Reed Valve Intake Design - Does Length Matter?

DrGeorgeTompson (Alan) /

So as far as i know, she's only into girth.

I need to build an intake for my Sachs 505 80cc Athena kit since there is no 24mm option readily available. I could probably chop something else up and weld it, but i figured I'd try my hand first.

Does the length of the intake make that much of a difference? All of the ones I've worked with are super short and straight, but I want my carb off to the right side of the motor with the intake facing backwards. Is this coincidence? There's not really any space forwards for das carbulator.

What about the angle that the intake tubing is welded to the flange? Ideally, i want it to be 90 degrees since hard angles make flowing fluids angry and turbulent and shit, Right? But a 180* bent intake would probably need a pretty big radius to minimize restrictions.

I've been reading Jennigs but i didn't see anything that's like "dont make the intake too long, you dumb shit". He mostly talked about primary compresion and case volume, which im not changing because reeds.

Any input on intake crafting for a reed intake is much appreciated. I'm pretty much just winging this shit.

Re: Reed Valve Intake Design - Does Length Matter?

more volume is less pressure, more bends is a poorer flow, more distance is weaker throttle response.

when in doubt about what is optimal, look at dirt bikes. Dirt bike intakes just hug the carb against the reeds.

Re: Reed Valve Intake Design - Does Length Matter?

DrGeorgeTompson (Alan) /

Shit. I was expecting that.

Hell, i should be happy. A short, straight intake is a metric fuckton easier to fabricobble.

How come many piston port intakes are long as shit ?

Re: Reed Valve Intake Design - Does Length Matter?

clearance issues and you cant be having a sideways carb on top of your cylinder

Re: Reed Valve Intake Design - Does Length Matter?

Overpriced Parts /

Wow 24 mm carb !

I destroyed my clutch on my Sachs 505 several times with 80cc reed kit and that was just running a 19 dellorto Phbg, the longest the clutch lasted was 48 miles before destroyed itself and I tried everything including 3 disks and spot weld on nut.

Not to get off topic but what are your plans for the clutch/clutch life for that Athena kit is a beast with a mid size carb let alone a 24mm one

Re: Reed Valve Intake Design - Does Length Matter?

Intakes do have a resonant frequency. Generally a longer intake will align with a higher rpm, and the shorter will match the pulses at a lower rpm, much like an exhaust. Depending on the rest of the build it may not make any difference though.

Re: Reed Valve Intake Design - Does Length Matter?

DrGeorgeTompson (Alan) /

No worries dude. Definitely want to talk to people about the clutch. As for plans, I don't know hahaha. I bought the "performance" clutch disks from L2S. Problem is the disks are thicker and that limits me to a 5 piece clutch. I'll probably try to find a 7 piece at some point or see if an Uber clutch pops up at some point.

Oh, it's also a manual clutch 2 speed so I'll probably be ok for a while. At least longer than 40 miles lol

Re: Reed Valve Intake Design - Does Length Matter?

Kevin Bishop /

My buddy races everything and we built a pp bike. He told me the longer the intake the better top end. Of course the intake will depend upon the pipe and there are limits to the length. A good long bend will reduce blowback even a 90* would slow a pulse wave. I prefer the 90* bend before the carb in the air filter, and long sweeping intake. That bike ripped BTW, 8" handlebar chopped and welded for 21 carb.

Re: Reed Valve Intake Design - Does Length Matter?

Piston port engines will benefit from less regurged fuel out the carb inlet (fuel fog) if the intake is loooong

Alan W Wrote:

-------------------------------------------------------

> Shit. I was expecting that.

>

> Hell, i should be happy. A short, straight intake

> is a metric fuckton easier to fabricobble.

>

> How come many piston port intakes are long as shit

> ?

Re: Reed Valve Intake Design - Does Length Matter?

DrGeorgeTompson (Alan) /

Makes sense. My Garelli shoots fuel everywhere with the airbox off. I should fix that some day. Plus I'd love to try and get some more revs out of that thing.

Re: Reed Valve Intake Design - Does Length Matter?

DrGeorgeTompson (Alan) /

I found the section that Jennings talks about this. P.92 Idk how i missed this. I am specifically interested in the testing process. Seems easy enough. I have a good bit of tubing to mess around with.

Also, stub pipe. Fuck you, neighbors. I'm stoked.

For intake-length experiments, you will need not only the stub-exhaust system,

but a tachometer on your motorcycle and either a long, straight road (or trail) or a hill.

These last are required because you must load the engine heavily enough to allow you a

good, seat-of-the-pants reading of the engine speed where the pipe has its effect on power.

And that effect will be unmistakable, for the engine will pull very strongly when

it comes “on the pipe”. You will also find that intake pipe length can move the stub

exhaust-equipped engine's power peak over a very wide speed range. 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 ½-inches in length they were able to move the power peak anywhere

from 3000 rpm to 8000 rpm. The 9 ½-inch length gave substantially the same power at

3000 rpm and 4000 rpm; at 5000 rpm, an 8 ½-inch was best; at 6000 rpm and 7000 rpm

the same power could be had with either a 9 ½- or 6 ½-inch length; and the same was true

at 8000 rpm, but with lengths of 5 ½and 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 ½-

inch 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. They are given here only to

illustrate that: A) it is possible to make a given intake timing work over a wide speed

range by varying intake tract length; and B) that the same length may produce more than

one peak, or conversely that more than one length may be effective at any given power

peak.

Re: Reed Valve Intake Design - Does Length Matter?

DrGeorgeTompson (Alan) /

So what about the thickness of the intake piping walls? All I could find on mcmaster with the proper ID was tubing that I think is too thin (.7 mm) and tubing that i think is too thick (2.1mm). I bought them both.

Do I want the thinner walls for better resonance/ less dampening? The thicker tube is also harder to bend and can't form as tight of a radius, but i think it will be sufficient either way.

Re: Reed Valve Intake Design - Does Length Matter?

DrGeorgeTompson (Alan) /

Fuck it. Let's see how she goes.

Re: Reed Valve Intake Design - Does Length Matter?

An intake that long is going to crack. You basically have a weight out on the end of a long lever. With an intake that long, you need to brace it somehow.

How much can your reeds flow? Those tiny ass reeds look like your restriction point now. If your reeds only flow enough for a 19mm carb, you'll just be reducing intake velocity with a bigger carb. That will make your bike hard to tune and will give you no benefit over a smaller carb that matches up to your reeds.

By the way, it looks like you need a new spark plug.

Re: Reed Valve Intake Design - Does Length Matter?

DrGeorgeTompson (Alan) /

I'm not really worried about it cracking. The pipe is 2.1mm walled steel. It'll only crack when I inevitably wreck it.

As for the reeds being a choke point: I have though about this and agreed. So i modified it. The reed block is no 4 petal monster, but it shouldn't be that much of a restriction. I have ground the block to match the square hole to the round intake. The inlet is plenty large enough imo.

I was just plugging the hole with that spark plug. That was on my keychain when i got hit by a car in like October. Kept it as a momento i guess.

Re: Reed Valve Intake Design - Does Length Matter?

those reeds can flow a 24 no problem. thats a pretty sweet sagnum, machs, kromachs, sinto, whatever it is it rules.

Re: Reed Valve Intake Design - Does Length Matter?

DrGeorgeTompson (Alan) /

Be still my heart. I love reading that people like my monster.

I think I'm going to calls it a Peuchs.

Re: Reed Valve Intake Design - Does Length Matter?

yeahhhhh

its gonna rip them shitty little m5 screws out is the problem, if it doesnt crack. might, might not, might just foam up your fuel and make it carbureate all wierd or it might be perfect, who knows mopeds. i had something similar but it was made out of fancy aluminum and i thought it was way overbuilt and it broke so fast it was scary.

its not so much the length by itself, the rubber mounts on the sachs motor make it shake like a sonofabitch and thats why intakes and exhausts are always peacing out. when i made the aluminum motor mount bushings it fixed the problem.

Re: Reed Valve Intake Design - Does Length Matter?

DrGeorgeTompson (Alan) /

I see a lot of chatter about aluminum mounts. I wish i had a lathe so bad. I really like making shit out of metal. It makes me feel cool. Hell, i might just be able to get something from mcmaster that will be fine. I fucking love that place.

As for the intake breaking, I REEEALLLY dont think it will. It's obscenely beefy and mostly vertical . maybe the bolts will tear out but idk. I'm pretty good about making sure shit stays torqued. M5 torque spec (106 in-lbs) is more than 50% of M6 (177 in-lbs) and there are twice as many intake bolts as i'm used to w/ M6. In a cumulative sense, i'm better off. idk maybe i'm stupid.

I thought my Garelli had a lot of vibration. This thing is another level. I've only started it a few times. still have some stuff to work out before it's "on the road" Only time will tell.

Re: Reed Valve Intake Design - Does Length Matter?

DrGeorgeTompson (Alan) /

Those torque spec numbers are totally fucked, but it's still applicable. m5 is more than 50% or m6 per bolt.

Re: Reed Valve Intake Design - Does Length Matter?

yeah i don't want to be an internet hater, it might be good to go, what it will do, if it does it, is just bash the threads all to hell in the little screws and they get real sloppy, strip out if you don't catch them

loctite would help but who knows.

i actually ended up making the motor mounts out of hard plastic and they worked even better, i'm going to make some more soon, keep an eye out.

Re: Reed Valve Intake Design - Does Length Matter?

I just fixed a morini today, the kits have m5 bolts for the exhaust. I was really confused when the 5mm tool didnt work. That being said, as long as they aren't getting shitty thread engagement cracking it should't be a problem. TM24 is a really light carb too. But as Graham said it might have issues fuel foaming in the high rpms, fast sachs shake like a mother. We need more pics!

Re: Reed Valve Intake Design - Does Length Matter?

carl jepsen /

Yes length matters. Most intakes are territorial, and they determine territory size horizontally rather than vertically. But the difference between a 20 long and a 30 isn't enough to gain you any real amount of "territory" when you consider that for most of these parts, "territory" is measured in mph. The main difference between the two intakes would be in the volume for chemistry stability and dilution, and the amount of surface area for exchange of O2 and CO2.

Re: Reed Valve Intake Design - Does Length Matter?

DrGeorgeTompson (Alan) /

I... I don't understand any of that. I'm also pretty sure that's all complete bullshit. Are you a robot?

Re: Reed Valve Intake Design - Does Length Matter?

DrGeorgeTompson (Alan) /

The TM24 is super light for how big it is. When you say foaming, do you mean in the float bowl or in the intake? The intake is close enough to the frame that it would be really easy to brace if need be.

I'm not sure if I'll hit high enough revs to really make the fuel foam on the 505. My Garelli hits around 11k rippems and i don't have foam issues with an even lighter 21 Polini CP. My garbage CDI box holds me back, really.

I'll get some more pics tonight.

Re: Reed Valve Intake Design - Does Length Matter?

DrGeorgeTompson (Alan) Wrote:

-------------------------------------------------------

> I... I don't understand any of that. I'm also

> pretty sure that's all complete bullshit. Are you

> a robot?

I think so too, but maybe it's onto something with the points about horizontal and vertical miles per hour. I must be doing it wrong if I only get miles per hour lengthwise!

Re: Reed Valve Intake Design - Does Length Matter?

DrGeorgeTompson (Alan) /

So I started it again today and ran it for 30 seconds or so, nothing crazy just trying to gauge how wrong my jetting is. Pretty sure i need a bigger idle jet, but i haven't really tinkered with the idle mix screw yet and I think the throttle isn't closing all the way. Also, I haven't put the exhaust together yet. You just get too excited you know?

Anyways, So many hand-tightened nuts fell off. The lower motor mount bolt was like half out cause i didn't put a nut on it yet. This bike will be a good time.

This is the only picture i got today. I'll get some more of the intake tomorrow. The first picture I posted was before i brazed the absolute fuck out of it since it leaked like a sieve with my shit weld. I am pleased with the result and it holds a good vacuum. Brazing is cool and surprisingly simple. I still don't think it'll break.

(edited)

Re: Reed Valve Intake Design - Does Length Matter?

any updates on this build?

Re: Reed Valve Intake Design - Does Length Matter?

A bit late but search Jennings for intake pulse. Basically the length changes the harmonics, as does the size of the carb and air filter.

Re: Reed Valve Intake Design - Does Length Matter?

That's not late. I said it almost 10 months ago.

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