If the "Ultimate Guide to Cylinder Heads" article showed us anything, it demon-strated that power production is a function of many things. In the case of cylinder heads, it's obviously linked to airflow. Advertisements, Web sites, and even word of mouth continues to tout peak airflow numbers, but there's much more to a good set of heads than just peak flow. Much like peak horsepower numbers, peak airflow numbers can be meaningless when not combined with additional data. The old adage that if 200 cfm is good, then 300, 400, or even 500 cfm must be better, is just not true.
The peak airflow offered by a cylinder head is but one of many factors that determines the worth of the head. Just as with the entire engine package, it's the combination of components that make a decent cylinder head. The big peak flow numbers combine with average flow figures, port volume and shape, and the valve job to produce a desirable head package. Let's 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.
The fouR-bolt main Dart 351W...
The fouR-bolt main Dart 351W block was stuffed with a steel stroker crank, forged rods, and forged pistons to produce a finished displacement of just over 428 ci.
To understand the importance of average (as opposed to peak) head flow, we can liken it to the average power production of a motor. After all, it's the average airflow that determines the eventual power output. We all like to talk about big peak power numbers, 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 leadfoot and flog your car at every opportunity, the motor sees maximum rpm for only a brief second or two. Only during top-speed runs (or the Silver State Open Road Race) does a motor run for any length of time at high rpm. Where the motor spends most of its time is revving from low-to-medium engine speeds at low-to-medium throttle angles. 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. Obviously, power production is very high on the scale, which is why we tested the many heads in our "Ultimate Guide to Cylinder Heads" on both the airflow bench and the engine dyno. It's true that power and airflow are related, but as the head testing demonstrated, the highest peak flow numbers did not always produce the highest peak power numbers. In the case of the TFS-R heads tested here, the high-performance aluminum castings offer much more than big peak flow numbers.
The Dart block offers more...
The Dart block offers more than enough bottom end strength for our needs. A little persuasion might be necessary to coax the oil pan rails over the four-bolt block.
Racers and street machines alike have run the TFS-R heads in confidence, as the design has always proven powerful. A portion of the credit for the impressive power production goes to the Twisted Wedge combustion-chamber design. Rotation of the intake and exhaust valves over the center of the bore helps to unshroud the valves, thereby improving the respective flow rates. The attending central location of the spark plug facilitates complete combustion by minimizing the required distance traveled by the flame front.
In addition to the altered valve angles, the R heads offer sizable port volumes and valve sizes. While Ford suggested we make do with a 1.78/ 1.46 valve combination in the stock E7TE heads, the TFS-R heads offer 2.08-inch intake valves and 1.60-inch exhaust valves. Naturally, the TFS-R offerings were of the stainless steel variety and, when combined with 61cc combustion chambers, 206cc intake ports, and raised exhaust ports, it resulted in the all-important peak flow numbers of 305 cfm for the intake and 222 cfm for the exhaust.