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Aftermarket Heads - Head to Head
Dyno Testing to Determine What Aftermarket Heads are Really Worth
Dec 5, 2005
Westech Performance Group
Mira Loma, CA
Air Flow Research
Valencia, CA 91355
Memphis, TN 38118
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Aftermarket Heads - Head to Head
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 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 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 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 the Bow Tie heads reveals mild porting as well as gasket matching.
Check out the intricate CNC-porting on the AFR 180's intake runners. The porting is stunning and the process ensures exactly the same dimensions from port to port and head to head.
A look at the AFR exhaust port shows more of the same. While many companies claim heads that are CNC-ported, often the work is on only a portion of the runner. The AFR heads feature CNC-machining on 100 percent of the port.
AFR's custom-designed combustion chamber improves airflow to the engine and is made even better with full CNC-porting. The 2.02-inch intake and 1.60-inch stainless steel valves work with hardened ductile-iron interlocking valve seats.
With valves removed, you can see the results of AFR's 5-axis CNC work and the three-angle competition valve job.
The AFR heads have pushrod guide plates, heavy-duty valvesprings, chrome-moly retainers, and 10-degree retainers as standard equipment. As an upgrade, AFR offers special-rate valvesprings (depending on your application) and lightweight titanium retainers.
Notice how the combustion chambers differ between the Bow Tie (left) and AFR heads. The AFR chambers don't shroud the valves as much as the Bow Tie design does.
Side by side, it's easy to see that the Bow Tie intake ports are larger than the AFRs'. With the smaller AFR intake ports (which outflow the Bow Tie ones), you'll realize improved throttle response and better low-rpm power.
A look at the exhaust ports shows the AFR heads to be more square, as opposed to the Bow Tie's more rounded shape.
Flow-testing the AFR 180 CNC heads revealed big intake and exhaust flow increases despite a smaller intake runner size. Proof positive that bigger isn't always better.
Hedman street headers (designed to fit a '62-67 Chevy Nova) vented to dual exhaust-wearing mufflers.
The 327 engine was warmed up and then given a dyno pull. Carb jetting was adjusted and ignition timing was advanced until power fell off. Once trimmed in, the final dyno pull generated 401.9 hp with the Bow Tie heads.
Westech's Steve Brule lifted off the Edelbrock Performer RPM intake and Barry Grant Mighty Demon 650-cfm carb in order to access the iron heads.
The deck was wiped clean before fresh 0.040-inch-thick Fel-Pro Perma-Torque MLS gaskets were installed. To hold consistency, the same style/thickness gaskets were used with both sets of heads.
Using our Craftsman torque wrench, we incrementally tightened the ARP bolts in sequence. Remember that aluminum often requires a different torque rating than iron, so contact the manufacturer for specifics.
The COMP Cams hardened pushrods and 1.52:1 roller rocker arms were reinstalled. Valve lash for the mechanical roller lifters was set at 0.016-inch.
After air/fuel tuning, ignition timing was advanced until power fell off. With the AFR aluminum heads, the engine liked a total of 37-degrees of ignition advance.
Once tuning was finished, the final dyno pull generated 442.8 hp at 6,800 rpm. Average power was up across the rpm range, along with a 14.5 lb-ft increase in torque.
Hit The Scales:
Bow Tie cast-iron cylinder head:
49.20 pounds each x 2 = 98.40 pounds
AFR 180 aluminum cylinder head:
32.81 pounds each x 2 = 65.62 pounds
Total weight savings: 32.78 pounds
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