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So I had a set of chebby heads laying around the shop, so I decided to take a gander at porting the bowls. I am pretty sure I screwed it up real good, lol. But can you take a peek and see where I went wrong the most?

For this set, I was just getting a feel for the tools and all that, I wasn't really concerned about doing it "right" (as it is I pounded the seats out with the chuck of the tool, haha). But looking at it now I figured a little bit of guidance/pointers would be awesome!

Anyway, I have attached a link to an album with a few pics if you would be kind enough to share your thoughts.

Head porting pictures by red-tail1 - Photobucket
 

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It's kind of hard to see some of the work in those pics, but a few things I'd recommend for your next one on that head. First make a vision of the flow through the port, bowl and seat. Big is not a concern - smooth transitions is. Air does not like to change directions quickly, and will flow faster if it doesn't have to make any fast turns as it's traveling. For example, do not hog the bowl/seat transition all the way out to the seat diameter. Keep it .040 to .080 smaller so the new seats have room for another angle cut to 'bend' the flow around the corner. The more you can keep the air following along the turn, the better it will flow - so no sudden corners. Air will try to follow the outside of a turn due to mass, and losing the flow at the inside of a turn will drastically hurt your flow. While more of the air is to the outside, if you force the air to pull away from the inside by turning too suddenly, it will push outward and kill all the flow in that area, causing the same effect as adding a big lump of iron there, pinching the port. Smooth.

The short turn is another example of this. Smaller turns are counter-productive, and larger but smoother is the goal. Think of airplane wings and how air flows around them. This also brings-in your valve guides. Which would go through air easier - a pipe or a wing the same thickness? Longer adn narrower, with a blunt leading-edge tear-drop shape will flow the air better than just small diameter. Keep thinking smooth transitions. Slowly and smoothly move the air around to get past crappy obstacles like the valve stem and guide. If this port were a tiny model of a slide at a water park, try to make it the most boring ride out there.

There are some simple tools you can make to home-port with purpose rather than guessing. A couple examples are flow strings and manometers. In both cases, you need an airflow source, such as a shop vacuum or a small electric leaf blower (too big and it will overheat with the limited airflow). Set a valve at various lifts and test for smooth flow with a heavy thread or light string attached to the end of a little stick. A flapping or wiggling string indicates turbulence and is bad for flow. Some port shapes will surprise you as sections may actually flow backwards. Minimize these areas.

The manometer is a simple vacuum sensing device (actually a pressure sensor that reads pressures below atmospheric). A tiny hose and thin brass tube for a probe works well. Measure the pressure along the runners and bowl areas as before, looking for lower and higher pressure areas. Do your best to even them, starting with a stock port and massaging the lower pressure areas out as they are a restriction until you hit the average pressure in the port. Again, in both cases, don't over-do it. Smooth is fast.

Finally, consider swirl in your porting. This is a tightly-held secret with many head porters, but is simple in concept. Low-pressure air flows spin like water down a drain. "Swirl-port" heads are actually the opposite, and are ported to help straighten the flow for better cylinder filling, whereas "normal" porting follows the flow for least resistance. Swirl chambers increase the effect at the end to decrease burn time and increase fuel vaporization and mixture consistency. Just like stopping the swirl in your drain and watching how the flow increases, flow-straightening can do wonders to increase port flows, but it takes a analytical mind, knowledge of fluid dynamics, and the ability to envision the airflow (along with proof-testing) to make gains with it. It's not something you should try if you only have one set of heads to port. In that case, follow the airflow with the string testing for least restriction. Hope that helps.

David
 

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So I had a set of chebby heads laying around the shop, so I decided to take a gander at porting the bowls. I am pretty sure I screwed it up real good, lol. But can you take a peek and see where I went wrong the most?

For this set, I was just getting a feel for the tools and all that, I wasn't really concerned about doing it "right" (as it is I pounded the seats out with the chuck of the tool, haha). But looking at it now I figured a little bit of guidance/pointers would be awesome!

Anyway, I have attached a link to an album with a few pics if you would be kind enough to share your thoughts.

Head porting pictures by red-tail1 - Photobucket
"wasn't really concerrned on doing it "right" " . then what did you learn ?
 

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Discussion Starter #4
It's kind of hard to see some of the work in those pics, but a few things I'd recommend for your next one on that head. First make a vision of the flow through the port, bowl and seat. Big is not a concern - smooth transitions is. Air does not like to change directions quickly, and will flow faster if it doesn't have to make any fast turns as it's traveling. For example, do not hog the bowl/seat transition all the way out to the seat diameter. Keep it .040 to .080 smaller so the new seats have room for another angle cut to 'bend' the flow around the corner. The more you can keep the air following along the turn, the better it will flow - so no sudden corners. Air will try to follow the outside of a turn due to mass, and losing the flow at the inside of a turn will drastically hurt your flow. While more of the air is to the outside, if you force the air to pull away from the inside by turning too suddenly, it will push outward and kill all the flow in that area, causing the same effect as adding a big lump of iron there, pinching the port. Smooth.

The short turn is another example of this. Smaller turns are counter-productive, and larger but smoother is the goal. Think of airplane wings and how air flows around them. This also brings-in your valve guides. Which would go through air easier - a pipe or a wing the same thickness? Longer adn narrower, with a blunt leading-edge tear-drop shape will flow the air better than just small diameter. Keep thinking smooth transitions. Slowly and smoothly move the air around to get past crappy obstacles like the valve stem and guide. If this port were a tiny model of a slide at a water park, try to make it the most boring ride out there.

There are some simple tools you can make to home-port with purpose rather than guessing. A couple examples are flow strings and manometers. In both cases, you need an airflow source, such as a shop vacuum or a small electric leaf blower (too big and it will overheat with the limited airflow). Set a valve at various lifts and test for smooth flow with a heavy thread or light string attached to the end of a little stick. A flapping or wiggling string indicates turbulence and is bad for flow. Some port shapes will surprise you as sections may actually flow backwards. Minimize these areas.

The manometer is a simple vacuum sensing device (actually a pressure sensor that reads pressures below atmospheric). A tiny hose and thin brass tube for a probe works well. Measure the pressure along the runners and bowl areas as before, looking for lower and higher pressure areas. Do your best to even them, starting with a stock port and massaging the lower pressure areas out as they are a restriction until you hit the average pressure in the port. Again, in both cases, don't over-do it. Smooth is fast.

Finally, consider swirl in your porting. This is a tightly-held secret with many head porters, but is simple in concept. Low-pressure air flows spin like water down a drain. "Swirl-port" heads are actually the opposite, and are ported to help straighten the flow for better cylinder filling, whereas "normal" porting follows the flow for least resistance. Swirl chambers increase the effect at the end to decrease burn time and increase fuel vaporization and mixture consistency. Just like stopping the swirl in your drain and watching how the flow increases, flow-straightening can do wonders to increase port flows, but it takes a analytical mind, knowledge of fluid dynamics, and the ability to envision the airflow (along with proof-testing) to make gains with it. It's not something you should try if you only have one set of heads to port. In that case, follow the airflow with the string testing for least restriction. Hope that helps.

David


Thank you, David! that does help. I have been talking to a friend of mine that is speaking the same language as you with fluid dynamics and so forth. he showed me a few tricks to a home made flow bench, and he suggested using smoke sticks to do the same thing your string idea has. He has a real flow bench at his shop, so I can test what I have done at a later date.

Anyway, your tips are much appreciated.

So, I shouldn't touch the area directly below the valve seat? Leave that as the machined roundness, just smooth the transitions to the port itself?

I only did one cyl, so I have a few more to try on :)
 

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Discussion Starter #5
"wasn't really concerrned on doing it "right" " . then what did you learn ?
How to use the tool... how to cut, and how to keep it from bouncing, how to smooth,how to hog out sections, ect. what burr to use to get what shape.
 

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So, I shouldn't touch the area directly below the valve seat? Leave that as the machined roundness, just smooth the transitions to the port itself?
You can touch them, but only if you can make the transition smoother. Many people will make the bowl/seat area as large as possible. This is a mistake, as a smaller area with smoother transition flow and less flow separation will flow more. See my quick drawing below for an example.

David

Airflow does not like corners or sharp bends. Note how the flow separates on the left, but follows the turn better on the right as it passes the valve seat:
 
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