Richard Holdener
February 1, 2009
Are CNC ported heads really worth the extra cost? The only way to find out is to stick a test motor on the dyno and run a back-to-back test.

One of the problems with running the proverbial ultimate cylinder head test that we did a few years back (a.k.a. MM&FF's Ultimate Guide to Cylinder Heads), is that the performance world continues to evolve. By that we mean that despite our very best effort to include every single cylinder head in our shootout, new 5.0L heads continue to hit the market.

Having been in the Mustang business for more years than I care to admit, I certainly remember when our only cylinder head option was a ported 351W casting. Times sure have changed, as now 5.0L enthusiasts have about a zillion different castings and configurations to chose from.

If there is one thing our all-inclusive head test proved, it certainly demonstrated that power production is a function of many things. Not surprisingly, (in the case of cylinder heads), power production is linked to airflow. Advertisements, websites, and even word-of-mouth continue to tout peak airflow numbers, but there is much more to the equation than the big flow number. If peak flow were the only criteria for choosing a cylinder head, then we'd all be running around with 400-plus cfm Yates, Neal, or Blue Thunder heads. Naturally, this choice would be ill advised on an otherwise stock 5.0L, but that doesn't stop individuals from making a cylinder-head choice based solely on airflow numbers.

Not unlike peak power numbers, peak airflow numbers can be all but meaningless when they're not combined with additional data. They are the proverbial tip of the airflow iceberg. The old adage that if 200 cfm is good, 300 or even 400 cfm must be better, should be taken out into cyberspace and shot. The peak airflow offered by a cylinder head is but one of many factors that determine its worth, to say nothing of the eventual power output of a motor.

Just as with the entire engine package, it is the combination of components that make a decent cylinder head. In other words, there is not a best head out there. The big peak flow numbers combine with things like average flow figures, port volume and shape, and even the valve job to produce a desirable head package. Lets not forget the combustion chamber design and volume, the valvespring package, and even small things like valve seals, as these can make or break the performance of a motor. Ruin the head by overlooking just one of these many components and the whole package suffers. The same holds true with a motor, as all the hot cams and high-flow heads will be of little use when you choke it down with a stock two-barrel carburetor.

Dissecting a head is helpful in understanding the importance of the individual components or attributes. One of these important attributes is average airflow.

To understand the importance of average, as opposed to peak, head flow, we can liken it to the average power production of a motor. It is after all, the average, and not peak, airflow that determines the eventual power production of a motor. We all like to talk about the big peak power numbers offered by a motor (a problem unfortunately aggravated by chassis dyno queens), but the reality is that this maximum value is rarely used.

Think about it for a minute-how often does your motor see the peak power rpm at wide open throttle? Even assuming you're a serious lead foot and flog your car at every opportunity, the motor only sees maximum rpm for a brief second or two. Only during top-speed runs, like the Silver State Open Road Race, does a motor run for any length of time at high rpm. By contrast, the motor spends most of its time revving from low-to-medium engine speeds at low-to-medium throttle angles-higher loads and throttle angles for various type of racing, but even those spend very little time at peak power. The same can be said of your cylinder heads, as the valves spend the vast majority of time running from zero-to-peak lift and back again, spending almost no time at the maximum lift value. This is not to say that peak numbers aren't important, they simply should not be the sole criteria for head selection.