Lynn,

I'm having a discussion off-list with a chap who is claiming the need for 2" runners for runways over 10,000 ft and hot days - in other words lower density situations where Max power is needed over 10,000 ft. I have suggested he look at velocity rather than size, but I feel he has a mind-set.

What do you think about his concerns for power at altitude, does this add another dimension with PP sizing?

I understand power will be down at altitude, but don't see how a bigger port compensates for that.

George ( down under)

That would be good if you are always in and out of this particular airport. Short 2" tubes to a turbo intake with about 4 pounds of boost.

 

You would be down to 55% power or less, without the turbo, so performance is going to be less than sparkling. You might want a bit more wing span as well. Altering tube diameters for pest power at 10,000 feet is reducing power at 6,000 where most folks cruise around. With the rotary, you can stay at full throttle and take out power by leaning well below peak EGT. 

 

A 12A bridge ported with small diameter runners 11" long has 173.3 HP. It actually uses the stock intake manifold gasket. That was with front EGT is 1515 rear is 1491. BSFC is (the devils own) 666. Oil temp 162 degrees. oil pressure 99.9 pounds and A/F of 12.3. This particular engine was dynode this low to get a good curve going. They are used between 7,500 RPM (218.2 HP) and 9,600 RPM (244.4 HP).

 

I would calculate the area of each stock port and add about 20% to each tube diameter to reduce flow drag. Make the tubes about 24" long to the manifold. The last 6" to the port I would reduce to match the port exactly. It is most important that the ends of the tube at the manifold look like the bell of a trumpet. You are probably looking for best power between 5,000 and 6,000 RPM. With your firewall forward built up and mounted on a shaft in pillow blocks. A bathroom scale one foot from the centerline of the pillow blocks would give you foot pounds of torque, and the RPM would give you HP. A couple of days of tuning (I bet) would have you over 190 HP.

 

Lynn E. Hanover