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Guest Message by DevFuse

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carb questions

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4 replies to this topic

#1 Guest_jon61484_*

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Posted 01 August 2003 - 11:10 AM

I don't really know much about how the fuel enters the car (carb or FI) and would *like* if someone could answer this question.

CFM. Cubic feet per minute... maximum volume of air allowable to speed through the carb in a minute. my carb from the caluclators I've used is 113CFM (EA81 hitachi carb).

If I put a 500CFM (overestimating of course, it's just an example, I don't plan on adapting a huge holley to my car!) carb on my car, is the rating for the CFM the MAX it can try to mix gas with?

the N/A of the engine sucking in air, is that the only force that sucks/puts the air/fuel mixture into the cylinders? So if I put a huge carb on my car, it assumes at full throttle it's sucking in 500CFM of air, and so it puts the correct mixture of gas for the 500CFM into the engine, despite the fact that it's not *actually* pulling 500CFM worth of air in a minute? *thereby havnig a whole crapload of gas in the ratio gas/air* and flooding the engine? That would be a massive flood.

that would suck, if I'm right. or not suck enough, actually. ;)

thanks for any help, just trying to learn.

#2 Guest_GeneralDisorder_*

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Posted 01 August 2003 - 12:44 PM

well - the CFM rating is based on volume - not density, so it doesn't matter if your getting 100% air, or 100% fuel, or 50/50, or whatever - anything that goes through the carb will be limited to 500 CFM. As for the fuel / air mixture, that is determined by the jet sizes in the carb, and the mixture needle settings. You could put a 10,000 CFM carb on you lawn mower, and if it was jetted correctly, it would probably work fine. Heck - you don't even need a carb at all - squirt the fuel right into the cylinder, and have an open hole to the outside for air - it's called fuel injection :) Hows that for CFM? The rating in CFM for a carb is simply the MAX that you will ever get though it - as long as that max is more than your engine can use, you'll be fine. If it's less, then your carb will be the "bottleneck" of the system. Just as if you use a carb that is too big, the cam, valve sizes, displacement of your engine, or exhaust will be the bottleneck (at least - one of them will be).

I think where your thought process has gone arwy is that you realized the air is getting *sucked* into the cylinder by the engine, but didn't realize that the gas gets in by the same method - the carb does not "decide" how much gas to put into the fuel, it only decides the ratio of gas to air. the ratio (for our simple discussion here) will always be the same, say 1/10 (1 part gas for 10 parts fuel), so if your sucking 500 CFM, then 50 CFM of that is gas, and 450 is air. if you drop to 200 CFM, then 20 CFM is gas, and 180 CFM is air. get the idea? In conclusion: The carb determines the ratio, and the engine determines the volume.

BTW, in a not-so-simple discussion, you would have to bring in the fact that as the throttle opens, that ratio may change, in mathematical terms, the ratio of fuel to air is not linear with throttle position.


#3 Guest_jon61484_*

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Posted 01 August 2003 - 02:19 PM

groovy. thanks.

I was contemplating what would happen if I got a dual weber setup. Just for kicks, a thought I had.

#4 Guest_Struculeus_*

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Posted 01 August 2003 - 02:20 PM

You go Proffessor!


#5 Guest_GeneralDisorder_*

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Posted 01 August 2003 - 02:36 PM

Hehe - I do tend to get a bit technical sometimes - just hope that helps some people out with their carb theory. I find it's useful to understand how a system works before trying to work on or even operate that system. I remember when I learned to drive a clutch - It took maybe an hour or so - because I understood how the internal operation of a clutch worked, and how the transmission relys on syncro's to allow you to shift with the clutch dis-enganged. It was clear in my mind what needed to be done, and so getting my appendages to move in the desired fashion took almost no time. The same knowledge has helped me teach others how to drive a stick - I have a complete routine that I make them go through, and I explain what is going on inside the car to them. Makes everything easier.


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