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I’ve been wondering for quite a while now about the dimensions of the intake ports of the 13B/20B and how they differ between the generations. The only thing I’ve found out is that the intake ports of the Cosmo 13B are the largest of the 13B variations. The main thing for me is to find out the area of the ports both Primary (Blue) and Secondary (Red)
been discussed many times on the forum; my understanding is that the Cosmo ports come in at a steeper angle and what you’re really seeing there is not a larger port, but a longer angular cut across the cross section of it. Think of it as cutting across the same piece of pipe or tube at two places; one cut at an angle across it and the other cut being square / perpendicular to it. The actual ID is the same, but the angled opening will appear much larger.
I have the REW dimensions here somewhere, but because of the angle involved would suggest that you consider it along with the circumference rather than just the area. To do it right imo, an elbow has to be cut across at a similar , near matching odd angle across the cross-section and then reworked into a matching oval shape.
On the REW I want to say that my often less than perfect memory recalls it equating to round sizes of 1.35” ID for the primary and 1.75” ID for the secondary.
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A while back, I was interested in learning about intake porting, runner length, flow , ect ect. While traveling down the rabbit hole I found some videos that were very informative. If you follow the series of SidneyCritic “Rotary port evolution” I think you’ll find it quite intriguing.
RX specialties had some interesting opinions. The first video series I linked doesn’t suggest that tho. It had a lot of really great information about port shape and runners. How they change from year to year etc. etc.. worth a watch. It’s a super deep topic that unfortunately not a lot of people test their theories. They just follow the regurgitated information that’s been passed around.
Even I wasn’t paying as close of attention as I had thought. Somebody shared something with me recently that really opened my eyes. It was right there in front of them all the time. In those very threads even.
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