extended intake for more low end power

jaguar

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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:
 

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jaguar

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Graphs

Here is a graph from the quoted research paper. The "tuned" length is what worked best with the engine they were experimenting with. For mine the tuned length is 8.5".
 

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jaguar

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I can climb inclined streets now that I had to pedal up before. I am very happy with this mod and highly recommend it if you want more torque.
 

DuctTapedGoat

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Wow Jag, I didn't even consider this! Nice. This in combination with an expansion chamber would really let you get some sick tuning options.
 

jaguar

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tuned intake and exhaust

Yes and no. An expansion chamber tuned for low rpm power is very long, around 85" which is why there are no low-rpm pipes available. You'd have to make your own. If you did that then yes you could have a torque monster. Figure 6 of http://2stroker.net/jennings/crankcase_volume.pdf shows that there's an additional 15% boost to be had when the exhaust is "tuned" (correct for the intended rpm range). So the two together show yield around a 30% gain!
 

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jaguar

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I bought a 58cc top end (which is ported better for low rpm power) and tested it on inclined streets. One was climbed at 11mph. After adding the intake extender I climbed it at 17mph!
Jennings reported that the power gains between a reed valve and an intake extension were about the same at rpm's under 3000.
 
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jaguar

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I used a long zip tie connected to the frame, but really hardly any support was needed because the hose was sufficiently stiff. I've rode it like that without the zip tie and it was fine.
 
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