Just wondering if there is any cylinder head guys here that would be interested in discussing port cross sectional area needs for given engine builds. I have had some education on cylinder heads and still working on gaining experience and would like to find people to share ideas with.
Cylinder head cross section
- Thread starter Brute Force
- Start date
There is a formula, which i don't know off the top of my head, to determine the area needed for peak RPM horsepower/ cubic inch.
Yes, I actually have a spread sheet to calculate that. What I'm more interested in is how the cross section area and where the minimum is at affects rpm after peak power. What got me thinking of this was two engines we recently did that are very similar in almost every respect accept the heads. One had a set of Pro Topline 220's on it with some minor bowl cleanup work, the other had a set of old 461 castings that had been ported and flowed approx. 250cfm @28". They made with in 3 hp of each other but the engine with the 220's only dropped a few horsepower past peak, and both engines peaked within 100rpm's of each other. But the smaller head made far better torque.
Here's my thoughts. Even if both heads flowed the exact same and made the exact same power the engine with the larger head (read larger cross section) made more power past peak because the airspeed due to the increased pressure differential was lower in the larger head and the smaller head reached it airspeeds over the short turn that were probably upwards of 400fps plus causing separation. As you know a smaller head flowing the same as a larger head will have much higher air speed at the same pressure differential. Plus the minimum CSA in the smaller head was further away from the valve, being the p/rod area. And both had 2.02 valves.
Incase your wondering, they both made the same power because of cam size. The engine with the 220's would have made more power in my opinion with more camshaft, but it was an effort to keep the rpm down for a street car. Both peaked around 6300 but the 220 motor only lost about 12 hp when pulled to 7000 rpms.
I believe that cross section size has a big effect on peak power and rpm, but its not the final say. Airflow in a given cross section is as important depending on how the rest of the engine is built. The average power and torque advantage up to 6500rpms went to the smaller head motor by a little. But as you know we dont race dyno's. In the car the larger head motor ran slightly quicker. But on the street the smaller head motor was better to drive around on cruise night.
As I said before, cross section area is important and should probably be looked at first, but airflow is also important.
Just my two pennies. Please, everyone share your thoughts or experiences
Here's my thoughts. Even if both heads flowed the exact same and made the exact same power the engine with the larger head (read larger cross section) made more power past peak because the airspeed due to the increased pressure differential was lower in the larger head and the smaller head reached it airspeeds over the short turn that were probably upwards of 400fps plus causing separation. As you know a smaller head flowing the same as a larger head will have much higher air speed at the same pressure differential. Plus the minimum CSA in the smaller head was further away from the valve, being the p/rod area. And both had 2.02 valves.
Incase your wondering, they both made the same power because of cam size. The engine with the 220's would have made more power in my opinion with more camshaft, but it was an effort to keep the rpm down for a street car. Both peaked around 6300 but the 220 motor only lost about 12 hp when pulled to 7000 rpms.
I believe that cross section size has a big effect on peak power and rpm, but its not the final say. Airflow in a given cross section is as important depending on how the rest of the engine is built. The average power and torque advantage up to 6500rpms went to the smaller head motor by a little. But as you know we dont race dyno's. In the car the larger head motor ran slightly quicker. But on the street the smaller head motor was better to drive around on cruise night.
As I said before, cross section area is important and should probably be looked at first, but airflow is also important.
Just my two pennies. Please, everyone share your thoughts or experiences
Good Reading from at freinds http://www.rehermorrison.com/
There are numerous high quality articles on Reher-Morrisons site which are copyrighted.
The one referenced here can be found at http://www.rehermorrison.com and then click on "tech talk" then"Flow Bench Fallacies"
There are numerous high quality articles on Reher-Morrisons site which are copyrighted.
The one referenced here can be found at http://www.rehermorrison.com and then click on "tech talk" then"Flow Bench Fallacies"
I'll share what I know of flow, doesn't come from cylinder heads, comes from designing gating for castings. But flow is flow, air or liquid.
Lets say you've got 2 pieces of pipe you're flowing thru.
One is 2.5 diameter, 3 feet long.
The other is 3.0 diameter, also 3 ft long, but necks down to 2.5 in the middle briefly, then flares back out to 3.0.
Both pipes have a the same minimum cross section of 2.5 diameter.
But every time, the 3" pipe that necks down to 2.5 briefly will flow more volume. Every time.
Thus flow is more than minimum cross section area, it's also relative to the length of the minimum cross section.
Can't really add any more to the conversation, I'm just happy to actually know something relevant in this forum
Lets say you've got 2 pieces of pipe you're flowing thru.
One is 2.5 diameter, 3 feet long.
The other is 3.0 diameter, also 3 ft long, but necks down to 2.5 in the middle briefly, then flares back out to 3.0.
Both pipes have a the same minimum cross section of 2.5 diameter.
But every time, the 3" pipe that necks down to 2.5 briefly will flow more volume. Every time.
Thus flow is more than minimum cross section area, it's also relative to the length of the minimum cross section.
Can't really add any more to the conversation, I'm just happy to actually know something relevant in this forum
The minimum cross section of the inlet path can and does have its effects on power production. Your on the right track with your current thoughts, and the pipe analogy was right on. Air is a fluid, and like any fluid it will become more dense when compressed. The higher the density the slower the speed. More times than not though the minimum cross section is rarely the biggest restriction in the flow of a head. A typical small block Chevy will have the min. C/S at the pushrod pinch. However this is not the critical choke point of the head. It's often the short turn transition or the throat size under the valve. Given two well designed ports, one with a std. pushrod location and one with an offset pushrod. they will make virtually the same power on the same combination. You've pretty much just experienced that. It's when very large displacements and cams are used that larger cross sections will be of the greatest benefit, even if there is minimal added flow observed on the bench. It's the simple demand for a large volume of air over an extended period of time that will require more space, even if there isn't much more detectable flow.
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