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