Nitrous and carburetors..... need some skooling

[russ67chevelle]

What makes a great carburetor? Are they like heads where you want most cfm with smallest hole?

Combination is 572 ci will spray 24-2500 pph in a fogger. So if you could pick between 2 carburetors that flow about the same, would you pick the one with smallest bore size, if all else was equal?

[NormS]

Hi Russ. A great carb is one that gives the engine good low speed/part throttle drivability, transitions to full throttle fast and clean, and meters fuel well over a broad rpm range at full throttle, particularly in the upper half of that range. For many engines, making good power in a wide rpm range, is usually aided by keeping the venturi and throttle bore sizes of the carb as small as possible, while also having the flow capability of the carb be correct for the normally aspirated HP of the engine.
I have been working on getting more flow through many popular sizes of Holley carbs, and like with cylinder head ports, shape is the name of the game. Venturi and booster shapes, throttle shaft and butterfly shapes, all become real critical. For example, by starting with a Holley HP 1050 dominator, it is possible to bore and reshape the venturis and retain the original 2" throttle bore size, and get the carb to flow 1375 cfm at 1.5" Hg.
Compare that to a modified HP1250 dominator with its original venturi shape and 2.125" throttle bores, flowing at 1415cfm. The modified 1050 is getting its flow with a venturi size that is .100" smaller and a throttle bore size that is .125" smaller than the modified 1250. At 1375 cfm, the modified 1050 is capable of handling about 1000hp, without the nitrous. And the smaller venturis and throttle bores should help make more midrange torque. Its a strong power curve over a wide rpm range that accelerates a car, not just top end hp. Food for thought. Norm/CFS

[stealth]

Nitrous carbs are unique animals. Overall flow is less important than booster response. As you fill the intake with ever increasing nitrous, there is less and less flow through the carb and much more demand that the boosters are responsive and deliver enough fuel without high velocity through the venturi. Basically the more nitrous used the more demand on the booster quality and single strength.

[jmarkaudio]

Booster response is key for any carb. A nitrous engine needs the same as any other, just a tad richer on the idle and transition to come up on the converter but not so much it fouls plugs. Other than that I tend to size the carb a bit smaller on nitrous as some of it's flow is displace with nitrous. Makes low speed response better.

[GARY C]

What makes a great carburetor? Are they like heads where you want most cfm with smallest hole?

Combination is 572 ci will spray 24-2500 pph in a fogger. So if you could pick between 2 carburetors that flow about the same, would you pick the one with smallest bore size, if all else was equal?

My personal experience going from an 850/4150 to an old school 1150/4500 on a 382 SBC was that the car ran the same NA but was around 2 tenths quicker in the 1/8th mile with nitrous.