Or I could have titled this "why mathematicians need engineers to do the math":-) Getting down to business, air flows in response to a pressure differential, like current flows in response to a voltage differential, however, unlike the linear relationship between voltage and current. Air flows in proportion to the square root of pressure drop. So if we double the pressure drop we only get 1.414 times more flow. When manufacturers measure flow rate they use a flow bench, which has a vacuum pump that can maintain a vacuum below the carburetor throttle valves. They'll take measurements, for example, every 0.2 inches of mercury pressure drop up to, let's, say 4.0 inches depression. If you then plot the curve you will clearly see that in order to double the flow the pressure drop must increase by a factor of four, or at double the pressure drop the air flow will increase by only 1.414 (the square root of two). For the purposes of sizing carburetors for specific engine applications, the industry decided to rate four-barrel carbs (and EFI throttle bodies) at 1.5 inches of mercury and two-barrels at 3.0 inches. Thus if we compare a multiple two-barrel setup to a four-barrel we must convert the two- barrel flow to four-barrel flow and we do this by dividing the two-barrel flow by 1.414 to get the flow at 1.5 inches instead of three. So the big block tripower setup flows equivalent to a 903 CFM four-barrel.
Now, the actual flow through the carb(s) on the engine is a function of the size of the engine and it's efficiency as an air pump. I computed the flow of a 427 solid lifter engine as 642 CFM at 6500 revs. Peak VE occurs at the torque peak and is typically about 90 percent, but like the torque curve, the VE falls off with increasing revs. Ol'Geezer came back and computed 803 CFM, but he forgot to multiply by volumetric efficiency, and VE of 80 percent is typical for our engines at peak revs (redline). Knowing the the engine is actually aspirating about 642 CFM I can go back to the square root relationship of air flow to pressure drop and compute an actual pressure drop across the the three carbs at about .79 inches Hg. at 6500 revs by punching a few numbers into my slide ru...I mean, calculator. You might notice that this is different than the pressure drop I calculated in the previous thread. Yeah, I screwed up. It's a good thing that I just work on Corvettes now, and not hydrogen bombs like I did back in my twenties. Sorry. Questions or comments?
Duke
Now, the actual flow through the carb(s) on the engine is a function of the size of the engine and it's efficiency as an air pump. I computed the flow of a 427 solid lifter engine as 642 CFM at 6500 revs. Peak VE occurs at the torque peak and is typically about 90 percent, but like the torque curve, the VE falls off with increasing revs. Ol'Geezer came back and computed 803 CFM, but he forgot to multiply by volumetric efficiency, and VE of 80 percent is typical for our engines at peak revs (redline). Knowing the the engine is actually aspirating about 642 CFM I can go back to the square root relationship of air flow to pressure drop and compute an actual pressure drop across the the three carbs at about .79 inches Hg. at 6500 revs by punching a few numbers into my slide ru...I mean, calculator. You might notice that this is different than the pressure drop I calculated in the previous thread. Yeah, I screwed up. It's a good thing that I just work on Corvettes now, and not hydrogen bombs like I did back in my twenties. Sorry. Questions or comments?
Duke
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