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Post by magoconnor on Jul 11, 2018 13:28:15 GMT -5
Hello.
I have been looking at different intake runner length, but I cant quite find a answear or a place to start.
So I hope you guys can help me out.
Well here we go..
A 139qmb engine normally reaches peak performance at around 7500 rpms.
Being that I have upgrade to 72cc and a performance cam should in-turn lower the peak performance to around 7000 rpms, I would think.
Does this seem right to you?
As far as I can read, the intake runner should match the pulse wave of the engine, there is a lengthy calculation you can make but the math seems to escape me.
A short intake runnner proves best at high rpms vs the longer version which got the moves at lower rpms.
If the intake runner is too long, it would be unable to supply enough air at high-speed.
I run with 4,5 grams rollers and a 15% stronger torque spring, so my rpms can get pretty high.
The trick is to be right in the middle too short and it will hurt the torque, too long the top speed.
What length do you think would work best for my setup?
Engine setup: 139qmb
72cc
stock head/ matched and ported.
performance cam
naraku cvk carb 18,5mm
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PirateLabs
Scoot Enthusiast
Posts: 296
Location: Bowling Green, KY
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Post by PirateLabs on Jul 11, 2018 14:43:57 GMT -5
I do not think this makes much difference on a single cylinder 4 stroke engine. Also, I think you might have it backwards...your performance cam will move the peak power up in rpm usually, not down. Also, in my experience, a longer intake increase optimal airflow at a higher rpm. Think high rise intakes and high lift cams on dragsters. They work for more performance but in a more condensed power band at higher rpms.
Anyway, this is my experience for what it is worth.
Bill
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Post by Happypancake! on Jul 11, 2018 15:09:15 GMT -5
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Post by magoconnor on Jul 11, 2018 15:22:21 GMT -5
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Post by Happypancake! on Jul 11, 2018 15:38:07 GMT -5
Without. Measurement is from the start of the intake to the intake valve.
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Post by magoconnor on Jul 11, 2018 15:59:01 GMT -5
Cool man I will get to work.
Do you also run with a bell mouth?
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Post by Happypancake! on Jul 11, 2018 19:32:03 GMT -5
No. Not on the diaphragm carb.
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Post by magoconnor on Jul 12, 2018 16:26:43 GMT -5
Which engine did you use?
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Post by Happypancake! on Jul 12, 2018 20:47:45 GMT -5
GET 49cc
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Post by magoconnor on Jul 13, 2018 14:30:11 GMT -5
Im quite unfamilliar with the honda engine, but acording to your diaphgram they can be spun up to almost 11000 rpm.
Would this be safe on my engine being a 139qmb ?
If say, I shoot to be in the 8000 rpm range, should I make the intake runner shorter ?
Because acording to your diaphgram a shorter runner would prove better at "lower" rpms, so maybe 8 inches would be better for my setup?
A little side question, do you think there is more power to be gained if I up the rpms?
Or would I just miss the sweet spot?
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Post by GrumpyUnk on Jul 14, 2018 12:24:15 GMT -5
mag, remember that when you 'tune' to a specific rpm setting, you will also affect other rpm range performance. You do not (I don't think) want a peaky engine that has power/torque only between 7700-8200 rpm. It would not be comfortable especially when running a CVT type transmission as it is quite difficult to 'select' the gear ratio for a specific speed. The contra, variator, and clutch springs will all enter into the equation, and it would be beyond my patience to try to get the 'best' performance. IOW, dont' reach for that max tuned output as it seems the other ranges will suffer. Or do so as you wish.
tom
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Post by magoconnor on Jul 14, 2018 13:46:27 GMT -5
No of cause not, my headache is getting worse just thinking about it  In the link to the ruchshop above, in the description to the bell mouth and intake runner: GET to perform tremendously through all rpm ranges, and to continue providing appropriate air fuel ratios all the way to 10,500 rpm. I have spoken with the ruckshop, they have informed me that from end to end (with the bell mouth) it messure 11,5 inches. This length is suitable for a get 49cc, I would just like to find a length that will perform best with my setup, as it says above "provide appropriate air fuel ratio all the way through the rev range" and "perform tremendously through all rpm ranges" Maybe brent could make one of his excellent and informing videos, to shine some more light on the subject, as it seems widly over looked. |
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Post by Happypancake! on Jul 14, 2018 15:58:53 GMT -5
Mag, My GET is tuned to maintain the rms powerband from the start then moves toward 10,000rpm. Here's some light reading. Once your variator is closed, up want that belt all the way to the edge and the belt buried deep in the clutch pulley. Correct belt length is also part of the equation. www.pbase.com/jdjetting1/ruckus_and_metropolitan&page=1 |
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Post by magoconnor on Jul 14, 2018 16:56:28 GMT -5
Hello Pancake.
Thanks for the link, great stuff.
I have just bought a new naraku carb and right now I'm focusing on getting it dial in to perfection.
After that I'm going to look over the cvt again, thou I have already done this and got it pretty close to where I want it.
But there is of cause always room for improvement.
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Post by magoconnor on Jul 16, 2018 8:20:17 GMT -5
Hello again. I have found a calculator to help narrow down the perfect length for my engine. Im having a little trouple filling out the formula, because I dont know the cam duration. The diameter of the hose I will use for the intake runner is 3cm As for the cam duration, does anybody know the spec for a stock 139qmb cam? Here is a link maybe you guys will have more luck than I :http://www.wallaceracing.com/runnertorquecalc.php Here is another one : www.velocity-of-sound.com/velocity_of_sound/calculator2.htmI also found this guy testing out different length on his motorcycle : www.phaedrus.me/id158.html |
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