sootie007
03-13-2009, 10:11 AM
One of the most interesting Holley tech articles I have read in awhile....I guess its not all about cfm ... Its amazing how tight the hp and tq curves were using 600,650,750,830,950,1000 sized carbs on the same engine there was no "huge" differences in Hp or Torque ......J
http://www.chevyhiperformance.com/techarticles/148_0403_seven_holley_carbs_test/index.html
http://www.chevyhiperformance.com/techarticles/148_0403_seven_holley_carbs_test/index.html
"Chevy High Performance.com Conclusion
The biggest difference in power was the change from a restricted 390-cfm carburetor to a larger 600-cfm unit. This part of the test showed just how much power was lost with too small a carburetor. The most interesting part of the test is how little difference in power there was out of the box between a 600-cfm carburetor through to a 1,000-cfm unit. As mentioned before, this had a lot to do with the air speed signal being picked up from the boosters. All of the tested carburetors featured down-leg boosters, which means the air speed is calibrated farther down in the venturi to help improve the signal. An annular booster design sits farther above the venturi so incoming air is not blocked, but at low engine speeds, the signal becomes extremely weak if the carburetor is too large. Judging from the test results, we'd say that the 750-cfm carburetor seemed to provide the best power curve and throttle response. In the event that the engine made 50 hp or more and was dedicated for racing, we'd be willing to say that the 830-cfm piece with down-leg boosters might add a few extra horsepower and still be livable on the street. As you can see, carburetor sizing isn't a science. As long as you are not all-out heads-up racing, simply balance throttle response, low-speed driveability, and peak power as close as you can, and the carburetor will do the rest. "
seperate Barry Grant carb generalities article said
" All things being equal, a bigger engine requires a larger carb. But for any given engine size:
• Higher rpm requires a bigger carb
• Higher horsepower requires a bigger carb
• Higher compression ratios require a bigger carb
• More distributor mechanical advance requires a bigger carb
• A manual-trans car can use a larger carb than an automatic-trans car
• Steeper (higher numerical) rearend gears tolerate a bigger carb
• Lighter cars can use a bigger carb
• Heavy cars need a smaller carb
• Too large a cam for the application requires a smaller carb
• With an automatic-trans car, too low a torque-converter stall-speed for the application requires a smaller carb
• Mild (lower numerical) rearend gears require a smaller carb
• Low compression requires a smaller carb"
http://www.chevyhiperformance.com/techarticles/148_0403_seven_holley_carbs_test/index.html
http://www.chevyhiperformance.com/techarticles/148_0403_seven_holley_carbs_test/index.html
"Chevy High Performance.com Conclusion
The biggest difference in power was the change from a restricted 390-cfm carburetor to a larger 600-cfm unit. This part of the test showed just how much power was lost with too small a carburetor. The most interesting part of the test is how little difference in power there was out of the box between a 600-cfm carburetor through to a 1,000-cfm unit. As mentioned before, this had a lot to do with the air speed signal being picked up from the boosters. All of the tested carburetors featured down-leg boosters, which means the air speed is calibrated farther down in the venturi to help improve the signal. An annular booster design sits farther above the venturi so incoming air is not blocked, but at low engine speeds, the signal becomes extremely weak if the carburetor is too large. Judging from the test results, we'd say that the 750-cfm carburetor seemed to provide the best power curve and throttle response. In the event that the engine made 50 hp or more and was dedicated for racing, we'd be willing to say that the 830-cfm piece with down-leg boosters might add a few extra horsepower and still be livable on the street. As you can see, carburetor sizing isn't a science. As long as you are not all-out heads-up racing, simply balance throttle response, low-speed driveability, and peak power as close as you can, and the carburetor will do the rest. "
seperate Barry Grant carb generalities article said
" All things being equal, a bigger engine requires a larger carb. But for any given engine size:
• Higher rpm requires a bigger carb
• Higher horsepower requires a bigger carb
• Higher compression ratios require a bigger carb
• More distributor mechanical advance requires a bigger carb
• A manual-trans car can use a larger carb than an automatic-trans car
• Steeper (higher numerical) rearend gears tolerate a bigger carb
• Lighter cars can use a bigger carb
• Heavy cars need a smaller carb
• Too large a cam for the application requires a smaller carb
• With an automatic-trans car, too low a torque-converter stall-speed for the application requires a smaller carb
• Mild (lower numerical) rearend gears require a smaller carb
• Low compression requires a smaller carb"