1965 300 h.p. AFB CFMs?
01-21-2018, 09:44 PM
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1965 300 h.p. AFB CFMs?
As per title. What CFM was the stock AFB on a 300 h.p.?
01-21-2018, 10:53 PM
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C1 of Year Finalist (performance mods) 2019
01-22-2018, 09:10 AM
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01-22-2018, 09:19 AM
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According to Colvin's Corvette by the Numbers (which is not entirely error-free but typically pretty reliable), it says (combo of quotes and paraphrasing):
"The first design Carter AFB was introduced in 1957 [but it] was 1962 before Chevrolet used [it] on the Corvette 327 through 1965 . . . the CFM rating of this carburetor was 500 cfm. This first design . . . has a 4.5 inch diameter air cleaner base, and used on some Corvettes through 1965 when Holley became the carburetor of choice on Corvettes.
The second AFB design was larger overall, with a 5.5 inch diameter air cleaner base, and was rated at approximately 600 cfm and was used on high performance 409s, as the Corvette only used the 4.5 inch air cleaner base.
If yours is original, send me the numbers and I'll quote what Colvin says specifically about it.
"The first design Carter AFB was introduced in 1957 [but it] was 1962 before Chevrolet used [it] on the Corvette 327 through 1965 . . . the CFM rating of this carburetor was 500 cfm. This first design . . . has a 4.5 inch diameter air cleaner base, and used on some Corvettes through 1965 when Holley became the carburetor of choice on Corvettes.
The second AFB design was larger overall, with a 5.5 inch diameter air cleaner base, and was rated at approximately 600 cfm and was used on high performance 409s, as the Corvette only used the 4.5 inch air cleaner base.
If yours is original, send me the numbers and I'll quote what Colvin says specifically about it.
Last edited by Easy Rhino; 01-22-2018 at 09:19 AM.
01-22-2018, 09:27 AM
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Some years ago a Corvette owner sent me a copy of a letter dating from the seventies from Carter that specified the Corvette AFB flow rate at 2.0" Hg depression.
For some years now the "industry standard" for 4-bbl. flow rate is 1.5" Hg depression (2-bbls are measured at 3.0 Hg depression), and since flow is proportional to the square root of depression it was easy to make the conversion, which turned out to be about 550 CFM @ 1.5" Hg depression.
There was a larger AFB back in the day that was used on the 409/400 that was essentially the same long block as a 409/409, but with the single large AFB rather than the two Corvette size AFBs. I recall we referred to the large AFB as "D-series" and the Corvette AFB as "C-series". The larger version had larger primary throttle bores and a larger air horn as recall, but I don't have any flow data for that version.
For a high performance road engine, the carb should provide the engine's maximum air demand at no more than 1.5" Hg depression, and no more than 1.0" depression on a racing engine. You can calculate maximum air demand by assuming 80-85 percent VE at the redline.
I recall back in the early days of the NCRS TDB there was a spirited discussion about the air flow of the L-71 and L-72. The total air flow for the L-71's three carbs is speced at something on the order of 1300+ CFM, but what the guys didn't understand is that this was measured at 3.0" Hg depression while the L-72's single four barrel spec of 800 was measured at 1.5".
Using the square root relationship between depression and flow it was easy to convert the 2-bbl. flow to 1.5" and I recall the final number was 906 CFM at 1.5".
And remember that specifying flow without the depression it was measured at is absolutely MEANINGLESS. You can get as high a flow number as you want... just keep increasing the depression, but once the flow through the venturi achieves sonic velocity, flow will not increase no matter how much you increase the depression.
Duke
For some years now the "industry standard" for 4-bbl. flow rate is 1.5" Hg depression (2-bbls are measured at 3.0 Hg depression), and since flow is proportional to the square root of depression it was easy to make the conversion, which turned out to be about 550 CFM @ 1.5" Hg depression.
There was a larger AFB back in the day that was used on the 409/400 that was essentially the same long block as a 409/409, but with the single large AFB rather than the two Corvette size AFBs. I recall we referred to the large AFB as "D-series" and the Corvette AFB as "C-series". The larger version had larger primary throttle bores and a larger air horn as recall, but I don't have any flow data for that version.
For a high performance road engine, the carb should provide the engine's maximum air demand at no more than 1.5" Hg depression, and no more than 1.0" depression on a racing engine. You can calculate maximum air demand by assuming 80-85 percent VE at the redline.
I recall back in the early days of the NCRS TDB there was a spirited discussion about the air flow of the L-71 and L-72. The total air flow for the L-71's three carbs is speced at something on the order of 1300+ CFM, but what the guys didn't understand is that this was measured at 3.0" Hg depression while the L-72's single four barrel spec of 800 was measured at 1.5".
Using the square root relationship between depression and flow it was easy to convert the 2-bbl. flow to 1.5" and I recall the final number was 906 CFM at 1.5".
And remember that specifying flow without the depression it was measured at is absolutely MEANINGLESS. You can get as high a flow number as you want... just keep increasing the depression, but once the flow through the venturi achieves sonic velocity, flow will not increase no matter how much you increase the depression.
Duke
Last edited by SWCDuke; 01-22-2018 at 09:34 AM.
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6T5RUSH (01-22-2018)
01-22-2018, 09:34 AM
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Some years ago a Corvette owner sent me a copy of a letter dating from the seventies from Carter that specified the Corvette AFB flow rate at 2.0" Hg depression.
For some years now the "industry standard" for 4-bbl. flow rate is 1.5" Hg depression (2-bbls are measured at 3.0 Hg depression), and since flow is proportional to the square root of depression it was easy to make the conversion, which turned out to be about 550 CFM @ 1.5" Hg depression.
There was a larger AFB back in the day that was used on the 409/400 that was essentially the same long block as a 409/409, but with the single large AFB rather than the two Corvette size AFBs. I recall we referred to the large AFB as "D-series" and the Corvette AFB as "C-series". The larger version had larger primary throttle bores as recall, but I don't have any flow data for that version.
For a high performance road engine, the carb should provide the engine's maximum air demand at no more than 1.5" Hg depression, and no more than 1.0" depression on a racing engine. You can calculate the air demand by assuming 80-85 percent VE at the redline.
I recall back in the early days of the NCRS TDB there was a spirited discussion about the air flow of the L-71 and L-72. The total air flow for the L-71's three carbs is speced at something on the order of 1300 CFM, but what the guys didn't understand is that this was measured at 3.0" Hg depression while the L-72's single four barrel spec of 800 was measured at 1.5".
Using the square root relationship between depression and flow it was easy to convert the 2-bbl. flow to 1.5" and I recall the final number was 906 CFM at 1.5"
Duke
For some years now the "industry standard" for 4-bbl. flow rate is 1.5" Hg depression (2-bbls are measured at 3.0 Hg depression), and since flow is proportional to the square root of depression it was easy to make the conversion, which turned out to be about 550 CFM @ 1.5" Hg depression.
There was a larger AFB back in the day that was used on the 409/400 that was essentially the same long block as a 409/409, but with the single large AFB rather than the two Corvette size AFBs. I recall we referred to the large AFB as "D-series" and the Corvette AFB as "C-series". The larger version had larger primary throttle bores as recall, but I don't have any flow data for that version.
For a high performance road engine, the carb should provide the engine's maximum air demand at no more than 1.5" Hg depression, and no more than 1.0" depression on a racing engine. You can calculate the air demand by assuming 80-85 percent VE at the redline.
I recall back in the early days of the NCRS TDB there was a spirited discussion about the air flow of the L-71 and L-72. The total air flow for the L-71's three carbs is speced at something on the order of 1300 CFM, but what the guys didn't understand is that this was measured at 3.0" Hg depression while the L-72's single four barrel spec of 800 was measured at 1.5".
Using the square root relationship between depression and flow it was easy to convert the 2-bbl. flow to 1.5" and I recall the final number was 906 CFM at 1.5"
Duke
01-22-2018, 02:46 PM
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I believe the first time an AFB showed up on a Chevrolet was on the 1959 348 police engine. Possibly even on late '58 police packages.
01-22-2018, 04:13 PM
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Race Director
The number of 450-500 CFM is what I remember from the early days. JC Whitney (remember them) used to sell various AFB carbs and the largest I saw advertised by them was 610 CFM
May even still have the old catalog.
Living on the South Side of Chicago, they were actually local to me..............but I rarely had any money available to buy their parts. 
Larry
May even still have the old catalog.
Living on the South Side of Chicago, they were actually local to me..............but I rarely had any money available to buy their parts. Larry
01-22-2018, 04:46 PM
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