For our purposes we'll run the engine carbureted on the dyno to see how close we came to our power goal. Carburetion is less expensive than fuel injection, unless, of course, you already have a tunable fuel-injection system you can modify to work at high rpm. That means a possible slight loss of driveability, definitely less fuel economy, and no hope on the emissions front.

All these parameters lead us to what the typical bracket racer would recognize as a good budget build. The first step to power is displacement--there is no replacement for displacement--and the best bang for the cubic-inch buck is, of course, a 347.

The next step is to pack as much air as practical into the (Procomp Electronics | Motorsport) cylinders, meaning a high-flow cylinder head and a really big cam, then squeezing it hard via elevated cylinder pressure to get some work out of it. To date, a turbocharger is the most efficient way to squeeze air, but that--and all other forms of supercharging--is off (budget) limits. Instead, we'll do it the old-fashioned way, with a generous dollop of compression.

What we've spec'd out is a 347 stroker packing a big, mechanical camshaft, decent heads, and a free-flowing intake manifold and carburetor combination. That's just the sort of thing Probe is used to putting together for its bracket customers, and we followed along as they demonstrated how they would put together a budget 347. This month we're showing the parts and highlights of how they went together; next month we'll strap the engine to one of Westech's dynos to see how the power comes out.

ST MACHINE COVER

PAGE 1

PAGE 3

 PAGE 4
 

GIVE AWAY PAGE

STROKER SPECIAL
PAGE 1

STROKER SPECIAL
PAGE 2

STROKER SPECIAL
PAGE 3

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.

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 CHP 408 stroker started out life as a late-model production 351 Windsor block equipped with 4.03-inch bores. The block was further machined to accept the forged reciprocating assembly from Coast High Performance. The CHP stroker kit included a cast 4.00-inch stroker crank swinging a set of forged I-beam rods and 22cc, dish-top forged pistons. In addition to the dish, the pistons featured valve reliefs for use with inline valve locations. The combination of the 4.03 bore and 4.00-inch stroker produced a final displacement of just over 408 ci.

The 408 was equipped with an XFI stroker hydraulic roller cam that offered 0.579 lift, both intake and exhaust, and a 236/248 duration split. The XFI cam was tailor made for stroker applications and featured aggressive ramp rates to maximize the opening time of the cam relative to the duration figures. The Comp cam was combined with a double roller timing chain, 1.6 ratio Gold Series roller rockers, and custom length pushrods to work with the Pro Comp heads and hydraulic roller lifters.

The remainder of the buildup included as-cast 190 cc Pro Comp heads installed using Fel-Pro MLS head gaskets and ARP ½-inch head studs. Both sets of heads were run with an Edelbrock Super Victor intake and a Holley 750 cfm Street HP carburetor. Also employed were 1¾-inch Hooker headers, an MSD small-cap, a billet distributor, and a CSI electric water pump.

The new CHP stroker was treated to a 20-minute break-in procedure where the load and rpm were varied to properly seat the rings and bearings. We ran Lucas conventional 30W oil for the break in, and then switched over to 5W-30 synthetic for testing. All testing was performed on 91-octane pump gas. Equipped with the as-cast 190 cc heads, the 408 produced 463 hp at 5,600 rpm, and 480 lb-ft of torque at 4,300 rpm. Swapping on the CNC-ported heads resulted in a jump to 505 hp at 5,900 rpm, and 487 lb-ft of torque at 4,600 rpm. The porting resulted in a sizable gain in power, as much as 54 hp at 6,000 rpm, but there was a slight trade off in the lower rev ranges. This can be attributed to the drop in static compression caused by the CNC work performed on the combustion chamber. Ideally, we should have milled the CNC-ported head to equalize the chamber sizes for this test, but it is obvious that the CNC-ported heads from Pro Comp offer significantly more power than their as-cast counterparts. With street prices near $650 for an assembled set of as-cast Pro Comp heads (via the Internet), these look to be pretty popular with 5.0L enthusiasts on a budget.