Can anyone tell me the total cfm the stock 427/435hp L-71 had? And also what the center carb had compared to the f/a carbs. Thanks GRR.#33570
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Re: 3x2 stock 67 L-71 CFM.
Hi G.R.
Some will correct me if I am mistaken, from memory here, 425 cfm each. All flow the same amount.
jerry -
Re: 3x2 stock 67 L-71 CFM.
center carb 350cfm end carb 466 cfm. ed lowther killed the class using this setup with mandated restrictor plates from scca one year because they never thought about the 3x2 set up when they made the rules.
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Trust in MM
Sorry Gr: I evidently really blew that one, however, while I wasn't cretain about the total CFM of each carb I was always under the impression that the primary and secondary carbs were the same rating. MM is seldom , if ever incorrect.
jerComment
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Re: Trust in MM
jerry my wife would give you a argument over that statement!
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Clarification on flow rates
It is industry standard practice to measure four-barrel air flow at 1.5 inches of mercury depression, but two-barrels are measured at three inches depression. Unless the actual depression is quoted with the flow rate I have to assume that it was measured at three inches. So to compare a two-barrel with a four, the two-barrel airflow must be divided by the square root of two because flow is proportional to the square of the pressure drop. Doing the math the flow at 1.5 inches works out to 246 for the center carb and 330 for the end carbs for a total of 906 CFM at 1.5 inches. Assuming a volumetric efficiency of 80 percent at 6500 for a 427/435, actual flow would be 642 CFM and the pressure drop across the carbs would be about 1.26 inches of mercury.
DukeComment
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Re: Clarification on flow rates
Hi Duke:
What I need to know is, if I divide the air flow by the square root of two for the primary carburetor and consider the secondary carburetors, i.e. two two barrels, a four barrel, I can measure the air flow at 1.5 inches of mercury depression. will the end result of the math make my answer correct or will you have to adjust the formula a bit ...Duke, ol buddy.
jer
BTW, I can't imagine any 427 that could be brought to an idle running on a primary carb that was flowing only 246 CFMComment
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Da "Duke of Earl" strikes again.....
right on the nose. Many, many light eons ago we had a heated discussion re: the tri power flow rate. We called Holley, and were informed that the "actual CFM" flow was no more than 900, but closer to 850. We questioned the wisdom of the techy on the other end, and asked how come it will only flow 850 when the carbs are rated at over 1100? Techy's answer was something to the affect that it was a complicated formula, and to trust him, the flow wasn't anywhere near the sum total of the individual carb ratings.Comment
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Re: I know I leave myself open to criticism but...
...you guys don't happen to wear "pocket protectors", do you? :-) TBarr #24014Comment
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Jerry...why not?
You don't need CFM to keep the motor at an idle, just the proper mixture of air/fuel. CFM is required at wide open throttle, when ya goin' down the road aka Eagles' "Take It Easy" an' ya have this wide grin on yer face when ya just dusted a Viper, or a Hemi, or a twin turbo Z.Comment
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Ya mean those white plastic liners.....
ya stick in your shirt pocket and then stuff it full with 50 pens and pencils of various lengths and colors? I don't know....Yo Duke...Tom, are you inferring that the Great Duke of Earl and my good buddy Jerry are geeks?Comment
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Re: Clarification on flow rates
Jerry:
Your question is not clear to me so I will just try to rephrase my first post. If we assume that the carb air flow is speced at 3" Hg. per industry standard for two-barrels, then we can approximate the air flow at 1.5 inches (the industry standard depression for measuring four-barrels) by dividing by the square root of two (for all three carbs) and the total is 906. If we measure each carb at 1.5 inches and then add the total, the answer should be about 906. Because two-barrel and four-barrel flow is measured at different a different pressure differentials, we can't compare them directily, but we can convert "two-barrel" air flow to "four-barrel" air flow by dividing the quoted two-barrel flow by 1.414. By making this adjustment we can directily compare a tri-power set up to a four-barrel. If we don't do the coversion, the tri-power will appear to have much more flow, but it's apples and oranges. We could also convert "four-barrel" flow to "two-barrel" flow by mulitplying the four-barrel flow by the square root of two, but this doesn't mean much. High performance engine carburetors are generally sized to provide peak engine air flow demand at 1.5 inches or less. Typical single two-barrel installations naturally see a higher pressure drop to provide peak air flow demand, which is why they are measured at three inches, and a designer would try to size a single two barrel to provide peak airflow demand at not much more than three inches depression across the carb.
DukeComment
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Re: Sometimes they're clear plastic.....
....but no, I wouldn't inject something mean spirited. Just poking fun at how serious we take our hobby. No offensivness intended. :-) TBarr #24014Comment
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Pocket protectors and complicated formulas
I wouldn't call dividing by the square root of two a "complicated formula". There are other minor factors involved, but it's a very good approximation. Then again, there are guys out there who still think that if you double the carburetor air flow you double the horsepower. Because of the tripower's thirteen percent higher flow rate than the 800 CFM Holley on the '66 big blocks, they provided peak engine air flow demand with a bit less loss through the carbs, but the horsepower improvement was small - on the order of about 2.5 percent according to Chevrolet's advertised numbers, but this is barely within the range of dyno test repeatability. Regarding pocket protectors, I'm a mechanical engineer. Only electrical engineers wore pocket protectors. On that note I gotta go before Jack finds me.
DukeComment
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Re: Clarification on flow rates
duke the numbers i quoted came from the holley book, but the numbers we use in the computerdyno programs are 250 and 370 cfm which came from air box numbers. sorry about the confusion.
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