The following question from Doug originated in the EMC thread and deserves it’s own post.
Doug T 10/26/2010
Ted & John and others, what do you think of the over/under ports? My personal estimation is that it is an advantage in the heads themselves but a disadvantage in the manifold. Estimating the effect in the combustion chamber is beyond me!
My two cents worth. The intake on the Y is where the advantages are strongest when dealing with the stacked ports. As Charlie brings up, the Y intake floor directly under the carb is level on both sides as opposed to most other engines with dual plane intake manifolds having the floors at distinctly different levels. This definitely helps in the tuning of the carb at all rpms. Besides the level floor feature, the Y intake is more conducive to having all the runners the same length and this is due to the stacked port placement on the heads. Having all the runner lengths optimized to the same as well as the correct length allows for a maximum torque output. On an intake with unequal runner lengths, short runners and long runners must average out for the torque which ends up being a detriment in achieving the maximum torque value. The short runners in this case pull down the long runners where overall output is being measured.
I’ll go out on a limb and state that the main problem with the oem Y heads would be in the difference in lengths of the runners between the short turn radius and the valves when comparing the upper and lower runners. The lower ports are at a disadvantage in this area. The turn in the ports around the head bolt would be secondary to this. John Mummert did an incredibly good job in raising the height of the short turn radius on the lower ports in the new aluminum heads which makes for a solid improvement over the stock oem iron heads. But even with the new heads, it’s still looks to be a challenge in keeping the flow balanced between the upper and lower ports when doing additional port work on the new heads. The problems between the lower and upper ports are simply more complex with the factory iron heads when attempting to port them for more flow.
In talking to Jon Kasse about the stacked ports and the problems with maintaining balanced flow between the upper and lower ports, his suggestion was to simply take those ports that could be modified for more flow and allow those particular ports to freely exceed those that couldn’t be modified any further. Overall horsepower and torque would still increase regardless of the mis-balance between the upper and lower ports.
Lorena, Texas (South of Waco)