"I took the measurement to be the ID = 2 9/16 X 25/4 = 65.0875 mm.
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According to info I encountered this morning (from a vw dunebuggy site), your butterfly would support airflow needed for approximately 402HP. The problem is that as you advance your throttle beyond the motors ability to create HP, you get nothing additional. This sounds like something I've heard John Slade and Paul say "I pushed the throttle further forward and there was nothing else".
At this website they said....
"If the air valve is too big, you don't control the air flow. For example, a smaller displacement engine will only flow X amount of air. If the engine is flowing all the air it can by the time the throttle is 1/2 open, then as you increase the throttle, there is no air flow change into the engine. The electronics needs to have a linear air flow to do the calculations correctly. Again, bigger is not always better. For turbo applications, don't go much bigger than the size of the turbo output."
They also go on to give a calculation of max butterfly size, based on some rules of thumb...
First, they say "a basic is to make 100 horsepower, you need 140 CFM of air."
(Remember Eds calculations showing the 13b consumes about 280CFM at 6000shaft-rpm, or about 2758 prop rpm using a 2.175 reduction unit. So approx, roughly, 200hp at 2758rpm? Looking like it's all in the ballpark except maybe ability of a 13b to actually do this.)
" The rule of thumb for naturally aspirated fuel injection, at a standard 1.5 inches of water column, is that for every square inch of butterfly, you will flow 140 CFM.
To calculate the air flow for a 2" butterfly, you need to calculate the area first. Area = pi X radius squared. So, 1/2 of 2" is 1 inch, squared is still 1, times pi (3.14) = 3.14 square inches of butterfly area. This means that it will flow enough air for approximately 440 CFM, or 314 horsepower. Now remember this is approximate, and there are losses, but this is where you have to be real and ask yourself just how much horsepower is (the motor) really going to make?"
So lets take your butterfly size of 2 9/16 inches.
Bobs butterfly area = (2 9/16 / 2) x 3.14 = 4.02 square inches
Max supported airflow based on their rule-of-thumb = 4.02 x 140CFM = 562CFM
Max HP based on these (100HP per each 140CFM) = 402HP
The difference between the actual HP your motor will make and 402HP is throttle-movement that would do nothing for you.
Here's the website:
Still learning stuff,
Tom