A hand-porting procedure is necessary to initially set up the CNC program, but once configured, it can replicate flow numbers with computerized precision. About the only thing limiting the effectiveness of the CNC porting is core shift of the cylinder head, as the program must orient itself on a known position on the head. Core shift can change the position of the port walls ever so slightly, but what ultimately happens is that the precision porting offered by the CNC program is applied to a not-so-precise wall location. This situation is unusual and can be handled with the proper setup procedure, something Dart seems to have a handle on if our heads are any indication of the quality consumers are to expect.
The chambers featured CNC...
The chambers featured CNC work as well along with a set of 2.08 intake and 1.60 exhaust valves. We liked that Dart taped the mating surface to minimize the chance of any unwanted scratches during shipping. Make sure to thoroughly clean the heads of any excess aluminum shavings prior to assembly.
Wanting maximum flow for our big-inch Windsor, we chose the big-boy 225cc Pro 1 CNC heads. According to Dart, these heads came with healthy 225cc intake ports and equally impressive 87cc exhaust ports. Size is one of the criteria that affects flow potential, but it's not the sole determining factor (remember, bigger isn't always better). To put the 225cc intake port into perspective, the intake port of a stock 5.0L Ford casting (model E7TE) checks in at a miniscule 127 cc. The disparity carries over to the exhaust port as well, with the Dart Pro 1 measuring some 43 cc larger than the stock 44 cc exhaust port. Dart stepped up on the valve sizes as well, with 2.08 intake and 1.60 exhaust valves. Naturally, the valves used in the Pro 1 heads are significantly larger than those in the stock Windsor casting (1.78 in/ 1.46 ex). When you add up the combination of sizable runner volume, precise CNC porting, and impressive valve sizing, you get one thing-big airflow. Compared to the stock intake ports, the Dart Pro 1 CNC heads offer twice as much flow (325 cfm vs. 160 cfm). The exhaust flow is even more impressive, with the Pro 1 heads coming in at 235 cfm compared to just 112 cfm for the stock heads, a gain of over 110 percent.
While the Pro 1 CNC heads certainly have a ton of impressive features and appear to offer exceptional airflow, how does that translate into horsepower? Loyal readers will remember the formula to convert maximum intake airflow into potential horsepower from our Ultimate Guide to Cylinder Heads. The word potential is import-ant here as head flow is but one of the many variables that ultimately affect the power output, but for now, let's look at the airflow/power formula. The formula states that horsepower is equal to the maximum airflow times 0.257 times the number of cylinders or:
HP = Max Airflow x 0.257 x Number of cylinders.
This airflow/power formula can be used to calculate the power potential of any cylinder head on any engine configuration. If we plug the airflow generated for the new Dart Pro 1 CNC head, we get the following:
HP= 325 cfm (max intake flow at 0.700-lift) x 0.257 x 8 (cylinders) or 668 hp
The formula suggests that the airflow offered by these 225 heads will support up to 668 hp in normally aspirated form-pretty impressive considering the power potential offered by a set of stock 5.0L heads is just 329 hp (160 cfm).
The 225cc heads flowed well...
The 225cc heads flowed well all the way to 0.700-lift. We obviously employed a clay entry for testing, but this shot illustrates just how impressive the intake ports looked after porting.
To properly illustrate the...
To properly illustrate the power potential of the Dart Pro 1 heads, we installed them on a 408 stroker supplied by Coast High Performance. The CHP 408 featured a cast steel crank but forged rods and flat-top pistons. The piston design helped push the static compression up past 11.0:1.
With plenty of displacement,...
With plenty of displacement, head flow, and compression, our stroker motor needed some aggressive cam timing. The solid roller cam supplied by Comp Cams featured a 0.622/0.627-lift split and a 264/268 duration split. Comp also supplied the necessary roller lifters.