http://2stroker.net/jennings/crankcase_volume.pdf shows how 6.3 and 10.3 inch intake lengths (from inside of cylinder to carb slide) can boost power by 15% between 1800 and 3300 rpm. The power boost is over what is had with a 4.3" inlet. Stock is around 3.5".
Gordon Jennings, in his 2 Stoke Performance Tuning book, wrote "My friends at McCulloch, who acquainted me with the stubexhaust testing technique, tell of tests they have performed with an engine having an intake period of 120degrees, and with pipes ranging from 5 to 9 1/2inches in length. They were able to move the power peak anywhere from 3000 rpm to 8000 rpm. The 9 1/2inch length gave substantially the same power at 3000 rpm and 4000 rpm; at 5000 rpm, an 8 1/2inch was best; at 6000 rpm and 7000 rpm the same power could be had with either a 9 1/2 or 6 1/2inch length; and the same was true at 8000 rpm, but with lengths of 5 1/2 and 8inches. From these tests, it was determined that the best compromise length for the entire range 30008000 rpm was 8inches; a 5 1/2inch length was slightly better for the range 60008000 rpm. It should be understood that these lengths only apply to this specific example, and would vary with any changes in intake timing, crankcase volume and intake tract diameter."
I experimented with different lengths on my bike and ended up with 8.5" (total intake length) as the most optimum. Look at this drawing to see what I did:
Gordon Jennings, in his 2 Stoke Performance Tuning book, wrote "My friends at McCulloch, who acquainted me with the stubexhaust testing technique, tell of tests they have performed with an engine having an intake period of 120degrees, and with pipes ranging from 5 to 9 1/2inches in length. They were able to move the power peak anywhere from 3000 rpm to 8000 rpm. The 9 1/2inch length gave substantially the same power at 3000 rpm and 4000 rpm; at 5000 rpm, an 8 1/2inch was best; at 6000 rpm and 7000 rpm the same power could be had with either a 9 1/2 or 6 1/2inch length; and the same was true at 8000 rpm, but with lengths of 5 1/2 and 8inches. From these tests, it was determined that the best compromise length for the entire range 30008000 rpm was 8inches; a 5 1/2inch length was slightly better for the range 60008000 rpm. It should be understood that these lengths only apply to this specific example, and would vary with any changes in intake timing, crankcase volume and intake tract diameter."
I experimented with different lengths on my bike and ended up with 8.5" (total intake length) as the most optimum. Look at this drawing to see what I did:
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