I have a 350 gm high performance crate engine in car. I don't know hp or rpm's that it is putting out. I have a 850 edelbrock carb on it now car runs fine, but burns alot of gas. I want to downsize carb. but not sure what size to go with. The carb on it has vacuum secondaries. Car is an automatic 4 speed. I am not racing the car.
My car has an automatic transmission.
Picking out the appropriate size carburetor is a factor of how much air your specific engine build will pump. This is measured in CFM’s (cubic feet per minute). You are most likely correct in needing a smaller CFM carburetor. Your typical 350 only needs a 450-600 CFM carburetor. Contrary to the popular belief, bigger is not better when it comes to carburetion. To enlarge a carburetor slows the air volume that moves through the intake manifold runners. This is an efficiency killer which hurts HP and fuel mileage.
Here is a good page that can guide you in picking the right carburetor for your motor, but you will need to know more about your motor; mainly, the maximum RPM the cam is designed for. Other factors that can affect carburetor choice are part of the cam specifications. Higher lift racing cams will affect your volumetric efficiency and therefore your carburetor choice.
By far, the most important consideration is the CFM rating. It needs to be matched to the engine build. As for what style of secondaries you get or what kind of choke you will get, these details are often decided for you depending on price and brand. If you have a choice, go with what you already have. There is nothing wrong with vacuum actuated secondaries.
You will likely see some improvement in power output and better fuel mileage with a smaller carburetor, but don’t expect too much. A 350 carbureted motor won’t give you much better than 15 miles per gallon. Especially if it is truly high performance.
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