Flow-testing 21 cylinder heads...
Flow-testing 21 cylinder heads is not an easy task. John Baechtel performed all of the cylinder-head testing on a SuperFlow 600 flow bench equipped with SuperFlow's latest computerized FlowCom digital data-capture feature. This not only allows quicker testing but helps eliminate test variables.
Ask any engine builder which engine component can make or break horsepower production from an engine, and they'll tell you straight out, the cylinder heads. That's why cylinder heads with proven port and chamber designs are so coveted.
How do racers determine if a cylinder head is good or not, besides actually bolting it on an engine and testing it on an engine dynamometer? The answer is by sucking or blowing air through the ports. The amount of resistance to the flow is used to determine the total amount of air passing through the port, and the results are corrected for ambient conditions. The quantifiable results for that port allow us to compare cylinder heads to determine which is best for our application. Unfortunately, variations in port tests usually taint the results, so to ensure the results are comparable, it's good to have one test from which to draw conclusions.
Not that long ago, a test was done on a large selection of small-block Chevy cylinder heads at John Baechtel's Westech Performance Group on a SuperFlow 600 flow bench, and we thought it would be good to list the results for you. All testing was performed at 28 inches of water. Most of the heads that were tested weren't full-on racing heads for a Super Late Model-type race car. These heads could be used in an unmolested Street Stock or IMCA Modified-type class that others may not have thought would work on the racetrack.
To make it easier to consume the information, the heads have been categorized based on port volume, measured in cc's, starting with Category 1: 179cc and smaller and Category 2: 180cc to 199cc. Next month, we'll show the results of Category 3: 200cc to 220cc.
This categorization isn't to say a bigger port volume will flow more than a smaller port. The shape of the port and chamber has a lot to do with how much air flows through the port at various valve lifts, which directly affects how much power the engine can produce. All of the heads were tested with the as-cast ports, but many of the chambers had a blend radius just under the seat where the valve seat tool made a cut. This radius does make a big difference in the flow figures, but that's how these few heads came from the manufacturers, so that's how they were flowed.
Interpreting the Info
In general, racers have a terrible time understanding anything less than peak numbers. This is bad, since peak numbers almost always result in an engine that does not produce good torque-and since torque is a major component of horsepower (torque x rpm = horsepower), that is not good. The numbers we've shown with each head include the port and chamber volumes, valve sizes, and intake and exhaust flow at various valve lifts. The exhaust was flowed with and without an Ameduri exhaust test pipe, and those figures are noted. Many people flow-test the exhaust side with the pipe, because they feel it is a better representation of the situation the port will operate under. We'll let you decide.
What you need to look at is the mid to max valve lift flow numbers on the intake and exhaust ports, the relationship of the exhaust to intake flow, which is called the E/I relationship (most agree it should be around 75 to 80 percent). A huge port with so-so numbers versus a midsize port with good numbers will be a no-contest program on the track. The smaller port will almost always have better throttle response, better torque production, and better efficiency. Every racer will have different needs and wants, so refer to this information, talk with your engine builder or fellow racers, and see if one of the heads listed here could work for your race engine.