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.
For the baseline test, we had the 327 wearing the GM Phase I cast-iron heads (see head flow sidebar). All testing was done on pump gas at normal operating temperatures, with Hedman street headers expelling through dual exhaust and mufflers. Numerous dyno pulls dialed in the Mighty Demon's jetting, air bleeds, and idle mixture. Distributor timing was incrementally increased until power fell off. Once trimmed in, a final dyno pull yielded a respectable 401.9 hp and 359.5 lb-ft of torque. Interestingly, the Bow Tie heads flowed better than expected on the flow bench, but dyno numbers immediately puffed a longtime myth about the 327--although it freely pulled past 7,000 rpm, power quickly dropped off after 6,500 rpm. Average power (321 hp) was decent, but certainly not much to brag about.
Off came the iron heads and the Fel-Pro Perma-Torque head gaskets. The deck was wiped clean before a fresh set of 0.040-inch-thick (see head gasket sidebar) Perma-Torque multi-layer gaskets were installed. We immediately noticed the AFR's drastically lighter weight (see sidebar). Note that the AFR 180s are a direct bolt-on and incorporate internal EGR passages for 50-state smog compliance along with external accessory bolt holes. The ARP head bolts were reused and incrementally torqued in sequence. Before reinstalling the intake and carb, we fit new Fel-Pro intake (PN 1205) gaskets that incorporate larger-than-stock openings to properly mate with the CNC'd intake ports. After the induction was in place, the MSD distributor was reinstalled, the engine was fired up, and the heads were allowed about 10 minutes of break-in running.A few dyno pulls revealed that the AFR heads warranted carburetor jetting changes. Once that was optimized, ignition timing was advanced until power fell off. With the AFRs, the 327 liked about 3 degrees more total ignition advance over the iron Bow Tie heads--likely due to the aluminum head's nature to reflect less heat into the combustion chamber. In general, more heat generates more power, but at the chance of detonation.
The dyno numbers showed that the AFR heads delivered 41 more peak horsepower and that average horsepower was up, too. Moreover, the AFR heads kept flowing strong past 7,000 rpm, while also delivering quicker, crisper throttle response. Interestingly, most high-flow aftermarket heads are known for increasing power, yet torque usually doesn't change much. With the AFRs, we were pleasantly surprised to see peak torque increase by a solid 14.5 lb-ft, a welcome addition with torque shy, small-cube V-8s such as our 327.
Hard numbers from the dynamometer show that aftermarket heads do make a notable difference; for this test we went with AFR's Competition Package heads that are regarded by many as the best-flowing 180cc heads on the market. Consequently, if you go with a less radical, smaller port, and/or non-CNC-ported aftermarket head, before-versus-after power may be less. Regardless, this test shows that although aftermarket heads can be expensive (AFR 180s list for $1,324-1,999 per pair, depending on options), the byproduct is a solid increase in power.Truth told, our motor combination wasn't optimized for the AFR heads, which ultimately hurt power. A more aggressive and/or open-plenum intake manifold would have been a much wiser choice for the high-flowing heads, but hood clearance issues eliminated that option. Because our 327 will soon receive a Vortech centrifugal supercharger, the previous 0.015-inch-thick head gaskets were replaced with much thicker 0.040-inch versions that knocked down compression by about 0.65 of a point. (Note: Going from our current 10.03:1 to the previous 10.68:1 would likely increase power by about 3 percent--about 11 hp in our case.) Moreover, the COMP Cams roller cam installed was ground with the upcoming supercharger in mind. Rather than running a 108- or 110-degree lobe separation angle, we went with a blower-friendly 112-degree LSA that ultimately hurt naturally aspirated power but should prove beneficial with forced induction. Regardless, our high-winding, 442.8hp 327 should prove plenty strong when it takes residence in the engine bay of a '66 Nova Super Sport. Besides the 41hp increase over the previous setup, the AFR's alloy construction shaves almost 33 pounds off the nose of the Nova, which should make the car feel even more energetic.