Venturi dia / engine capacity

[kadora]

hello friends
For my new engine i need to find suitable carb .
Does exist some relation //formula// between venturi diameter and
engine capacity./4 stroke/ or i have to use try and error method.

Thank you a lot.
Kadora

 

[jixxerbill]

Cubic inches * Max RPM = X
X divided by 3,456 = Y
Y divided by Volumetric Effiency (on normal size engines this number is around .85) = Z
Z = Required Carb size in CFM

The size of your carb will have to flow at least this much air.. Hope this helps...Bill

 

[kadora]

Hello Bill
Thank you for your help but i still do not understand how i get carburetor
venture diameter from your formula. Please by patient with me i am math dumb.
Say i have 0,85 Ci engine capacity so

X = 0.85 ci x 3000 rpm = 2550
Y = 2550 / 3,456 = 737,84
Z = 737,84 / 0,85 = 868,05 CFM

CFM is probably air volume flowing trough a carburetor /i am metric guy/
I do not understand how i can find right carb on the base of 865,05 CFM.
Would you help me more please?
Thanks Kadora

 

[canadianhorsepower]

Hello Bill
Thank you for your help but i still do not understand how i get carburetor
venture diameter from your formula. Please by patient with me i am math dumb.
Say i have 0,85 Ci engine capacity so

X = 0.85 ci x 3000 rpm = 2550
Y = 2550 / 3,456 = 737,84
Z = 737,84 / 0,85 = 868,05 CFM

CFM is probably air volume flowing trough a carburetor /i am metric guy/
I do not understand how i can find right carb on the base of 865,05 CFM.
Would you help me more please?
Thanks Kadora

something is totaly wrong 865.CFM is good for 1000ci motor
what's youre bore and stroke specify Metric or imperial

 

[rythmnbls]

The error is in the calculation for Y

Y = 2550 / 3456 = 0.73784

therefore Z = 0.868 CFM, which sounds about right.

Regards,

Steve.

 

[123RWO]

Kadora, you can go to the Tower Hobby site and find an OS, 4-stroke, R/C engine that has approx the same displacement as yours and note the carb model # listed in the specs. Tower Hobby usually sells the carbs separately as spare parts and sometime lists the carb throat dia. in the specs. Even if they don't list the carb throat dia, you could just buy the same carb and be resonably assured that it will work. Of course, this only works if your engine is similar in displacement and RPM the R/C engine you pick. This is another site that can provide some data points: http://www.perrypumps.com/prod01.htm

RWO

 

[jixxerbill]

something is totaly wrong 865.CFM is good for 1000ci motor
what's youre bore and stroke specify Metric or imperial

That would be true if only turning the engine around 2500 rpm. Also we are talking normally aspirated engines. If you add nitrous and or blower/supercharger totally different formula. Bill

 

[kadora]

thank you guys
i will buy a carb from model hobby shop for my 14 ccm vee tween
However to know some theories is always useful for engine builder.
Happy to be member of this community.
Best wishes from Slovakia
Kadora

 

[t.l.a.r. eng]

A very easy method to get into the ball park is to make the carb bore diameter .25% of the cylinder bore dia. Use only one bore's diameter on a multiple cylinder engine as well.

If you look at some model engines and measure them you will see it is pretty close.

 

[Banjoe]

t.l.a.r. eng - is that 25% or 0.25% of cylinder bore?

 

[t.l.a.r. eng]

Bore size divide by 4

e.g. 1.000 inch bore divided by 4 = .250 inch carb bore diameter at the throttle plate.