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Formula for calculating carb size

Old 06-06-2003, 10:37 PM
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ANeat
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Default Formula for calculating carb size

Here is a formula I located for determining the correct carb venturi size or at least to get in the ball park.


D = K x Square Root(C x n)

where:
D=carb diameter, in millimeters
K= constant between 0.65 and 0.90 (determines the smaller and the biggest diameter to be tested at the specific engine)
C=cylinder displacement, in cubic centimeters
n=RPM at peak power/1.000 (be realist, dreaming only will make you try carbs far bigger than the correct, with results below your needs)

Here are a couple of examples

For a 46cc running @8000 rpm
46*8=368 (8 for 8000 rpm)

Square root of 368=19.18

19.18*.65=12.47
or
19.18*.9=17.62

So that says a carb between 12.47mm and 19.18mm should work, After plugging in some values on known combinations that work a constant of .7 seems real close so

19.18*.7=13.42mm

Which is a value that is real close to what comes on the Poulan 46cc saw that seems to work well.

For a Ryobi 31cc running @ 8000

31*8=248

Square root of 248=15.75

15.75*.7=11.02

So 11.02mm should work on the Ryobi, Ill have to try that and see what happens.

One more for Bob (av8tor1977) and his 16cc Echo

16*8.5 = 136 (8.5 for 8500 rpm, that little piston should rev a little bit)

Square root of 136= 11.66

11.66*.7=8.16

So 8.16mm would be the size here.

I hope this helps. We will all need to do some serious testing to see how good all this is

Adam
Old 06-07-2003, 03:32 AM
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av8tor1977
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Default Formula for calculating carb size

Thanks Adam,

Nice work. Sounds like we just about hit it on the button with 7.94mm for my little guy. I've copied your post for reference. I love things like this!!

Take care,
AV8TOR
Old 06-13-2003, 03:03 AM
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canadianjosh
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Default Formula for calculating carb size

I don't know too much about carbs, but i was wondering, if you were to open up the throttle barrel part would that not allow more air to flow past the venturi allowing more fuel to come in?? therefore increasing the rpm?. Basically i have an OS .40 LA that i experiment with, and if you look into the carb, and open the throttle barrel all the way, the throttle barrel hole is smaller than the carb opening, by about i would say .3 of a mm, that adds up to a decent amount of area, so if i was to pull the throttle barrel out and drill a hole to the size of the carb hole would that increae performance?
Old 06-13-2003, 12:36 PM
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Default Formula for calculating carb size

Hi,

More air isn't necessarily more fuel, but our carbs are adjustable with the needle valves, so if you increase the air inflow, you can just open the needles a little to get the proper ratio again.

I've never tried it on a carb like yours, so I can't advise you directly on that, but I have done this before. I once had a Fuji .19 engine that had a really tiny carb and I bored it out. Gained quite a bit of power. You have to be careful that you don't get too big for your engine size, as it will affect the fuel draw from the tank as well. If the carb is too big, it might need crankcase pressure or a pump to draw fuel. You might try comparing it to other .40 size carbs, and see what range you have to work with. Another option is the Perry high performance carbs for model engines.

Hope this helps,
AV8TOR
Old 06-19-2003, 05:28 AM
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Red Baron Mike
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Default Formula for calculating carb size

Opening the carb without opening the exhaust will have little positive results and possible negative results. The LA engine is a very modestly ported engine and is not designed for high performance. The opening in the carb coincides with the porting and the size of the exhaust outlet. Changing all three will have an affect. However, changing only one will not give you what you want and may destroy easy starting or idling.

Think of the engine like your breathing. The body needs to be able to inhale and exhale a similar volume of air. If your ability to exhale is restricted, even though you can inhale a massive volume of air, you will obviously have problems. An engine
performs in a similar manner.

Regards,
Old 12-19-2005, 09:46 PM
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captinjohn
 
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Default RE: Formula for calculating carb size

Red Baron Mike: If a person inhales more than he exhales...is the pressure going to build up somewhere in the end? no name!
Old 12-19-2005, 11:22 PM
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davewallace
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Default RE: Formula for calculating carb size

Hi Josh,
It sounds like you have an air bleed carb which is like that by design. If you alter this, you will hurt idle and transition. You may also loose fuel draw, since the engine relies on venturi suction to draw fuel. Gasoline engines usually have a builtin fuel pump in the carb.

Dave
Old 12-20-2005, 10:55 PM
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The_Pipefather
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Default RE: Formula for calculating carb size

To add to what ANeat posted:

This formula is originally from Gordon Jennings' Two Stroke Tuners handbook.

He says that the constant C is a "gulp factor" that depends on other parameters like intake port shape, duration, crankcase volume and expansion chamber characteristics. He gives a range of 0.65 to 0.9 for C, and says that the lower numbers are for motocross type engines and the higher ones for GP engines.

I am inclined to think that our conversion engines are rather mildly ported (from the port duration and RPM numbers I have seen on RCU) and therefore will need a 'C' closer to 0.65. So ANeat I think your 0.7 number is on-the-dot.
Old 12-27-2005, 07:00 PM
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Default RE: Formula for calculating carb size

I agree 100 percent. I have taken the factors and made a spreadsheet with them. Works perfectly. Also just check to see what Zenoah, BME etc. use for carbs and run the formula. Right on the money.

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