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Old 05-17-2012, 02:51 PM   #1
Carnival Ride
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Question Cylinder head runner size vs airflow cfm

After buying a set of heads last year and doing some clean up on them, having them flowed and lurking on this site I have a few questions. Recently in the cylinder head show and tell thread Scott Foxwell showed some work on some heads he was working for a blown pump gas 588. When asked about the runner size he originally guessed about 320-325cc, Chad Speier commented that the runners seemed way small to him. Later Scott replied they actually poured 335cc.

Now this makes me question cfm and velocity. I assume and have heard that you can actually have too much velocity and get a dry center with the fuel tending to go to the outside which causes a non uniform air/fuel mixture in the cylinder is this true?

For example say we have something like a fully cncíd AFR 290cc head and say a larger 320cc runner head both flowing identical numbers in the cam lift range to be used. And they were going on something like a 572 and the owner wanted to turn at least 6500rpm but stay with a hydraulic roller cam. Would the smaller higher velocity head be better or could it possibly have too much velocity in an application like this?

Just asking a question and I guess make the assumption that the cam would be tailored for each head.
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Old 05-17-2012, 03:07 PM   #2
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Default Re: Cylinder head runner size vs airflow cfm

subscribing. have wondered the same thing. also the intake being used would have a big part in all this correct?
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Old 05-17-2012, 03:20 PM   #3
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Default Re: Cylinder head runner size vs airflow cfm

The average velocity must match your intended RPM.

In that thread, I mentioned that would be a good head for a low engine speed, which was exactly what it was going on.

Now Iím a big believer that the velocity profile vs. the port volume vs. the air moved, is key to a efficient port that will perform as it should. CFM isnít the first thing to use in choosing a cylinder head, and itís not tops on the list, but it is critical. When it comes to cylinder heads, port shape, localized velocity, average velocity and then CFM are in the order of importance.
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Old 05-17-2012, 03:42 PM   #4
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Default Re: Cylinder head runner size vs airflow cfm

Quote:
Originally Posted by Carnival Ride View Post
After buying a set of heads last year and doing some clean up on them, having them flowed and lurking on this site I have a few questions. Recently in the cylinder head show and tell thread Scott Foxwell showed some work on some heads he was working for a blown pump gas 588. When asked about the runner size he originally guessed about 320-325cc, Chad Speier commented that the runners seemed way small to him. Later Scott replied they actually poured 335cc.

Now this makes me question cfm and velocity. I assume and have heard that you can actually have too much velocity and get a dry center with the fuel tending to go to the outside which causes a non uniform air/fuel mixture in the cylinder is this true?

For example say we have something like a fully cncíd AFR 290cc head and say a larger 320cc runner head both flowing identical numbers in the cam lift range to be used. And they were going on something like a 572 and the owner wanted to turn at least 6500rpm but stay with a hydraulic roller cam. Would the smaller higher velocity head be better or could it possibly have too much velocity in an application like this?

Just asking a question and I guess make the assumption that the cam would be tailored for each head.
Do both heads have the same port center line length?

Stan
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Old 05-17-2012, 04:46 PM   #5
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Default Re: Cylinder head runner size vs airflow cfm

The cylinder head guy Johnny, goes by CFMCNC on here told me an interesting thing when I was talking to him and I believe it. He said the cfm number is not the thing that is most important, port shape, velocities, runner taper, ST shape, and VJ is more important then the cfm number, BUT when all of those factors are correct the cfm number is usually there.

This made so much sense to me and I think alot of the cylinder head guys on here will agree. Chad is the last person to discuss cfm numbers and always like to compare his heads based on cross section and design on the port, but his cfm numbers are still up there with the best of the heads. It sucks that the industry has dumbed cylinder head rating down to a simple number when it is much more complex then that.
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Old 05-17-2012, 05:23 PM   #6
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Default Re: Cylinder head runner size vs airflow cfm

Quote:
Originally Posted by shawns engines View Post
The cylinder head guy Johnny, goes by CFMCNC on here told me an interesting thing when I was talking to him and I believe it. He said the cfm number is not the thing that is most important, port shape, velocities, runner taper, ST shape, and VJ is more important then the cfm number, BUT when all of those factors are correct the cfm number is usually there.

This made so much sense to me and I think alot of the cylinder head guys on here will agree. Chad is the last person to discuss cfm numbers and always like to compare his heads based on cross section and design on the port, but his cfm numbers are still up there with the best of the heads. It sucks that the industry has dumbed cylinder head rating down to a simple number when it is much more complex then that.
It does suck...It's a reason why most people pick a cylinder head based soley on flow also. To beginners, an engine is an air pump so they see "more" CFMs and pick that one. Sucks that marketing did that...
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Old 05-17-2012, 05:36 PM   #7
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Default Re: Cylinder head runner size vs airflow cfm

I think more people are seduced by cc's than CFM these days. Have a look at how many heads are marketed by the cc size, such as one brand I will not mention for fear of the "ruin your head thread" gang dropping in. How many time do you see the phrase "you cant put a BIG enough head on that engine"
I would have thought that shape, cfm and airspeed were higher on the list than runner size? Wouldnt a badly designed big runner perform worse than a well designed small runner?
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Old 05-17-2012, 05:54 PM   #8
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Default Re: Cylinder head runner size vs airflow cfm

The correct way to label head sizes should be by MCSA,only Chad does that in his marketing,It is easier to label heads by port volume but not the ultimate criteria.All of our heads are designed with the manifold crossections and runner length included.Maybe Johnny will log on later and comment.Bill C.
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Old 05-17-2012, 06:17 PM   #9
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Default Re: Cylinder head runner size vs airflow cfm

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Originally Posted by Innovative225 View Post
I think more people are seduced by cc's than CFM these days. Have a look at how many heads are marketed by the cc size, such as one brand I will not mention for fear of the "ruin your head thread" gang dropping in. How many time do you see the phrase "you cant put a BIG enough head on that engine"
I would have thought that shape, cfm and airspeed were higher on the list than runner size? Wouldnt a badly designed big runner perform worse than a well designed small runner?
When Ive used those words in the past.... Its been with cross section in mind.

Imo, when the port is designed right.... Theres a direct corelation between the mcsa of the port and how many cc's it pours.

Of course the kicker to that whole thing.... Is the port being designed right.
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Old 05-17-2012, 06:24 PM   #10
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Default Re: Cylinder head runner size vs airflow cfm

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Originally Posted by LilJohn View Post
When Ive used those words in the past.... Its been with cross section in mind.

Imo, when the port is designed right.... Theres a direct corelation between the mcsa of the port and how many cc's it pours.

Of course the kicker to that whole thing.... Is the port being designed right.
Yep, you can't have the correct MCSA and everything else big or small. It's all based around the valve size.

When you talk cc's, your really talking average velocity, they just don't know it!
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Old 05-17-2012, 07:00 PM   #11
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Default Re: Cylinder head runner size vs airflow cfm

You can only get so much air through a X.XX" size hole. It's how well it get's there for the intended use.
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Old 05-17-2012, 09:03 PM   #12
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Default Re: Cylinder head runner size vs airflow cfm

Quote:
Originally Posted by Chad Speier View Post
The average velocity must match your intended RPM.

In that thread, I mentioned that would be a good head for a low engine speed, which was exactly what it was going on.

Now Iím a big believer that the velocity profile vs. the port volume vs. the air moved, is key to a efficient port that will perform as it should. CFM isnít the first thing to use in choosing a cylinder head, and itís not tops on the list, but it is critical. When it comes to cylinder heads, port shape, localized velocity, average velocity and then CFM are in the order of importance.
Okay, thanks.
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Old 05-17-2012, 09:03 PM   #13
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Default Re: Cylinder head runner size vs airflow cfm

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Do both heads have the same port center line length?

Stan
Hypothetical just trying to increase my understanding.
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Old 05-17-2012, 09:13 PM   #14
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Default Re: Cylinder head runner size vs airflow cfm

what we have learned in the superstock world, is..................................how quickly you can get that air in there and the shape of the port. But we are regulated to a certain runner volume. I've seen a car run faster with the same runner volumes but different port shapes..........
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Old 05-17-2012, 09:38 PM   #15
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Default Re: Cylinder head runner size vs airflow cfm

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what we have learned in the superstock world, is..................................how quickly you can get that air in there and the shape of the port. But we are regulated to a certain runner volume. I've seen a car run faster with the same runner volumes but different port shapes..........
Very true statement right there. You can have equal volume and two entirely different ports. The architecture of the head has a lot to do with how a port is designed.
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