigger it should flow more and velocity stay about the same.
If the valve IS the restriction and you make the port bigger it should make the velocity lower.

Size is relative. What is a big port or small port. A small port for a 434ci engine could be a very big port for a 283. On the flowbench they could either one be fast or slow depending how they were ported. Average flowbench velocity is an indication of port efficiency and NOT an indication of suitability for a particular engine size. I would never have a "target" airspeed. Shape the port right at the right size. A pitot can help you shape a port by indicating slow or fast areas to size a port area relative to the rest of the port but not the size of the overall port.

Rick

[statsystems]

Rick, i thought IF the valve is NOT a restriction , and you increase the CSA. Or. MCSA the airspeed would slow down ?? Mark

If the valve is NOT the restriction and you make the port bigger it should flow more and velocity stay about the same.
If the valve IS the restriction and you make the port bigger it should make the velocity lower.

Size is relative. What is a big port or small port. A small port for a 434ci engine could be a very big port for a 283. On the flowbench they could either one be fast or slow depending how they were ported. Average flowbench velocity is an indication of port efficiency and NOT an indication of suitability for a particular engine size. I would never have a "target" airspeed. Shape the port right at the right size. A pitot can help you shape a port by indicating slow or fast areas to size a port area relative to the rest of the port but not the size of the overall port.

Rick

You just need to keep reading this over and over until it makes sense.^^^^^^^^^^^^^^^^^^^^^^^^^^^^


Otherwise you'll spend too much time chasing an air speed number that doesn't mean much in the final port size/shape. I pay as much attention to the sound of the air moving through the port as I do pitot measurement.

[GARY C]

Ok. 1 question at a time
#1. If an intake port is to small the air speed will be faster than if it was bigger? Mark

On the same engine and the same rpm the airspeed in a smaller port will be faster.

On a flowbench ... Nope. It might be faster or slower.

Rick

Wouldn't that depend on how much cfm the port is moving?

[Rick360]

Ok. 1 question at a time
#1. If an intake port is to small the air speed will be faster than if it was bigger? Mark

On the same engine and the same rpm the airspeed in a smaller port will be faster.

On a flowbench ... Nope. It might be faster or slower.

Rick

Wouldn't that depend on how much cfm the port is moving?

Yes, velocity is the result of flow and csa. I tend to use cfm/sqin more than velocity values. Velocity and cfm/sqin are proportional. When you use cfm/sqin you (and anybody listening/reading) know it's flowbench numbers. When you express it as velocity it can get mixed up if not stated exactly what velocity. I see a lot confusion online about bench or engine velocity.

Rick

[swampbuggy]

To everybody posting.........first i must say i am learning as we go, and i am REALLY interested in this stuff. I know that i am asking questions that may be stupid to some of you, but as has been said by my friend Kevin "there is no such thing as a stupid question. Thanks for all the time given it is genuinely appreciated!!!!

[GARY C]

On the same engine and the same rpm the airspeed in a smaller port will be faster.

On a flowbench ... Nope. It might be faster or slower.

Rick

Wouldn't that depend on how much cfm the port is moving?

Yes, velocity is the result of flow and csa. I tend to use cfm/sqin more than velocity values. Velocity and cfm/sqin are proportional. When you use cfm/sqin you (and anybody listening/reading) know it's flowbench numbers. When you express it as velocity it can get mixed up if not stated exactly what velocity. I see a lot confusion online about bench or engine velocity.

Rick

Yes I never could get my velocity probe math to match my cfm/sqin math but I may have been calculating wrong... BTW I like the new term sqin.

[paulzig]

Shape the port right at the right size. A pitot can help you shape a port by indicating slow or fast areas to size a port area relative to the rest of the port but not the size of the overall port.

Rick

If you could do that, would you even need the CFM Flow numbers? Shape and size 1st, probe for localized velocity and call it good.