Areas of Importance
Considering the flow through the intake port as a whole, the greatest loss must be downstream of the valve due to the lack of pressure recovery (or diffusion). This loss is unavoidable on intake ports due to the nature of the poppet valve. On the exhaust ports the opposite condition exists and one can control the geometry down stream of the highest speed section, namely the valve seat. This allows the possibility of good pressure recovery and is the reason exhaust ports flow better than intake ports of equal size.
Accepting the expansion into the cylinder loss as unavoidable, the rest of the port becomes that much more important. The critical areas are those that pass the most air at the highest speed for the longest time.
The valve seat configuration on the port and on the valve together form one of the most critical areas. The highest speed within the port is at or near the valve seat for most or all the duration of the cycle. After that, the throat area and short turn radius become critical at higher lifts in the middle of the cycle. The valve seat and valve head angles require careful study in each design.
The bowl area and the rest of the length of the port have important functions in controlling some of the dynamic behavior of the waves that traverse the system as well as setting up the air for a good entry to the throat. Shape, cross section, volume, cylinder swirl or tumble, and surface finish must be considered together with the overall design of the rest of the engine and vehicle to achieve good results.
The port is shaped to allow the maximum use of the available cross sectional area as the flow velocity should be optimized for the conditions the engine is expected to encounter. Well-shaped ports have few dead spots.
Some typical losses and their sources on a small block Chevrolet intake port.
1. Expansion exiting valve to cylinder 31%
2. Expansion, 30° (bowl) 19%
3. Short turn radius bend 17%
4. Expansion, 25° (valve seat entry) 12%
5. Bend at valve guide 11%
6. Expansion behind valve guide 4%
7. Wall friction 4% * (For sand cast surfaces. This falls to 3% for smooth surfaces)
8. Contraction at push rod 2%
Total = 100%
Read more about this topic: Cylinder Head Porting
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