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Post by repherence2 on Jun 10, 2018 23:08:28 GMT -5
To all the air box gurus and wizards, looking for suggestions on inlet port diameter.
Years ago when I was running 70cc chinarelli set ups, I modified my stock rear facing air box inlet. Those Veneto zip type air boxes. I opened up the inlet port to a rectangle about 1" x 1 1/2". I didn't know any better back then about air boxes, I thought bigger intake port, berrer performance.
I am currently a no-air-box person on a forward facing OKO carb for a 54mm bore.
I'm building another block right now. Motoforce mid race 70. 10 pin crank large spline large taper Ruima. Polini bearings and seals. Polini 24 cp carb.
From responses in other threads, I have come to see that there is a reasonable science behind the explanations of those that understand air boxes. I don't understand the science yet, so I am looking for suggestions for inlet port diameter.
Thanks in advance.
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Post by ThomasTPFL on Jun 11, 2018 0:24:08 GMT -5
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Post by Lucass2T on Jun 11, 2018 4:43:35 GMT -5
Helmholz is a formula regarding resonation. Those can be used for calculating airbox volumes and subwoofer housings volumes and such.
I made my own airbox for the 70mph axis a few years ago. You can find some info in my build thread. Didnt calculate anything really, it was merely for inlet sound muffling. Ran a huge open twin-air foam filter first and with that airbox no jetting changes were needed. Thus no power loss. Only less noise.
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Post by ThaiGyro on Jun 13, 2018 1:34:51 GMT -5
Hi repherence, there are very few, true guru's regarding resonent boxes. Only many who have tried and failed.
My views for your project are this: If you are using a 49cc OEM box for starters, consider that it is likely a bit over designed from the factory, as most engineering standards call for. Maybe. So it should be OK for a 70.
Your idea to allow more inlet flow, IMO, is the right thinking. My Honda 49cc mod motor has an OEM box with bigger diameter snorkel and more direct flow. It snorkels in front in low air flow space and flow reverses inside the box, but my filter is at the snorkel inlet, not inside the box. High flow K&N. That is my current thinking, as I want the box to contain the alternating pressure wave, without centered filter dynamics.
I have yet to try a rear facing setup, but can see merits, depending on ambient air flow/pressure at the snorkel. FYI, my OEM inlet port from Honda is bottom mounted, oval and measures near what you mentioned. From memory, however...about 13mm height by 30mm wide oval. May be wider...can measure when back in scooter ville.
If you know anyone with a Rotax or other 2T kart racer. In the US and Europe they are required to have a resonent box. The Rotax Max Evo 125cc makes 34+hp. Look it up. My next attempt will be to copy the triangulated design. They have 5 variations of the 125...with the same box, I think. No snorkel, but dual bottom velocity stacks...depending on hp, they use different diameter velocity stacks. (For a kart, the bottom is low air flow, as it is close to the engine pan, though I have seen folks cut an air hole...not my thinking)
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Post by eclark5483 on Jun 13, 2018 2:44:05 GMT -5
Another thing to consider, is how a carb works and how a venturi works. When you pull back the throttle on a carb, you are not letting in more gas (per say), you are actually letting in more air. Remember, the gas is just sitting there in a bowl in the carb, vacuum pressure from the air box is helping to create a suction that pulls that gas up and through the venturi. When you change the dynamics of what is coming in through the air box, you are also changing the atmospheric pressure used to create that suction that pulls up gas. Thus, the larger the hole/air mass, the lower the vacuum pressure. This is why you see people making changes to larger jets when using a free air cone or pod type filter. They reduced vacuum and need to compensate for the loss with a bigger jet.
IMHO, when it comes to drilling or adding more air into the air box, it's best to experiment with very tiny increments and several jets. It's kind of a long drawn out process, but usually, what you are going to find, is whomever created the original stock engine, probably already has it tuned on the air intake and jets to what is best for that engine. That's not to say you can't get more out of it, but typically, you will get better life and a better run if you can keep your engine as close to stock as possible, especially when it comes to air intake.
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Post by Lucass2T on Jun 13, 2018 5:12:42 GMT -5
Another thing to consider, is how a carb works and how a venturi works..... They reduced vacuum and need to compensate for the loss with a bigger jet. Makes sense. typically you will get a better run if you keep your engine as close to stock as possible, especially when it comes to air intake. Go wash your mouth!
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Post by benji on Jun 13, 2018 9:13:35 GMT -5
The "best performance' is kind of a matter of opinion. W the oem airbox you get much smoother fuel delivery but restricted volume of fuel and air (depending on airbox size and inlet opening) which gives the motor a decent amount of torque and limits top speed to a safe number for most riders.
If top speed is what your looking for, look no further than dirt bike technology. 2t motors that live at high rpm. A kx80 has a 24mm carb and does like 10-12krpm stock. Sounds like right where we want our stuff to be. A dirt bikes oem airbox would be a good bet I think.
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Post by repherence2 on Jun 13, 2018 11:17:39 GMT -5
Thank you to everyone so far for all of your explanations and insight.
So the air box that I am going to use has a modified larger air inlet. It was once a hole about 3/4" in diameter stock. I opened it up to a rectangle about 1" x 1 1/2". I'm thinking about closing that modified inlet port with a piece of lexan polycarbonate. I'm going to drill a hole in the lexan to recreate an inlet port.
It's kinda the reverse of drilling extra holes in the air box, I'm going the opposite direction. I'm trying to close it back up. I'm using a stock rear facing intake manifold mated to a polini cp 24 carb. The intake manifold is the biggest restriction, it's only about 5/8" to 3/4" diameter on the carb side. So, I was thinking of making the "new" inlet port for the air box around the same diameter. I guess I'm trying to achieve a vacuum on the carb mouth side and within the air box.
Most of my tuning experience is with UNI pod filters or just wide open carb, no filter.
Thanks again to everyone who responded. Any ideas are welcome. All opinions and suggestions are welcome too.
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Post by ThaiGyro on Jun 21, 2018 21:13:53 GMT -5
Hey repherence...let us know what you come up with! Many of us play in the mystery box. Easy to modify, not so easy to measure improvements.
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Post by repherence2 on Jun 22, 2018 0:50:35 GMT -5
Hey repherence...let us know what you come up with! Many of us play in the mystery box. Easy to modify, not so easy to measure improvements. So here is my logic. The limiting factor is the inner diameter of the rear facing stock manifold. I plan on making the air box inlet port the same diameter. Intake inner diameter is 18-19mm. Polini cp 24 manifold side inner diameter is 26mm. Carb mouth inner diameter is 42mm. Air box to carb rubber transition piece removed. Modified to black ABS pipe short radius elbow inner diameter of 40mm. The current air box inlet is the stock location opened up to a 28x38mm rectangle on a Vento type rear air box. So to keep it simple, I'm going to make a restrictor plate with a 3/4" diameter hole to cover over the modified air inlet rectangular port from the inside of the air box. I was thinking about cutting a stub of 3/4" diameter PVC and gluing it to the restrictor plate aligned with the 3/4" hole that I will drill to be the new inlet. A stub around 1/2", 3/4", or 1" in length. Kind of like a subwoofer box port. Just make any kind. Guess and test. Trial and error. That's just how my madness works. I understand that mopeds are both: an Art and a Science.
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Post by ThomasTPFL on Jun 22, 2018 0:56:24 GMT -5
Air is moving faster at the manifold than at the mouth of the carb.
On a long wide open run I think you'd be choking yourself some if the airbox inlet matched the manifold.
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Post by repherence2 on Jun 22, 2018 1:08:30 GMT -5
I have main jets from 100 to 158 in my tuning kit. So I'm hoping that the Motoforce mid-race 70 mated to a Yasuni c21 will at least need a 100 main jet. If not I gotta get main jets like 98 and below.
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Post by repherence2 on Jun 22, 2018 1:13:42 GMT -5
Air is moving faster at the manifold than at the mouth of the carb. On a long wide open run I think you'd be choking yourself some if the airbox inlet matched the manifold. You recommend a larger port? For real, I'm open to suggestions.
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Post by ThomasTPFL on Jun 22, 2018 1:16:08 GMT -5
I'm of the opinion tune it to rip with no filter or box, then put an air cleaner assembly on there that doesn't wreck your tuning. I want as much as I can get, and I don't want to seize an engine because I lost my airbox when I caught air hitting those railroad tracks at 50 mph.
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Post by repherence2 on Jun 22, 2018 1:32:47 GMT -5
I'm of the opinion tune it to rip with no filter or box, then put an air cleaner assembly on there that doesn't wreck your tuning. I want as much as I can get, and I don't want to seize an engine because I lost my airbox when I caught air hitting those railroad tracks at 50 mph. Roger that. That's the way my 54mm 90 block is set up. But it's too dang loud. It's super fun, but Loud. With this build its focus is on quieting the beast. I'm moving away from the roaring wide open carb to more stealth/incognito muffled air box.
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