Aftermarket Heads - Head to Head
Dyno Testing to Determine What Aftermarket Heads are Really Worth
From the February, 2009 issue of Chevy High Performance
By John Kiewicz
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.
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.
Check out the intricate CNC-porting...
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...
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...
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...
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...
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...
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...
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...
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...
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...
Hedman street headers (designed to fit a '62-67 Chevy Nova) vented to dual exhaust-wearing mufflers.
The 327 engine was warmed...
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...
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...
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...
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...
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...
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,...
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...
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
|Ported GM Bow Tie AFR 180 CNC |
|Lift (Inches) ||Intake Flow (cfm) ||Exhaust Flow (cfm) ||Intake Flow (cfm) ||Exhaust Flow (cfm) |
|0.200 ||116 ||107 ||140 ||112 |
|0.300 ||169 ||143 ||201 ||152 |
|0.400 ||212 ||166 ||244 ||190 |
|0.500 ||239 ||169 ||260 ||208 |
|0.550 ||225 ||172 ||262 ||214 |
|0.600 ||228 ||174 ||263 ||219 |
|Head Gasket Thickness vs. Compression Ratio |
|Although bore size affects compression ratio, Fel-Pro reports that a general formula for calculating how head gasket thickness affects compression ratio is as follows. For every 0.005-inch thickness increase/decrease, gasket volume changes by about 1 cc, which translates to slightly less than 1/8 (0.125) of a point of compression ratio. We did the exact math on our 327 V-8 and came up with these figures: |
|Gasket Thickness ||Compression ||Notes |
|(inches) ||Ratio || |
|0.015 ||10.68:1 ||Steel shim previously on 327 |
|0.020 ||10.61:1 || |
|0.030 ||10.31:1 || |
|0.040 ||10.03:1 ||Fel-Pro gasket used on this test |
|0.050 ||9.76:1 || |
| || |
| || |
AFR 180 CNC
|RPM ||HP ||Torque ||HP ||Torque |
|3,000 ||194.5 ||340.4 ||194.6 ||329.5 |
|3,100 ||198.8 ||336.9 ||194.9 ||329.8 |
|3,200 ||205.3 ||336.9 ||201.0 ||329.9 |
|3,300 ||214.1 ||340.8 ||209.7 ||333.7 |
|3,400 ||223.1 ||344.6 ||217.7 ||336.3 |
|3,500 ||230.5 ||345.9 ||225.0 ||337.6 |
|3,600 ||236.3 ||344.7 ||230.6 ||336.4 |
|3,700 ||242.0 ||343.5 ||237.0 ||336.5 |
|3,800 ||246.8 ||341.1 ||244.4 ||337.8 |
|3,900 ||252.5 ||340.0 ||251.8 ||339.2 |
|4,000 ||260.9 ||342.6 ||260.3 ||341.8 |
|4,100 ||270.4 ||346.4 ||270.8 ||346.9 |
|4,200 ||280.0 ||350.1 ||282.5 ||353.2 |
|4,300 ||288.8 ||352.7 ||293.4 ||358.3 |
|4,400 ||296.6 ||354.1 ||303.4 ||362.2 |
|4,500 ||304.5 ||355.4 ||312.5 ||364.8 |
|4,600 ||312.5 ||356.7 ||321.7 ||367.2 |
|4,700 ||320.4 ||358.1 ||331.0 ||369.8 |
|4,800 ||328.5 ||359.4 ||340.5 ||372.6 |
|4,900 ||335.4 ||359.5 ||348.8 ||373.9 |
|5,000 ||341.1 ||358.3 ||356.0 ||374.0 |
|5,100 ||345.6 ||355.9 ||363.2 ||374.1 |
|5,200 ||350.0 ||353.5 ||369.1 ||372.8 |
|5,300 ||355.6 ||352.3 ||375.0 ||371.6 |
|5,400 ||361.1 ||351.2 ||382.2 ||371.7 |
|5,500 ||367.8 ||351.3 ||389.3 ||371.8 |
|5,600 ||373.3 ||350.1 ||396.5 ||371.9 |
|5,700 ||377.3 ||347.7 ||402.3 ||370.7 |
|5,800 ||381.3 ||345.3 ||406.7 ||368.3 |
|5,900 ||383.8 ||341.6 ||412.4 ||367.1 |
|6,000 ||387.5 ||339.2 ||416.7 ||364.7 |
|6,100 ||392.6 ||338.0 ||420.8 ||362.3 |
|6,200 ||397.6 ||336.8 ||424.9 ||359.9 |
|6,300 ||399.8 ||333.3 ||430.3 ||358.8 |
|6,400 ||401.7 ||329.6 ||435.8 ||357.6 |
|6,500 ||401.9 ||324.8 ||439.6 ||355.2 |
|6,600 ||400.3 ||318.5 ||442.0 ||351.7 |
|6,700 ||394.2 ||313.6 ||442.5 ||346.8 |
|6,800 ||385.6 ||308.1 ||442.8 ||342.0 |
|6,900 ||380.8 ||304.4 ||439.6 ||334.6 |
|7,000 ||374.4 ||300.1 ||438.9 ||333.3 |