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Head porting with draw through

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sowle

Member
Joined
Apr 4, 2003
Messages
154
I'm guessing there is very little to no benefit from ported cylinder heads with the draw through setup. As I get more in tune with completing two of my projects I am getting closer to being able to pick and choose from my parts hoard and let others benefit from the rest of it. I imagine the turbo mount flange to intake restriction cancels any benefit of head porting until boost pressure takes over. Then of course with draw through boost levels are low- on an 83 I had I was able to run 12psi on the right day but I would not consider that again. I don't remember anyone sharing their experience with cfm flow or other greatest hits with these draw through systems on forums before. Just interested in any thoughts.
 
Here's a hot air that did well. Not sure if any details are available.
 
Porting is a benefit regardless of N/A, draw or blow thru!
Not just for volume of flow, but for quality also. If you look at the intake valve bowl size and area in relation to the elbow size and area, and figure that only 3 cylinders intake per crank revolution, and they are staggered, your premise of area doesn't hold up in the real world operating conditions. Also the turbo discharge air speed is higher than bowl speed due to skin friction and changes in port shapes and area so the smaller "neck area" into the manifold isn't as bad as it would seem.
Port the intake bowls and port match the head and manifold. You can open up the neck area, and leave the 90 degree turn there, but you should smooth it.
The exhaust ports were designed to flow only the N/A engine size flow, not boosted flow. Figure the pressure ratio multiplied by the cubic inches of the engine, and exhaust porting makes good sense also. 14.7 lbs of boost nearly doubles the exhaust flow! And 7.5 lbs makes the exhaust equivalent of a 346" engine! All with a port designed for a 231" N/A engine.
Removing sharp edges and blending transitions are a good place to start in intake and exhaust ports. The 90 degree turn from the turbo to the intake manifold is not a design flaw! It is there to straighten the "corkscrew" of the flow out of the turbo into a more laminar flow and also to reintroduce the fuel that was "centrifuged" to the outside of the port walls back into the airstream. You can blend the short side radius of the neck and it will improve airflow, but leave the 90° wall alone except for smoothing it.
Unshrouding the chamber wall around the valves is a big deal too!
If the chamber wall is say, .080" away from the valve edge (intake or exhaust), then that part of the valve cannot flow any more than that flow, regardless of lift!
Just some ideas and trends from someone that has "lived" on a flowbench for over 40 years!
P.S. air doesn't flow where YOU want or think it should, it flows where IT wants to flow!
And if you think it flows straight down into a carburetor or injection stack, you are WRONG!
That's why porting costs so much money, it takes years of study and flowbench experience to do it correctly.
TIMINATOR
 
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