Volume III, Issue 10, Page 2

There’s an old hot rodding adage: “Airflow is everything.” But is it really? If you consider that in its most simplistic 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 even more important that port volume? For that we headed to the dyno to do back-to-back testing to determine exactly where myth crosses reality. All tests were done on the same small-block V-8, on the same day, using the same dyno—so gone are the variables that often creep up in other such tests. Going head-to-head would be 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 Mule

The test mule for this tech comparo was a moderately healthy 327 V-8 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 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 Cam 1.52:1 roller rockers were actuated by a GM Duntov 30-30 mechanical lifter camshaft. A ‘67-‘69 Z/28 dual-plenum aluminum intake manifold and a Carter AFB 625cfm carb funneled the fuel. The combo sounded tough and revved past 6,000 rpm, but was a little shy on torque.

As time progressed, the 327 was upgraded with more modern parts. A MSD billet distributor, 8mm wires, and 6AL ignition replaced the stock wares. The carb and intake were shelved in favor of an Edelbrock Performer RPM aluminum intake teamed with a Barry Grant Mighty Demon 650cfm 4-barrel. Power increased, but it still wasn’t stunning. Recently, the old school 30-30 cam was replaced with an aggressive mechanical roller cam/lifter/timing chain kit from Competition Cams. Power was up, but there was one key element of old school that needed to go—namely the heavy, old-tech cast iron heads.

The Big Question

Everybody talks about how aftermarket heads make big power; but what are we really talking about here—25, 50, 100-horsepower? We wanted real answers to this expensive question so we did back-to-back dyno testing. We’ve seen too many tests where aftermarket heads make big power over boat-anchor, smog-o-rama heads—but c’mon, that’s a no brainer. Of more interest, we wanted to know how well aftermarket heads stack up against decent design, mildly ported heads. Now that’s a real comparison!

To further keep the playing field level, the AFR 180s were decked to generate 70cc combustion chambers to match the 70cc chambers on the iron Bow-Tie heads. While the AFRs come standard with 74cc or 68cc chambers, you can custom order your heads cut 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 more power would have been achieved. However, the focus of this dyno test was to keep variables to a minimum to maximize consistency and testing accuracy.