Stacked Ports Discussion


http://209.208.111.198/Topic51335.aspx 		Print Topic | Close Window

By Ted - 15 Years Ago

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.

By jepito - 15 Years Ago
I worked on a midget a few years back that had different length air horns. (Mechanical injection). Two 4 1/2" two 5 1/2". The engine builder did this to broaden the power band. Since midgets and sprints are direct drive you need power from 3000 to 9000rpm. So if you require an engine with a wide power range the stacked ports maybe a positive.
By pcmenten - 15 Years Ago
jepito beat me to it. I've read discussions about regarding a V8 with a dual plane intake as if it were two V4s. The V4s would probably have slightly different torque curves because of the different tuning of the intake, but the net torque curve would be less peaky and broader. But I could be crazy.



With regard to the turn in the port around the head bolt; I thought I read that the effect of that turn was to cause fuel droplets, especially the bigger ones, to be directed toward the center of the cylinder because of their larger mass compared to air. The momentum of the larger fuel molecules prevents them from changing direction as quickly as air. The result is a better mixture distribution and more even temperatures across the combustion chamber. But I might be nuts.
By John Mummert - 15 Years Ago
I can't say that there are any great advantages to stacked ports.There are times when I think that it was an idea that should have never gotten past the cocktail napkin after a 3 martini lunch. But stacked ports are what we have and what makes a Y-Block a Y-Block in my mind. I can remember when any other design looked strange to me.

From an air flow stand point the biggest problem is that the lower must rise from near the deck surface to a reasonable height of around 1" in order to make a well shaped turn toward the back of the valve. The air must approach the back of the valve at close the valve angle of 18 degrees. Some early Y-Block heads have a lower port that follows a gentle arc and approach the valve nearly flat. This is what limits the lower port air flow in heads such as the ECZ-C.

A problem arises in trying to make the lower port rise. It is under the upper port so only so much can be done before the divider is breached. This is why the upper ports in ECZ-G heads rises just passed the gasket surface, to make room for the lower port. In the case of the aluminum head we decided to design a lower port with as close to the shape we wanted as possible and fit an upper around it. Over it might be a better term.

In actual air flow numbers the lower port flows more than the upper to .450" lift. From that point on the upper is better. This is interesting to me because everyones answer to designing a new head is to raise the ports. But with the same gasket opening, valve and valve job the lower is better over much of the flow curve. Perhaps if the upper port was not compromised to fit around the lower it would be better.

I think that stacked ports fit into the Ford engineers plan for the engine. I believe that they intentionally put turns with sharp edges in the ports to increase turbulance, thereby keeping the mixture homogenous. Having the ports wide and low forced them to turn more than a side-by-side port head. Again the turns have corners that are sharper that they need to be. These turns ensured that the majority of the flow would pass by one side of the valve guide. This produced a great deal of swirl in the chamber. The chamber being offset to the bore increased squish area to increase mixture motion during ignition. Even the valve seats were recessed intentionally to produce turbulance as the mixture entered the chamber. Speed-o-motive found that recessed seats  work well in nitrous oxide engines and probably didn't realize that Ford did it 55 years ago and were following Ricardo's findings from nearly 90 years ago.

I think that the engineers did all this in an effort to reduce detonation and improve fuel economy. Unfortunately, at high RPM these turns hurt the power.

Due to the interaction of the ports with the stacked design I do feel there is a definite wall in regards to airflow and horsepower. There are still avenues to explore that would help straighten the ports and lead to increased power but there is only so much that can be done with the limited time and budgets of Y-Block development.