holley carb question ?

[millerracing]

what is the difference between a holley 750 and an 830 ?

[big.ed]

750 cfm: Still Gaining


Jet Size: Primary-73; Secondary-73


The important thing to mention here is that of all the carburetors tested, only the 750-cfm HP Holley came with a vacuum port below the base of the carburetor. We would have liked to include engine vacuum in the entire testing sequence, but due to a lack of accessible portable vacuum sources, we were unable to do so. Although the Air Gap manifold does have a tapped source for vacuum, it is inaccessible unless there is a spacer below the carburetor.



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To our amazement, the average numbers were almost negligible: 1 lb-ft of torque and 1 hp. Peak torque was still up by 21 lb-ft and horsepower remained the same at 4 hp above the advertised peak. This test also showed optimum air/fuel ratio results, which revealed how well the HP Holley carburetors are calibrated from the factory. If you are wondering how the 390-cfm carburetor was able to supply as good an air/fuel ratio as the 750-cfm unit using smaller jets, it's because the smaller carburetor created a stronger airflow signal, causing the fuel discharge ports to deliver the maximum amount of additional fuel. The 750-cfm carburetor may have had an equally good air/fuel ratio with larger jets, but the air speed signal was slower; therefore, it drew less fuel from the jets and delivered nearly the same air/fuel readings.



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830 cfm: Peaky Power


Jet Size: Primary-86; Secondary-86


We were sure the engine would stumble and rattle at idle with the 830-cfm carburetor, but we were wrong. Surprisingly, the larger carburetor ran considerably well down low. There was a definite throttle response difference between the 750-cfm and 830-cfm off-idle feeling, but both idle quality and power worked out well.



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The 830-cfm carburetor posted the best peak numbers and average numbers of the entire test. While the average power gain only increased 2 lb-ft and 1 hp, it was nice to see some new peak numbers. This time around, the 383 made 22 lb-ft of torque more than its rating and made an additional 9 hp. The 500-rpm increments remained about the same as power increased with engine speed. Also, the air/fuel ratio was right on.


950 cfm: Super-Sized



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Jet Size: Primary-78; Secondary-78


We were hoping to see even more power with the 950. It definitely produced a low-speed stumble, and we thought about tweaking the idle-air bleeds but refrained since we had come this far without touching any of the other carburetors. As soon as the engine passed the 1,200-rpm mark, the air speed signal through the carburetor improved and the engine ran smoothly. Typically, a large carburetor like this one is only run on an extremely high-horsepower engine, where the camshaft depletes most of the low-speed vacuum. With the idle speed set a little higher, we began the next pull in hopes of more power.



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Unfortunately, the powertrain had maxed out, and we were now merely maintaining the best power curve we could. The peak numbers still posted 19 lb-ft of peak torque better than the advertised engine, and the horsepower was still up by 3 hp. In comparison to the previous test with the 830-cfm carb, the numbers reveal a loss in power at every point. The air speed signal through the carburetor slowed even at high engine speeds and caused the air/fuel mixture to become slightly less than ideal. While we could have altered the carburetor to promote a better power curve and perhaps equal the previous tested power, we would have been bandaging the situation rather than doing ourselves a favor.



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1,000 cfm: Big Daddy


Jet Size: Primary-84; Secondary-84


After experiencing the 950-cfm carburetor, we weren't too sure of what to expect from an even larger piece. Usually, once you reach the limit of beyond, it's common sense to head in the opposite direction. However, since we're talking about carburetor sizing, we thought it would be interesting to see what happens way beyond big. Now, into Dominator cfm, we figured the engine would hardly run at all below 1,500 rpm.



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Again, the Holley HP series amazed us with its ability to meter incoming air speed and adjust for the proper amount of fuel needed to run the engine. Of course, the idle was extremely choppy below 1,000 rpm, so we raised it to 1,200, where it seemed happy. The touch of the dyno handle had changed as well. Throttle response went out the window from anything below 2,000 rpm, and at that point, we just wanted to see what the extra cfm would do to our previous power curves. Amazingly, we saw one more peak lb-ft of torque and still managed to stay at a 5hp peak above the advertised horsepower rating. Reviewing the 500-rpm incremental test points, the 1,000-cfm carburetor did better than the 950-cfm unit but was still making considerably less power than the 830-cfm piece. Perhaps the air speed signal through 1,000-cfm carburetor boosters was stronger than that of the 950-cfm, which is why it did a little better. Either way, the 950-cfm and 1,000-cfm carburetors were too big for our engine.



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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.

[50j]

what is the difference between a holley 750 and an 830 ?

To answer your question, the 830 has a bigger venturi and a chamfered base plate bore. 830's were designed for restrictor plate use and are sort of backwards from what you want to use without one.