225 cid/6 = 37.5 cid
At 5000 rpm that translates to approximately 54 CFM at 100% VE, so it is actually probably closer to 40 CFM (@ 5000 rpm...)
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| 2bbl vs. dual 1bbl revisited https://slantsix.org/forum/viewtopic.php?t=18410 |
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| Author: | slantvaliant [ Thu Jul 06, 2006 9:29 am ] |
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I understand that there is a difference between how four- and two-barrel carburetors are rated, related to the vacuum used on the flow benches. Are one-barrels rated at the same vacuum as two-barrels? And are the advertised ratings to be believed, or used as a basic comparison for similar carburetors? |
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| Author: | emsvitil [ Thu Jul 06, 2006 12:39 pm ] |
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Yes the 1bbls are rated the same as 2bbl.......... 3.0inHg vs 1.5inHg for 4bbls |
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| Author: | CStryker [ Thu Jul 06, 2006 2:40 pm ] |
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Quote: Say the 1bbl carb is rated 100cfm, the 2bbl is rated at 200cfm.....
Ah ha, I see. I was (retardedly) thinking that the front and back sets of cylinders fired together, rather than the 180* apart that they truly are. (or something along that thought process.) Duh. Realistically, how much air is one cylinder going to pull?
With the split manifold, one of the cylinders needs air every 240degrees of engine rotation (out of the 720degrees need for complete 4-stroke cycle). So each cylinder pulls on that 100cfm carb all by itself, and the max flow to that cylinder is limited to the 100cfm With the single manifold, one of the cylinders needs air every 120deg of engine rotation. And although there's overlap of cylinders needing air, the 1 cylinder does at some point have most of the flow capacity of 2 barrels available to it during the intake cycle (or slightly less than 200cfm). |
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| Author: | Super6 [ Thu Jul 06, 2006 2:45 pm ] |
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225 cid/6 = 37.5 cid At 5000 rpm that translates to approximately 54 CFM at 100% VE, so it is actually probably closer to 40 CFM (@ 5000 rpm...) |
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| Author: | CStryker [ Thu Jul 06, 2006 3:19 pm ] |
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Hmm.. yeah.. I don't think enough before I ask questions. I was trying to figure the math behind it and /almost/ asked another question before I realized that it revolves twice for each intake stroke. Once again, duh. |
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| Author: | emsvitil [ Thu Jul 06, 2006 4:29 pm ] |
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Quote: 225 cid/6 = 37.5 cid
At 5000 rpm that translates to approximately 54 CFM at 100% VE, so it is actually probably closer to 40 CFM (@ 5000 rpm...) But the flow isn't even (think sine wave)...... So the instantanous flow (or peak flow) will be higher so that on average it's 40cfm.... |
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| Author: | emsvitil [ Fri Jul 07, 2006 5:18 am ] |
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Quote: Quote: 225 cid/6 = 37.5 cid
At 5000 rpm that translates to approximately 54 CFM at 100% VE, so it is actually probably closer to 40 CFM (@ 5000 rpm...) But the flow isn't even (think sine wave)...... So the instantanous flow (or peak flow) will be higher so that on average it's 40cfm.... Follow this........... 1 cylinder needs 54 cfm at 5000 rpm. Assume the intake stroke lasts 240 degrees. So to average 54cfm during the 4-stroke cycle, the carb needs to flow (720/240) * 54cfm = 3 * 54cfm = 162cfm to average 54cfm..... During that 240 degrees of intake flow, the flow isn't constant.... As an approximation, figure the flow rate is like a sine wave (think AC current, where the average is 120volts, but the peak is higher (1.4 times higher)) so at peak flow you need a carb that can flow 162 * 1.4 = 227cfm......... BTW, with the same 240 degrees intake duration, the 227cfm carb would work with 2 and 3 cylinders as there's no overlap during intake cycles. BTW2, and the 227cfm carb would even work with 4,5,6 cylinders with 240 degrees duration because when 1 cylinder needs the 227cfm, other cylinders need 0cfm. BTW3, a 6 cylinder would have constant flow of 227cfm |
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| Author: | 64'4$peed [ Fri Jul 07, 2006 10:12 am ] |
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so what your saying is a dual 1bbl setup that flowed 227 cfm out of each carb would meet the peak requirements for the motor. or are you saying a 2bbl setup that flowed 227 cfm would have sufficient flow. what are the carter bbs carbs rated at? I'm starting to understand thanks emsvitil |
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| Author: | RossKinder [ Fri Jul 07, 2006 2:40 pm ] |
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Quote: so what your saying is a dual 1bbl setup that flowed 227 cfm out of each carb would meet the peak requirements for the motor.
I think he's saying dual singles that could deliver "113.5" each would get the job done.
or are you saying a 2bbl setup that flowed 227 cfm would have sufficient flow. what are the carter bbs carbs rated at? I'm starting to understand thanks emsvitil |
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| Author: | emsvitil [ Fri Jul 07, 2006 3:30 pm ] |
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Quote: Quote: so what your saying is a dual 1bbl setup that flowed 227 cfm out of each carb would meet the peak requirements for the motor.
I think he's saying dual singles that could deliver "113.5" each would get the job done.or are you saying a 2bbl setup that flowed 227 cfm would have sufficient flow. what are the carter bbs carbs rated at? I'm starting to understand thanks emsvitil Nope...... At peak flow 1 cylinder needs 227cfm (with the above 5000rpm example).. On average 1 cylinder needs 54cfm, but flow isn't constant, so a carb rated at 54 would choke the engine. I think a BBS is about 200-220cfm. Any carb with 227cfm will meet the flow requirements on a single or split manifold (with 6 or less cylinders). The advantage of the 2bbl carb is that they flow more air, and there's less restriction to the flow getting into the engine. Less restriction, better volumetric efficiency, more power. |
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| Author: | 64'4$peed [ Fri Jul 07, 2006 6:18 pm ] |
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Last question I promise. Is their any real advantage to go with 2 1bbl's over a single 1bbl? Besides the fact that they look cool and would be easier to install on my 64' then a 2bbl. |
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| Author: | SlantSixDan [ Fri Jul 07, 2006 6:25 pm ] |
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Don't forget that the twin-carb manifolds have balance pipes such that they are not truly "split". |
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| Author: | RossKinder [ Fri Jul 07, 2006 7:12 pm ] |
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Quote: Last question I promise. Is their any real advantage to go with 2 1bbl's over a single 1bbl? Besides the fact that they look cool and would be easier to install on my 64' then a 2bbl.
Look cool? Here's an idea  Get 3 carbs and put 2 of them on believable mockup manifolds - one on each side of the real one. If I could find some really cheap (read plastic) carb stacks I'd put 6 of them on a mockup just to see the first look on a couple faces when I pop the hood. 
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| Author: | SlantSixDan [ Fri Jul 07, 2006 7:40 pm ] |
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Quote: I'd put 6 of them on a mockup just to see the first look on a couple faces when I pop the hood.
Been done
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