318 carb requirements
03-30-2009, 08:56 PM
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318 carb requirements
i found a formula for figuring out the cfm requirements for an engine. the formula was
Cubic inches x maximum used rpm / 3456 = ideal cfm.
Ideal cfm x voulumetric efficiency = actual cfm
so i put my 318 into the equation. maximum hp is at around 4000 rpm so i went with 5000 rpm cause thats the highest i ever had it. Mopar muscle magazine said that racing only vehicles had a volumetric efficeincy of 100%
a high performance street/strip was about 90-95% VE
a stock to slighly modded motor runs at 80-90% VE
im going to assume with the 302 heads that i gasket matched, the headers, and a 4bbl intake manifold that im running around 87% VE
318x5000=1590000 1590000/3456=460 ideal cfm
460x.87=400 needed cfm.
in the magazine article they said you dont want to go less than the needed cfm but i figured i need between a 390 and 450 cfm carb for my application. I was wondering if anyone knew of a carb that came stock on something that i could use instead of shelling out 300 bucks for a new one. I was shocked to see that i needed a carb this small but i assumed 600 or 650 was too big.
Cubic inches x maximum used rpm / 3456 = ideal cfm.
Ideal cfm x voulumetric efficiency = actual cfm
so i put my 318 into the equation. maximum hp is at around 4000 rpm so i went with 5000 rpm cause thats the highest i ever had it. Mopar muscle magazine said that racing only vehicles had a volumetric efficeincy of 100%
a high performance street/strip was about 90-95% VE
a stock to slighly modded motor runs at 80-90% VE
im going to assume with the 302 heads that i gasket matched, the headers, and a 4bbl intake manifold that im running around 87% VE
318x5000=1590000 1590000/3456=460 ideal cfm
460x.87=400 needed cfm.
in the magazine article they said you dont want to go less than the needed cfm but i figured i need between a 390 and 450 cfm carb for my application. I was wondering if anyone knew of a carb that came stock on something that i could use instead of shelling out 300 bucks for a new one. I was shocked to see that i needed a carb this small but i assumed 600 or 650 was too big.
03-30-2009, 09:45 PM
#2
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Put a 600 cfm Carter/Edelbrock on it, & forget it. While the old cfm formula makes sense, it rarely works in the real world. You can get a manual choke 600 Edelbrock on sale for around 200 bucks, if you're really on a budget, get a used one & rebuild it.
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