extended intake for more low end power

jaguar

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#1
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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#2
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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#3
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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#4
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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#5
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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#6
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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#8
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.
 



jaguar

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#11
Pipe length

Did you know that the exhaust pipe length affects power also?
Look at this graph plotting changes in delivery ratio (.8 means the cylinder is filled 80% with air/fuel mixture which equates to engine power) and pipe length. It's from the same crankcase_volume pdf.
I measured my stock pipe to be 9" from cylinder to muffler.
Between 30 and 40 inches really boosts the delivery ratio for mid-range rpms and probably 20 inches would be best for high rpm power.
I'm going to start experimenting on pipes.
Also some airplane engines utilize straight pipes that slowly taper outward. They supposedly give the best power over the whole spread of engine rpms.
 

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Kpi890

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#15
Man I wanna revive this post! I have been looking to extend my intake as well just like everyone on this thread. I was told I can use heater hose 3/4” ID 1” OD for my extension. I am just looking to get a good boost at my low end but will my top end suffer a lot?
 

Timbone

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#16
I doubt it would alter top end by much. Do it and see! You can always undo your change. That’s the value of this hobby: we are all experimenters and we can learn from others. Let us know your results. Thanks!
 

Kpi890

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#17
I doubt it would alter top end by much. Do it and see! You can always undo your change. That’s the value of this hobby: we are all experimenters and we can learn from others. Let us know your results. Thanks!
Alright will do thanks!
 

Kpi890

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#18
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Okay so I got the heater hosing and cut myself a 4.5” section of it just to test out first. I took a spare intake manifold and cut the mounting bracket off the end because Furry was correct there would not have been enough room for the hose clamps to bite onto. Here is how I currently have it rigged. I’ll have to cut a longer piece of fuel line to attach to the carb but I think it’s looking pretty good so far. Do you all see any issues with how it’s setup? The fuel line won’t be running downhill as much now is that alright? I have it zip tied to the frame for the moment just until I come up with the length of hose I’d like to use and then I’ll come up with a better way to stabilize the carb on the frame. And I’m not sure why my photos are posting sideways I’m sorry!
 

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#19
Ok, so, what would be an ideal intake length for a zeda 80 (ported, 40mm intake, 40mm stroke, 40mm exhaust), nt speed carb, 36t sprocket and a zeda 66 pipe for the 3k-8k rpm? Would it still be at the 8 inch total length?
 

Timbone

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#20
You’ll just have to experiment! Start at about 4 1/2” and work up. You’ll hit a point of diminishing returns when your intake gets too long.
 


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