Testing The Newest Dart Aluminum CNC Heads - Pro Street 408 Stroker

Stop for a minute and take a close look at the air we breathe. OK, so you can't see it, and most of us walk around without giving it a second thought, but air is one of those critical things that helps plants and humans sustain life. It's also one of two critical components in the production of horse-power, the second being fuel.

Naturally, air and fuel must be supplied to each cylinder in the correct proportion and at the proper time, but hot rodders have taken to the task of creating additional power by improving airflow with aftermarket induction components. As we've said time and again, once airflow is improved, additional fuel is required, but the first item on the horsepower priority list should always be more air.

If you're an avid reader of MM&FF, the connection between airflow and horsepower should come as no surprise. That additional airflow can produce more power is hardly a revelation; the difficulty comes from how to actually go about improving it. When it comes to airflow, we're most concerned about the big three: the intake manifold, the cylinder heads, and the camshaft. Given the dynamic equation that is an internal combustion engine, the displacement, carb, and header sizing all play a part in the eventual power. But it's the intake, cam, and cylinder heads that determine the eventual power output and the overall shape of the power curve.

Though all of the components are covered in this buildup, we'll concentrate most of our efforts on the heads, more specifically the Pro 1 CNC aluminum cylinder heads from Dart.

A recent introduction by Dart, the Pro 1 CNC heads for the Ford Windsor family (including the 5.0L Mustang) offer a number of important features designed to help improve the flow characteristics. Checking out the impressive Pro 1 castings, the first thing you'll notice is its lightweight aluminum (355 T6) casting. This helps performance by improving the all-important power-to-weight ratio. Who wouldn't want the improved acceleration, handling, and braking associated with dropping a good 45-50 pounds off the nose of your Mustang?

Along with the weight savings, the aluminum helps raise the detonation threshold. Credit the heat dissipation qualities of aluminum for the reduced sensitivity to detonation. The final consideration-one you hope you never have to take advantage of-is the ease of repair. Damaged aluminum heads are more easily repaired than their cast-iron counterparts. We obviously never set out to hole a piston or torch a combustion chamber, but it does happen, and it's nice to know that such damage is much more easily repaired on an aluminum head than its cast-iron counterpart.

All the features of the aluminum construction are nice, but aluminum heads for 5.0L motors are a dime a dozen. What really makes the Dart Pro 1 heads attractive is the airflow potential offered by the CNC porting program. For the uninitiated, the acronym CNC means computer numeric con-trolled, basically a fancy name for computerized porting. Computer-controlled porting allows for precise, consistent machine work, and this translates into time savings and consistency between ports, even over dozens or hundreds of heads. The art of hand porting takes years of experience to master, and short of the tedious procedure of porting and flow testing each runner numerous times during the process, it's difficult, if not impossible, to replicate identical flow characteristics for each port.

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.

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).

While the power formula can be used as a guide, the reality is that the eventual power output of any motor is not solely determined by the flow rate of the intake port on the cylinder head. It should come as no surprise that adding these Pro 1 CNC heads to your stock 302 would never result in over 600 hp. The remainder of the combination (displacement, cam, and compres-sion) will obviously affect the power output as will the exhaust flow, carburetion, and even header configuration. Suppose you had a head that offered big-time intake flow but suffered with a poor exhaust-port design. You can't expect that head to offer as much power potential with a poor exhaust port regardless of the intake flow (though cam timing can be used to offset some of the disparity). With 235-cfm exhaust ports, the Pro 1 heads offered an impressive 72 percent intake-to-exhaust flow relationship, so the exhaust ports wouldn't hold the power potential back on these heads. It's important to note that we have on occasion exceeded the power output suggested by the airflow formula, but this is usually the exception rather than the rule.

At MM&FF, we would never simply provide our readers with airflow numbers and call it a day. Knowing how our readers live and breathe for dyno numbers, we decided to see just how close we could get to the numbers suggested by the power formula. Since we went looking for something over 600 hp, we decided to build a motor capable of utilizing every bit of airflow offered by the new Dart Pro 1 heads. Our Pro Street Stroker short-block came from Coast High Performance. The CHP stroker features a 351 Windsor block stuffed to the gills with a 4.0-inch stroker crank. When combined with a 0.030-inch overbore, this upped the displacement from the standard 351 ci to a full 408 cubes.

The big-inch Windsor featured 11.7:1 compression thanks to a set of forged flat-top pistons and connecting rods from Probe Racing. Exceeding 600 hp requires a healthy cam profile, so we went right to the experts at Comp Cams for a solid roller profile. The cam for our Pro Street Stroker featured 0.688 intake lift, 0.672 exhaust lift, and a 264/268 duration split at 0.050. The aggressive roller cam also featured a tight 106-degree lobe-separation angle. The Dart heads were topped off with a ported Edelbrock Super Victor intake and a 950 HP Holley carburetor. Also present was a set of Hooker Super Comp headers, an MSD ignition, and a CSI electric water pump.

After installing the heads using Fel-Pro 1011-2 head gaskets and 1/2-inch ARP head studs, we installed the intake and carb and warmed up the 408-inch beast on the Westech Superflow engine dyno. After bringing the motor up to temperature, we gave it a full 30 minutes of break-in time before subjecting it to full throttle loads.

We then changed out the conventional oil used during the break-in procedure for Lucas Synthetic 5W-30. The preliminary loads at 4,000 rpm looked promising, as the 408 thumped out over 500 lb-ft of torque. After some jetting and timing sweeps, we were finally rewarded with some big numbers, as the Dart-headed 408 produced peak power numbers of 618 hp at 6,600 rpm and 562 lb-ft of torque at 5,100 rpm. Torque production from the big Windsor exceeded 550 lb-ft from 4,600 rpm to 5,700 rpm, and exceeded 500 lb-ft from 4,000 rpm (or lower) all the way to 6,500 rpm. Though we failed to reach the potential power output suggested by the power formula, we suspect that a larger carb would have helped (there was vacuum present at W.O.T.) as would a larger set of Hooker headers. An oil pan with additional windage control would surely improve power as well, but all in all, we were plenty pleased with our 618hp Pro Street stroker.

CHP 408-Dart Pro 1 225 Heads
Obviously, this stroker buildup was not for the timid, as the elevated static compression, wild cam timing, and healthy 950 HP Holley carb were designed with speed in mind. After topping the CHP 408 stroker with the Dart Pro 1 225 CNC heads, a wild solid roller cam from Comp Cams and a Holley 950 HP carb and Super Victor intake, our Pro Street Stroker thumped out 618 hp at 6,600 rpm and 562 lb-ft of torque at 5,100 rpm. Torque production from the big Windsor exceeded both 550 lb-ft from 4,600 rpm to 5,700 rpm and 500 lb-ft from 4,000 rpm (or lower) to 6,500 rpm. The Dart Pro 1 heads offered plenty of airflow to work with the long-duration cam to help produce plenty of high-rpm power. On the wish list for this combination is a set of 1 7/8-inch Hooker headers, a racing oil pan with better windage control, and even a Holley Dominator carb (there was vacuum present with the 950 HP).