Author Topic: Port Flow Questions  (Read 1199 times)

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Cyclone03

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Port Flow Questions
« on: February 04, 2021, 10:50:01 PM »
Reading Jay’s cylinder head development thread sparked a few questions for me.
I’m not building or porting anything like some of you have done these are just question for discussion.

What is the downside to port flow going sonic? I’m assuming this referred to intake flow carrying fuel.

The next is port flow stacking,or backing up. Again my assumption is this relates to intake flow where the air/fuel column slams into either a port wall or the back of the valve and swirls back into the port.

I know a few here can “see” the air inside ports, can you help a guy see some of that picture?
Lance H

blykins

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Re: Port Flow Questions
« Reply #1 on: February 05, 2021, 06:27:56 AM »
If an air/fuel charge is going too fast, it's hard for it to make the turn into the valve.  It will separate. 

There could be two situations that you're referring to with "backing up".  Sometimes on the flow bench, a port will flow well and you will see it pick up flow to a specific lift, then if you go to a higher lift, it will flow less.   That's typically what I refer to as a port "backing up".   The other scenario could refer to a situation where a big fat port doesn't have velocity and it's hard to get it moving.   An air/fuel charge has inertia and momentum, just like anything else.  If it's hard to get it moving, then you can't fill the cylinders and can often run into a reversion situation. 

There is also something called "wave harmonics" where you're looking at the "timing" of the air fuel charge hitting the valve.  This is determined by the port volume, the length of the intake runner, etc. 
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Falcon67

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Re: Port Flow Questions
« Reply #2 on: February 05, 2021, 10:49:43 AM »
>If an air/fuel charge is going too fast, it's hard for it to make the turn into the valve.  It will separate.

I ported a set of 351C 2V heads and we put them on a flow bench.  At just above .450 lift, at around 250 CFM the bench made a loud BANG and air flow dropped 30 CFM.  The bang was the air failing to make the short turn since the 2V heads port floor is really low with a sharp short turn. Flow only caught up and maxed out at 254 around .550 but didn't sound all that great.  Low lift flow was nice though. 

Joe-JDC

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Re: Port Flow Questions
« Reply #3 on: February 05, 2021, 11:13:48 AM »
Ideally, you want the incoming air/fuel to center in the port and hit the back of the valve head dead center so that it spreads around the valve a complete 360*.  I keep coming back to the point that the shortest distance between two points is a straight line, and above a certain speed in the port if the port shape is not conducive to keeping the airflow centered, the air will shear and take the shortest path over the valve face.  That is the reason for the "pop" and reduction of flow in ports.  Most of the air then flows over the short side, and the top of the port is dead.  The only way to alleviate that issue is to kick the airflow back up into the top of the port so that it goes over the front of the valve into the chamber.  As the valve opens further, the flow over the short side will begin to fluctuate and go over parts of the valve that is unshrouded.  A combustion chamber that has a wall close to one side of the valve will kick the airflow in the opposite direction, and airflow will decrease or stagnate.  The more you take the short turn down, the more you create a waterfall affect and the air will take that shortest distance, especially with a very short turn like the FE or SBF.  The higher the short turn, the easier it is to kick the airflow up into the port and have it flow over the complete valve face.  Velocity helps with keeping the airflow slamming into the front of the port and going over the front of the valve head and not shearing over the short turn.  Joe-JDC
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Cyclone03

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Re: Port Flow Questions
« Reply #4 on: February 05, 2021, 08:11:19 PM »
So a reason “port plates” work so well in 4v C  head is not that they reduce volume ,it’s they raise the floor and help center the flow over the valve?

I have only had C1OE heads,428CJ heads and my current Edelbrock heads (First set up by Berry as his X head,then 10years later freshened and touched locally by Mission Auto in San Antonio) Besides the aluminum the noticeable change between the 3 is the intake port floor. Am I seeing an evolution of flow “control” maybe even shape v. Max airflow?
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Re: Port Flow Questions
« Reply #5 on: February 05, 2021, 08:48:47 PM »
"Yes" - the port floor is an improvement.  There are several characteristics in a port that do not necessarily show up on a flow bench, and "valve targeting" is a big one.  We tend to fixate on the flow numbers because that is a number we can generate and use for comparison, but its not "everything" by a long shot.