If peak airflow is not the sole criteria for determining the worth of a cylinder head, then what is? What about power production? Obviously, power production is very high on the scale. It is true that power and airflow are related, but the highest peak flow numbers do not always produce the highest peak power numbers. In the case of the Pro Comp CNC heads tested here, the high-performance aluminum castings offered much more than big peak flow numbers when compared to their as-cast counterparts. The CNC versions of the Pro Comp aluminum heads offered larger port volumes, larger intake valves, 2.05-inch vs. 2.02-inch, and different size combustion chambers, which is something that actually hurts power. The combination of these variables can help produce higher flow values as measured on the airflow bench, but the ultimate test is actually on the dyno, and then on a track if quicker e.t.'s and higher trap speeds are the ultimate goal.
Not surprisingly, either one of the Pro Comp offerings, as cast or CNC, represents a dramatic jump in performance over the stock 5.0L iron heads. Ford suggested 5.0L owners make due with a 1.78/1.46 valve combination in the stock E7TE castings, but even the as-cast 190 cc heads featured the more common 2.02/1.60 valve combination. Naturally, the valves were of the stainless steel variety and when combined with 60 cc combustion chambers, 190 cc intake ports, and raised exhaust ports, resulted in the all-important peak flow numbers of 258 cfm for the intake and 180 cfm for the exhaust.
While everyone touts the 0.700-lift airflow figure, the reality is that most enthusiasts run cams of less than 0.700 lift, especially with as-cast Pro Comp heads. The more realistic lift value for most applications is 0.550 or even 0.600, as it is important for the airflow not to go static or drop off at higher lift values. The cam run on our test motor checked in at a tad under 0.600 lift on the exhaust side, so the 0.600-lift number was more of a concern than at 0.700 lift.
While we all want heads that flow 350-plus cfm on our stock 302, the reality is the larger port volumes are best suited to larger displacement motors. The often used calculation for the relationship between airflow and horsepower is hp = peak airflow x .257 x number of cylinders. Obviously, this formula is helpful for determining the power potential of a set of cylinder heads, but it is not a predictor.
The airflow formula does not take into account the other engine components that may affect the power output, nor does it consider average airflow. Just for grins, we applied this formula to predict that these new CNC-ported Pro Comp heads can support just under 600 hp in normally aspirated trim, a great deal more if equipped with forced induction. Again, just because you run these heads on your 302, 331, or, as in our case a 408 stroker, doesn't guarantee that your combination will produce anywhere near the 600 hp potential offered by the new CNC-ported heads. Basically, this test was going to reveal not the absolute power potential of the heads, but rather how much additional power they offered compared to the as-cast heads on this mild stroker combination.
To illustrate what the new CNC-ported heads from Pro Comp had to offer, we installed them on a 408 stroker from Coast High Performance. We wanted a combination that was both representative of what can be found on the street, as well as something large enough to take advantage of the additional flow offered by the porting. What better combination could there be for the impressive head flow and large port volumes than a large-displacement stroker?
Testing these same head configurations on a smaller and milder 302 would yield considerably different results, likewise for a wilder 427 stroker.
The two-bolt block was more...
The two-bolt block was more than sufficient for the intended power output. Note the HV oil pump, Canton oil pump pickup, and Fel-Pro pan gaskets.
To ensure plenty of airflow,...
To ensure plenty of airflow, we chose a ported Super Victor intake from Keith Wilson. The intake would allow the stroker to take full advantage of any airflow supplied by the cylinder heads.
Holley supplied one of its...
Holley supplied one of its 750 cfm Street HP series carburetors. We tried a larger carburetor, a 950 HP Holley, on the combination run with the CNC heads, but the larger carb offered no increase in power.
Ignition chores were handled...
Ignition chores were handled by an MSD billet small-cap distributor, along with MSD plug wires and a Digital 7 ignition amplifier. The distributor was set up with a blue bushing and springs to improve the timing curve.
The exhaust system consisted...
The exhaust system consisted of Hooker Super Comp headers flowing into a set of 3-inch MagnaFlow mufflers.
Run on the Super Flow engine...
Run on the Super Flow engine dyno, the 408 produced peak numbers of 463 hp at 5,600 rpm, and 480 lb-ft at 4,300 rpm. Note that Pro Comp also offers both cast and these trick fabricated aluminum valve covers.