An old hot rodding adage is "airflow is everything." Well, if you consider that in its simplest terms--an engine is nothing more than an air pump--then airflow is everything.
For years, gearheads have been porting cylinder heads and installing oversized valves in the name of increased flow, but just how much horsepower is realized by way of better-flowing ports? Moreover, is port shape more important than port volume? For that we did back-to-back dyno testing to determine exactly where myth meets reality. All tests were done on the same small-block, on the same day, using the same dynamometer. Going head-to-head are early-'80s GM Bow Tie cast-iron heads versus state-of-the-art Air Flow Research (AFR) 180 CNC-ported aluminum heads to determine if, in fact, airflow is everything.
The Test Mule
The mule motor for this comparo is a moderately healthy 327 that was originally built in the mid-'80s using old-school speed secrets. TRW pop-up pistons swing from stock GM 5.7-inch rods that are moved via a GM 3.25-inch-stroke steel crank. The rotating assembly is balanced, but is of only decent strength, with the exception of the Milodon heavy-duty rod and main bolts. Up top, GM Phase I Bow Tie iron cylinder heads. In '86 the heads were sent to Engine Prototype Development (EPD) for a competition valve job, pocket porting, gasket matching, and the installation of necked-down stainless steel 2.055-inch intake/1.60-inch exhaust valves. COMP Cams 1.52:1 roller rockers were actuated by a GM Duntov 30-30 mechanical lifter camshaft. Later, an Edelbrock Performer RPM intake teamed with a Barry Grant Mighty Demon 650-cfm four-barrel funneled the fuel/air mix. An MSD billet distributor, 8mm wires, and 6AL ignition replaced the stock wares. Recently the Duntov had been replaced with an aggressive mechanical roller cam/lifter/timing-chain kit from Competition Cams. Power was good but certainly not stunning, and there was one key element of old school that needed to go--namely the heavy, old-tech Bow Tie castings.
The Big Question
Everybody talks a big game about how aftermarket heads make big power, but what are we really talking about here--25, 50, 100 horsepower? We've seen too many tests where aftermarket heads make big power over smog-o-rama boat-anchors, but that's a no-brainer. Particularly, we wanted to know how well aftermarket heads stack up against decently designed, mildly ported heads. To keep the playing field level, the AFR 180s were decked to generate 70cc com-bustion chambers to match the 70cc chambers on the Bow Tie heads. While the AFRs come standard with 74cc or 68cc chambers, you can custom-order them with a specific deck height, and consequently a specific combustion chamber size. Without a doubt, if the combustion chambers were smaller, compression would have been higher and the engine would have achieved more. However, the focus of this dyno test was to keep variables to a minimum in order to maximize consistency and testing accuracy.
If airflow is everything, then it would make sense to run the largest, highest-flowing intake ports possible, right? Well, not exactly. As flowbench testing has proven, port shape is just as important as port size in determining overall flow. Carefully shaping an intake port not only provides better peak flow, but it more evenly delivers the air/fuel mixture over the valve head, resulting in better cylinder filling and improved power. Interestingly, our ported Bow Tie heads actually had larger (cc) intake runners and larger valves, but they flowed less than the smaller-runner, smaller-valve AFRs. Another thing to remember is that smaller runners deliver better velocity (resulting in quicker throttle response and better low-rpm power), so for the street it is always advisable to employ the smallest runner possible while still fulfilling your cfm needs.
AFR generates its outstanding flow numbers for a variety of reasons. First, it has spent hundreds of hours in R&D and dyno testing to create port shapes that perform. Moreover, all AFR heads feature CNC-ported intake and exhaust runners. The CNC-porting maximizes the runner shape and volume and is absolutely consistent from port to port and head to head. All too often, hand-porting delivers a fabulous first port with increasingly worse results as the man with the grinder tires throughout the day. Many aftermarket heads can be had with optional CNC-porting, but AFR offers 5-axis CNC porting for more fine-tuned work.
In the following chart, compare the intake and exhaust port flow between the ported Bow Tie head and the AFR casting. Although the AFR 180s flow more peak cfm, of equal importance is that they also flow better in the lower lift areas, which translates to better low-rpm power and crisper throttle response.
Superflow Flowbench Head Flow Test
The same operator evaluated both sets of heads on a Superflow SF600 flow bench. All data was measured using the (industry standard) 28-inches water depression baseline. Also, exhaust flow was measured with a 1 3/4-inch-diameter primary pipe in place for both heads.
The ported Bow Tie Phase I...
The ported Bow Tie Phase I cast-iron cylinder heads were inspected and then flow-bench tested to determine intake and exhaust flow at different valve-lift heights.
The combustion chambers on...
The combustion chambers on the Bow Tie heads have been smoothed out and opened up around the edges of the valves. For increased flow, oversized 2.055-inch intake and 1.60-inch stainless steel valves (with undercut stems to increase airflow) were installed with a competition valve job.
With the valves removed, you...
With the valves removed, you can see some of the bowl blending and porting. Also note that the Bow Tie heads feature an angle-plug spark-plug design said to increase power by directing the spark path more toward the intake valve. The AFR 180 heads, however, feature a straight-in plug design.
Using a burette, the Bow Tie...
Using a burette, the Bow Tie combustion chamber size was measured at 70 cc. To keep the field level, the AFR 180s were decked to deliver 70cc chambers, ensuring the same compression ratio.
Eyeing the intake ports on...
Eyeing the intake ports on the Bow Tie heads reveals mild porting as well as gasket matching.