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Registered:
October 2003
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carb cfm to engine matching?
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Sat, 02 April 2005 05:11
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i got this formula off the web and was wondering if it was correct and applied to sidedrafts, downdrafts, mech and vacuum secondaries.
Engine Size (CID) X Maximum RPM / 3456 = CFM @100% Volumetric Efficiency
i calculated for my engine a 13t that i only needed around 190 cfm at 100% efficiency
i was wondering how you match carbs with engines. given that a dcoe 40 phh carb can suck in 180cfm per cyl.d a 22r carb gets 326 cfm. how does one factor in if its a side draft or downdraft when choosing a carb? do you ultiply the 180cfm by 4 if you have twin dcoe's? i'm just plain confused, any help would be greatly appreciated
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Registered:
October 2003
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Re: carb cfm to engine matching?
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Mon, 04 April 2005 07:10
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bump
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Registered:
October 2003
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Re: carb cfm to engine matching?
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Sat, 16 April 2005 12:27
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bump bump. any input please
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On Probation
Location:
North East NSW
Registered:
December 2002
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On Probation
Location:
North East NSW
Registered:
December 2002
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Re: carb cfm to engine matching?
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Sun, 17 April 2005 11:12
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basically, no engine will never reach 100% volumetric efficiency. the manifold and head and throttle butterflies and combustion and exhaust etc etc all take away from the full potential of the amount of air the engine can draw in. on a fairly normal street motor, you're likely to end up with max 85%VE (at your torque peak rpm) perhaps falling as low as 50%VE at idle. on a well modified motor, you can achieve 95%VE and higher. so multiply your 190cfm by 0.85, thats really about how much air your engine needs
yes, the formula is correct and real. it's mathematics
i think you're misunderstanding the formula, it calculates airflow requirements based on the efficiency of the engine, how you then deliver that air is a whole different kettle of fish. you can never OVER-AIR an engine, but you definately can SLOW THE AIR DOWN too much, by installing a carb with too-big throats. you can also restrict the air too much, by making the throats too small to flow the engine's max potential
if you put a big 750 double pumper on your 13T, it would be pathetic until you reached enough rpm to get the airflow up (likely 4000rpm or more), then it would really launch! if you put a nicely sized stock carb on there, sure it doesn't flow as much up top as the 750, but the air flows alot faster so your car idles and has response at 1500rpm and so on
the idea is to find a carb that gives good flow up high, without slowing down the air too much at low rpm, and hence reducing engine response and driveability. you may also want something that doesn't use alot of fuel if that's a concern to you
however, it goes without saying that a downdraft carb is likely to be less efficient at its air delivery than a sidedraft, as the air has to turn a whole 90 degree bend. so a 200cfm sidedraft is going to be more beneficial than a 200cfm downdraft
by the way, a dcoe sidedraft flows 180cfm PER THROAT, so one dcoe40 flows 360cfm. two flow 720cfm, etc
there are lots of different carb types to choose from. a downdraft is easy because it's what the engine came with, so you usually don't need to change much (manifolds, hoses, etc) and they are cheap and plentiful. a race downdraft like an IDA or IDF would go well, but requires a different manifold and will chew alot of fuel. a sidedraft DCOE or DHLA gives ideal air delivery, however they're expensive to aquire and tune properly
all these carbs can have the throats modifed (some in ways easier than others) to provide better airspeed at low rpm. fuel delivery is tuned by use of different size fuel jets, emulsion tubes (which mix air and fuel at a different rate as the vacuum changes) and sometimes air jets. some carbs atomise fuel better than others, so you'll get a better burn (and hence more power) for the same amount of air and fuel out of a better carb (say compare the mixture quality of a good DCOE to a crappy stock carb)
an SU is a variable-choke carb, so it always provides ideal airspeed, then you just tune fuel delivery by a needle and seat. often expensive to aquire, time consuming to setup but very simple when working
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Registered:
October 2003
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Re: carb cfm to engine matching?
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Sun, 17 April 2005 13:41
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thanks!
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