Minimum required airflow for performance These calculations explain the relationship between horsepower and CFM, and max RPM and CFM. Horsepower potential at a given CFM is the same regardless of engine size, however CFM required to achieve RPM varies because the larger the engine the more air is moved per revolution at a given RPM. These calculations were also based on known 195-220cc heads on the market today. (I have included Vortec, which is a 170cc runner head because it is a popular upgrade from stock L98 heads, camel humps, etc) This is important, because for a street car, that will operate from 1500 - 6000 RPM you need to maintain good port velocity or low rpm use (street) will suffer. A head that flows 250 CFM on a runner that is 195cc will have the same horsepower potential as a head that flows 250 CFM on a runner than is 220cc, however because the cross sectional area is smaller the velocity is higher and low RPM operation is much better. These are MINIMUMS, not guaranteed specifications. For example, a 350 cubic inch motor with Vortec L31 heads will not make 472hp just by bolting the heads on. In fact, the relationship to RPM is similar in that to use 230cfm and make 472hp a 350 might need to spin close to 6900 rpm. In theory, a properly tuned engine (naturally aspirated), with decent intake manifold design could reach a VE of 127%, which for the example 350 would be roughly 255cfm at 6,000 RPM. This would be a roughly 520 horsepower 350. 350 cubic inches, Head flow at 28" of water: Cylinder Head Flow of 220 is 452.33 at a RPM of 6,611.32 (Patriot, Procomp, Vortec L31) Cylinder Head Flow of 230 is 472.89 at a RPM of 6,911.84 (Vortec L31) Cylinder Head Flow of 240 is 493.45 at a RPM of 7,212.35 (Sportsman II) Cylinder Head Flow of 250 is 514.01 at a RPM of 7,512.87 (TFS 23*) Cylinder Head Flow of 260 is 534.57 at a RPM of 7,813.38 Cylinder Head Flow of 270 is 555.13 at a RPM of 8,113.89 Cylinder Head Flow of 275 is 565.41 at a RPM of 8,264.15 (AFR 195 eliminator) 383 cubic inches, Head flow at 28" of water: Cylinder Head Flow of 220 is 452.33 at a RPM of 6,041.68 (Patriot, Procomp, Vortec L31) Cylinder Head Flow of 230 is 472.89 at a RPM of 6,316.30 (Vortec L31) Cylinder Head Flow of 240 is 493.45 at a RPM of 6,590.92 (Sportsman II) Cylinder Head Flow of 250 is 514.01 at a RPM of 6,865.54 (TFS 23*) Cylinder Head Flow of 260 is 534.57 at a RPM of 7,140.16 Cylinder Head Flow of 270 is 555.13 at a RPM of 7,414.79 Cylinder Head Flow of 275 is 565.41 at a RPM of 7,552.10 (AFR 195 eliminator) 400-412 cubic inches, Head flow at 28" of water: Cylinder Head Flow of 220 is 452.33 at a RPM of 5,616.41 (Patriot, Procomp, Vortec L31) Cylinder Head Flow of 230 is 472.89 at a RPM of 5,871.71 (Vortec L31) Cylinder Head Flow of 240 is 493.45 at a RPM of 6,127.00 (Sportsman II) Cylinder Head Flow of 250 is 514.01 at a RPM of 6,382.29 (TFS 23*, Procomp 220 CNC) Cylinder Head Flow of 260 is 534.57 at a RPM of 6,637.58 (EQ23) Cylinder Head Flow of 270 is 555.13 at a RPM of 6,892.87 Cylinder Head Flow of 275 is 565.41 at a RPM of 7,020.52 (AFR 195 eliminator) Based on these calculations, and a typical street redline of 6,000 RPM, the real bare minimum head we can run on a 383 engine is 230 CFM. On any 400+ cubic inch motor, we would need a head in excess of 240 CFM minumum. On larger cubic inch engines we can get away with heads with larger runners, such as a 220cc head on a 400-412, or a 210cc on a 383. These larger heads would fall flat on any 350 engine under 4500 RPM. They don't have the velocity because of the sewer pipe runner size. In theory, a head such as an EQ23 220cc would support 550+ hp on a 400-412 cubic inch small block however the much less flowing Procomp 220 CNC head would support low 500s on the same engine. Felpro 1255 port size 2.11 x 1.08 Felpro 1205 port size 2.09 x 1.28 Felpro 1206 port size 2.21 x 1.31 Why is this important? Remember, velocity is important. Larger ports will have less velocity than smaller ports at the same CFM. Head flow data: Vortec L31 (170cc, Stock, Felpro 1255 port) $300 used Lift Int Exh .200 118 111 .300 177 148 .400 217 160 .500 226 164 .600 n/a n/a These numbers were taken from factory heads, with no porting or mods. A lot of guys on the forums like to claim the L31 heads flow 240 CFM, and that is probably true after a few hours with a grinder. Procomp 195 (195cc, New version, Skip White Performance, Felpro 1205 port) $649 NEW Lift Int Exh Velocity .200 113 97 .300 159 132 .400 199 161 .500 227 180 .600 244 189 Sportsman II (200cc Iron, Felpro 1205 port) $900 NEW Lift Int Exh Velocity .200 132 102 .300 187 131 .400 225 150 .500 240 164 .600 243 162 Procomp 210 (210cc Alum, 64cc chamber, Felpro 1206 - 58CC FLOWS LESS!!) $679 NEW Lift Int Exh Velocity .200 n/a n/a .300 161 131 .400 206 148 .500 240 152 .600 260 152 Trickflow 23* (Rev 2 / Kenny D Felpro 1205 port - up to 500hp per mfg) $1100 NEW Lift Int Exh Velocity .200 136 98 .300 191 136 .400 230 163 .500 253 177 .600 254 190 Procomp 220 CNC (220cc full CNC, Felpro 1206) $1000 NEW Lift Int Exh Velocity .200 n/a n/a .300 195 150 .400 236 175 .500 258 193 .600 274 203 Engine Quest EQ23 220BA (220cc iron, Felpro 1206 I think?) $700 NEW Lift Int Exh Velocity .200 141 115 .300 196 145 .400 238 169 .500 265 189 .600 280 201 AFR 195cc Eliminator (195cc, Felpro 1205 port) $1300 NEW Lift Int Exh Velocity .200 146 119 .300 201 166 .400 247 197 .500 275 213 .600 280 218