Cylinder Head Flow Data At 28" H2O

Flow Numbers and What They Mean

 1) Every flow bench is different. One flow bench will not flow the same as the next. Other factors include: when and how often the bench is calibrated and what condition the equipment is in. As a result, you may not be getting an "apples-to-apples" comparison when comparing flow numbers from different flow benches.

2) Bigger is not always better. A large flow number doesnÕt always mean more power. This is especially true with street cylinder heads and in some cases, race heads. Velocity is just as important as flow. A smaller port volume generally equates to higher velocity for better street performance. The speed of the mixture determines how tightly the combustion chamber is packed. The more tightly packed the combustion chamber, the more pressure is developed when the mixture is ignited, pushing the piston with more force for more power. For example: A large port and a big flow number at 0.600" lift means low velocity (especially off-idle to 3500 rpm) and results in less throttle response. For the street, velocity is the key to overall performance.

3) Comparing the low lift flow numbers is a good indicator of how well the port will produce velocity. As a general rule, the larger the flow number at low valve lifts, (ie: 0.200"), the more velocityÉ see rule #2. 4) Type of valve influences flow. An undercut stem valve has less material around the stem for less flow restriction and therefore allow a port to flow better, which equates to more mixture making it to the chamber for more power.

5) Generally, in a street application, the smaller the valve diameter, the better the velocity and flow will be with the correctly sized port. This holds true for both the intake and the exhaust ports. When you can equal the flow of a larger port and larger valve with a smaller port and smaller valve, you have a much more efficient port, which will generally make more power and use less fuel.

6) What pressure differential were the heads flowed at? All Edelbrock heads are flowed at 28" of water, which more closely represents what an engine will see. Beware of companies flowing heads at anything less than 28" of water, because it is not an apples-to-apples comparison with Edelbrock heads.

Note: All flow numbers are in cfm. Intake flow numbers are long and short ports for big-block Chevys with right and left ports for Oldsmobile.

Part numbers in black are not legal for sale or use on pollution controlled motor vehicles.

Part numbers in red are 50-state street (emissions) legal for 1995 and earlier vehicles (E.O. #D-215-33).

Footnotes:

(A) Width of slot in guide plate
(B) Recommended valve sizes, not maximum
(C) Approximate figures
(D) Valves .120" longer than stock