Carburetor cfm ratings are generated by flowing them at a specific level of vacuum in inches of mercury. Holley and most carburetor companies rate their four-barrel carbs at 1.5 inches of mercury. For one- and two-barrel carbs, 3 inches of mercury. To give you some idea comparing this to cylinder head flow-testing, if you convert 1.5 inches of mercury to inches of water, you get 20.43 inches of water. And the 3 inches of mercury is 40.86 inches. When Holley rates its carburetors, they're tested on a wet flow bench. If you only run them on a dry flow bench, the flow would be much greater without the fuel being added into the air stream. If you take a performance air/fuel ratio of 12.5:1 and break it down into percentages, 8 percent of the total flow is fuel. It would be very safe to say that if you flowed a carburetor on a dry flow bench like a SuperFlow SF600 flow bench, you would realize at least an 8-percent increase in flow over the factory rating. Now, this brings us to another factor in trying to flow carburetors. My SF600 flow bench is rated at 600 cfm at 28 inches of water-this is if you feed it with 230 volts of electricity. Our building only has PG&E's finest of 208 volts on a good day. When all the welders fire up their torches and the robots are humming, my voltage suffers! What I'm trying to say is my bench probably pulls around 550 cfm. If you throw a 750 Holley on the bench you wouldn't be able to pull it up to test pressure. We've seen people block off three of the four barrels and flow them one at a time. This isn't bad, but you really need all four barrels flowing at the same time.
As for booster design, I prefer the down-leg boosters on the 4150 carbs and the straight-type boosters on the Dominators. What sets the flow with the different boosters is the diameter and at what height they are set in relationship to the choke point of the venturi. The down-leg boosters and the straight boosters both reach the same point in the venturi choke point, but with annular boosters it's a whole other story. The booster is much larger in diameter, but there is much more airflow through the center of the booster. Hope this has answered your carb questions. Good luck bracket racing your big-block Impala!
Which Heads?Q I'm building a 377 for my '83 Camaro. The car sports a manual T5 with 3.42:1 gears (will be changed to 4.10s later on) and a posi. It also has open twin 211/42-inch exhaust and headers. The engine is a destroked 400 small-block with hypereutectic pistons and 10.7:1 compression with 64cc heads. I run 95-octane fuel or even 99-octane (both standard in Sweden). I currently have a Comp Cams Magnum 280H camshaft with 1.6:1 roller rockers and a Performer RPM intake. I want to run the engine to about 7,000-7,500 rpm. (Nothing sweeter than a high-revving small-block!)
My problem is that I'm not sure about what heads to use. I've been looking at the AFR 210s and 195s. I'm also looking at the Canfield 220s. I would also appreciate any ideas on carbs. I'm going to use the car mostly as a weekend warrior. Have you guys ever made a 377 buildup?Aron HolmbergStockholm, Sweden
A Aron, you must have a lot of fun over there with your Camaro. What's up with 95- to 99-octane pump gas? Here in California we only get to drink 91 and it doesn't taste very good. We're jealous!
The 377 engine combination is a good mix of the 400 small-block and the standard stroke of the 350. Yes, the larger cylinder bores will allow the engine to breathe well with your choice of cylinder heads. Over the years there have been many buildups of this engine combo. However, with the availability of aftermarket blocks and crankshafts at reasonable prices, it doesn't make much sense to not stretch the displacement out to the full 400 or more. Yes, the 377 revs nicely, but that doesn't make up for the loss in power over the inches.