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CFM Selection Chart / Volumetric Efficiency assumed to be 100%
Engine	Maximum Engine RPM
C I D	1000	1500	2000	2500	3000	3500	4000	4500	5000	5500	6000	6500	7000	7500	8000	8500	9000
100	29	43	58	72	87	101	116	130	145	159	174	188	203	217	231	246	260
200	58	87	116	145	174	203	231	260	289	318	347	376	405	434	463	492	521
225	65	98	130	163	195	228	260	293	326	358	391	423	456	488	521	553	586
250	72	109	145	181	217	253	289	326	362	398	434	470	506	543	579	615	651
300	87	130	174	217	260	304	347	391	434	477	521	564	608	651	694	738	781
325	94	141	188	235	282	329	376	423	470	517	564	611	658	705	752	799	846
350	101	152	203	253	304	354	405	456	506	557	608	658	709	760	810	861	911
400	116	174	231	289	347	405	463	521	579	637	694	752	810	868	926	984	1042
425	123	184	246	307	369	430	492	553	615	676	738	799	861	922	984	1045	1107
450	130	195	260	326	391	456	521	586	651	716	781	846	911	977	1042	1107	1172
500	145	217	289	362	434	506	579	651	723	796	868	940	1013	1085	1157	1230	1302
525	152	228	304	380	456	532	608	684	760	836	911	987	1063	1139	1215	1291	1367
550	159	239	318	398	477	557	637	716	796	875	955	1034	1114	1194	1273	1353	1432
575	166	250	333	416	499	582	666	749	832	915	998	1081	1165	1248	1331	1414	1497

Carb Size, CFM Calculator

Most factory, normally aspirated engines have an efficiency of about 80%. High performance tuning can increase it to close to 100%. To go beyond this requires a pressurised induction system like supercharging or turbocharging. The default entries below are from the glory days of the muscle car! Enter Your Engine Information Select Inches or CC? (1=CI 2=CC)    Enter Engine Displacement   CI / CC Enter Your Maximum Engine RPM    Enter Volumetric Efficiency Of Engine   %       Required Carburetor Size In CFM Carb CFM Required 100% VE Carb CFM Required Your % VE

Calculate your own carburetor needs:
CID	x	RPM	/	3456	x	VE %	=	CFM
455	x	3000	/	3456	x	90	=	355

Notes: Most Street engines are capable of achieving only about 80% VE (Volumetric Efficiency). A modified street engine with ported heads, headers, intake and carburetor can achieve about 85% VE. With extensive head and valve work and a new cam an engine may reach 95% or even more. It's normally not possible to achieve VE's over 100% without some form of forced induction.

If using dual carburetors each carburetor should be of the same CFM as listed for a single carburetor above. In other words if using two four barrels double the CFM.

Rochester big block carbs tend to be over-rated by ~30 CFM.

The above chart represents the minimum CFM required, to support a certain cubic inch engine at the maximum RPM potential of that engine. The number represented on the chart does not account for obstructions such as air filters, or inaccurate CFM ratings found on some carburetors. Also note that the Buick engines tend to like a big carburetor.

          Page information contributed by Bob K. Mando

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