D
Deleted member 12676
Guest
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 stub-exhaust testing technique, tell of tests they have performed with an engine having an intake period of 120-degrees, and with pipes ranging from 5- to 9 1/2-inches in length. They were able to move the power peak anywhere from 3000 rpm to 8000 rpm. The 9 1/2-inch length gave substantially the same power at 3000 rpm and 4000 rpm; at 5000 rpm, an 8 1/2-inch was best; at 6000 rpm and 7000 rpm the same power could be had with either a 9 1/2- or 6 1/2-inch length; and the same was true at 8000 rpm, but with lengths of 5 1/2 and 8-inches. From these tests, it was determined that the best compromise length for the entire range 3000-8000 rpm was 8-inches; a 5 1/2inch length was slightly better for the range 6000-8000 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 stub-exhaust testing technique, tell of tests they have performed with an engine having an intake period of 120-degrees, and with pipes ranging from 5- to 9 1/2-inches in length. They were able to move the power peak anywhere from 3000 rpm to 8000 rpm. The 9 1/2-inch length gave substantially the same power at 3000 rpm and 4000 rpm; at 5000 rpm, an 8 1/2-inch was best; at 6000 rpm and 7000 rpm the same power could be had with either a 9 1/2- or 6 1/2-inch length; and the same was true at 8000 rpm, but with lengths of 5 1/2 and 8-inches. From these tests, it was determined that the best compromise length for the entire range 3000-8000 rpm was 8-inches; a 5 1/2inch length was slightly better for the range 6000-8000 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: