2 carbs or one?

ok so i did some math.

2x15mm ~ 1x21mm

so do i want one carb or two?

how much of a benefit would i get from either?

Re: 2 carbs or one?

What math did you do? Are you talking crosssectional area? or cost?

Re: 2 carbs or one?

like the surface area of 2 15mm circles is almost equal to one 21mm circle.

Re: 2 carbs or one?

1 carb of 21mm is better then 2 of 15mm... unless you can make a system that closes one of them. In low-rpm you use one for more torque, and open 2 of them in hihg rpm for more power. I think that would actually work... but for normal use: take a 21mm ;)

Re: 2 carbs or one?

the two 15s will flow alot more air than athe single 21 wil tho

someone had the math for this

Re: 2 carbs or one?

i did math.

its the same.

its just that 2 small holes pull air faster then one big one (i thing :P)

Re: 2 carbs or one?

derbeez ryderz /

yup you should get way more velocity from 2. probably way more torque.

Re: 2 carbs or one?

it has to do with the ratio of flow area and the permiter of the flow area. for the same flow area one circle will flow better than any other configuration. id like to see the math behind multiple carbs if you can find it.

Re: 2 carbs or one?

i just did the surface area of a 15mm circle and multiplied by 2 (2 x 353.25mm2 = 706.5mm2)

and the surface area of a 21mm circle (692.37mm2)

Re: 2 carbs or one?

derbeez ryderz /

if its pulling the same amount of air through 2 small holes apposed to 1 gaping hole would that create a stronger vacuum?

Re: 2 carbs or one?

#CrazyWayne™ rocks. #CrazyWayne /

Go with too 13mm's or 14mm's.

Put stiffer Reed's on one for top end kick.

With soft reed's on the other for low sped power & toque.

That will help make tuning & jetting geezer.

Fas Bat out

Re: 2 carbs or one?

thats the theory

Re: 2 carbs or one?

your engine will work harder to pull through 2 carbs with half the area than one carb with the same equivelent area. you would see this as a stronger vacuum if you were measuring the vacuum across the venturi.

geometry is one thing, this is fluid dynamics.

Re: 2 carbs or one?

i dont know fluid dynamics lol

i have a 21 oko.

ill keep that i think haha

Re: 2 carbs or one?

math smath. there have to be at least 10 people on this board right now with 2 15 bings laying around, a 21 phbg on their running piston port 70 kit, and a cheap welder or a cool bud. try that shit.

hippy? webcast? <3

Re: 2 carbs or one?

if you ran straight piston port, it'd probably be easy.

reeds would be a whole different set of numbers.

figure the intake construction, if you're running piston port, you can just run a t junction with carbs on both sides of the bike, however the intake on the cylinder would probably have to be 21mm+ for your two 15mm carb situation.

then you have to assume that the cylinder will pull an equal amount of fuel from each carb, and figure out the air intake for each carb.

if you ran reed intake, you could either run one set of reeds with a T for two carbs, or devise some ridiculous sort of dual reed setup, which on a reed intake cylinder would be ridiculously difficult.

HOWEVER, it has been done with a vespa case, one rotary valve intake, and one reed valve intake. that setup was on the warbux ebay for sale a while back, it was just a engine case half, and the goodies youd need to split fuel/throttle i believe.

all in all, good luck.

Re: 2 carbs or one?

Re: 2 carbs or one?

Aera isn't the only factor to how much flow you get in a hole. Some air clings to the sides of the hole, some air breaks off.. You get turbulant air around the edges that effectively makes the carburator smaller. So two holes, with with the same area as the larger hole will flow much less. Especially when working with very small holes like those in our carburetors.

When the hole area gets twice as big, it flows four times as much fluid. Say your 15mm carb flows 1 unit of air, the 21mm carb will flow 4 units of air for a given vacuum level. Two 15's would flow uh, 2 units of air. If i had my graphing calculator handy i'd tell you exactly what size carb that would be the equivalent of... but it's not at arms length. and I'm unwilling to do the algebra ;-)

Re: 2 carbs or one?

"When the hole area gets twice as big, it flows four times as much fluid."

did you mean:

When the hole DIAMETER gets twice as big, it flows four times as much fluid.

its somewhat close as the area does go up by 4. but i can tell you for sure that 2x area does not equal 4x flow.

Re: 2 carbs or one?

get some tubing ,1 or 1.25 inch for the base and 3/4 or 1/2 for the carbs, and try it. personally i would go with 2 13mm carbs so you could just do 1/2 inch to the carbs and inch-15 in. tubing to the cylinder after they join.

Re: 2 carbs or one?

Two small carbs would have the same velocity as one big carb with the same cross sectional area. Think of it this way, the case pulls a certian ammount of air into it over a certain amount of time amount of time. That volume has to travel through the cross sectional area of the carb to get in into the case over the same ammount of time as it takes to fill the case.

Basically the same volume of air is pulled into the case regardless of what size the carb is, so a smaller carb has higher velocity because the same volume of air is moving through a smaller hole over the same amount of time.

Now at some point the air reaches it's maximum speed and can't flow any faster which is what makes bigger carbs desirable for higher rpm/displacement engines, also 2 small carbs have a larger surface area which would cause more friction creating less flow at max speed.

Re: 2 carbs or one?

No, I'm fairly sure I got that right. Lets check. ;-)


Plug in Q = 25 x 0.049 x 0.62 x 4.47

works out to Q = 3.4 gallons per minute

0.049 is the area of the hole. In this case a .25" hole. Double the area...

Q = 25 x 0.098 x 0.62 x 4.47

Q = 6.8 Gallons per minute.

And... my memory failed me. I stand corrected.

Okey, so the math works out. I'd still use a single 21mm carb over two 15's. The big problem with multiple carbs is synchronization. Especially right near idle. With a single carb, that isn't a problem You also have a single intake port, there's no reason to complicate things further by making a split intake.

Re: 2 carbs or one?

Looks like I was confused between diameter and area. The math is just ideal stuff corrected for the shape of the orafice. That ignores the behavior of fluids at high speeds. The layer of turbulent air around the sides of a pipe is the same thickness for a given air velocity. That's to say, two small carbs have a smaller working area than a single larger one. ;-) Not that I expect any of our motors to reach that sort of limit, even on single 18mm carb.

Re: 2 carbs or one?


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