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"
Another carb article noting the measured effects of cfm sizing on a 440 Mopar with ~ 550 hp. http://www.moparmusclemagazine.com/parts/engine/mopp_0111_demon_carburetor_dyno/dyno_results.html kind of contradicts the first article .
Which goes to show you that the "right carb" has more to do with the particular engine and build, than any arbitrary "rules" derived from just one test with one engine. What you find out in that case, is what that engine wants.. JW
The thing that most guys miss about carb size is this..... a given engine size will react to the carb size by how quick it reaches its peak hp/torque..... big carbs, multiple carbs ect. dont add that much power,,,, they do make a engine reach the peak power quicker.... and that is a performance advantage....and that factor is enhanced by hotter cams/free flow exhaust systems/cold air pkgs, ect...
One of the oddest things I have had experience with was with Holley carburetors. I had a Camaro with a 400 small block years ago that would run quicker and faster at the track with a 650 DP than it would with a 750 vacuum. Now I have a 350 small block that runs quicker and faster with the 750 vacuum than it does with the 650 DP. I should also mention that these are the same carburetors that I used, not different ones that are the same size, although the jetting was changed to suit the needs.
That is pretty interesting. I also have a 650 DP and 750 vacuum setting on the shelf for the LQ9. I just assumed that the 650 DP would outperform the 750 vacuum. Now I am curious to see which runs better.
My best guess is because the 400 was built more for torque than horsepower. It was pretty much done making power at 5,000rpm. I shift the 350 at 1,000 more rpm than I did the 400. I also had a looser converter behind the 400 than I do the 350 so maybe the vacuum secondary carb is more beneficial with the tighter converter o No: Stranger things have happened I suppose. Like Jim said, it's all about what the car/engine likes.
Interesting doc I had never thought of the "rate" of reaching optimum hp with multiple carbs ....I have however on many occasions noticed a tandem dual quad setup "sounds" like it revs quicker when somebody raps the accelerator.
Yep,,,, the way I got my first 3 barrell,,,,, a guy had it on a big block chevy in a 68 camero.... it would bog bad and he could not tune it out.... ''too much carb'' everybody said..... I had a 850 vac secondary holley on my 401 nailhead... he offered to trade.... I traded with him and everybody laughed at me for being so ''stupid''.... BOTH CARS RAN MUCH BETTER.....:Brow: :Brow:
