With the supercharger testing complete, HPP felt it was time for The Mule to explore some new horizons. Up until now, the stroker 455 retained all of its original build parts and we worked within those confines. In every instance, including the rocker arm ratio and cam phase test, we discovered that our original combination yielded the best results and thus, the engine was returned to that state for the next phase of research and development.
 With the 6X cylinder head removed, an examination of the combustion chamber reveals th
Within the traditional Pontiac community there are two schools of justifiable thought when it comes to cylinder heads. These are: retain and fully port the stock castings or switch to an aftermarket aluminum design, such as the Edelbrock offerings or others. We chose the first ideology, since it made the most sense. The 6X castings used in the mid ’70s have become well respected and widely used, so it was a logical place to start our engine build.
These factory heads when treated to RaceKrafters’ porting expertise and state-of-the-art equipment produced stunning results. The Mule consistently returned dyno numbers of between 492 and 494 hp and was very insensitive to octane. We were able to make that power on regular grade gasoline, though at times we did try higher-octane fuels with little to no gain.
In addition, the combustion event when qualified for efficiency by the amount of ignition lead required was also quite impressive. With a 9.1:1 compression ratio, the Poncho with the 6X cylinder heads only required a maximum spark timing of 32 degrees BTDC. By any measure The Mule was an impressive engine. But we knew that the cylinder heads were at their maximum power potential, if the dyno needle were to move upwards, we needed to increase the volumetric efficiency of the engine.
Volumetric efficiency (VE) is a measure of cylinder fill. On our test level we cannot confirm or accurately assign a number for it. But the more air throughput the engine has, the greater the VE will be. In an engine, peak cylinder fill coincides with peak torque and in most applications the VE is around 80 percent. A simple way to understand this is that at the most the bore is 80 percent filled to capacity with charge. That is defined as the combustible mixture. A well-defined race engine can experience VE nearer 100 percent and beyond. Once forced induction is applied, the VE potential becomes almost limitless depending on the airflow of the power adder and the pressure realized.
There is a simple equation to calculate power from airflow that HPP has introduced in previous installments of this series, but it bears repeating.
Horsepower = flow at 28 inches/H2O x 0.257 x number of cylinders.
 The piston crowns were evenly discolored and showed no signs of carbon accumulation. C
Our RaceKrafters modified 6X cylinder heads flowed 236 cfm at 0.500-inch lift and 250 cfm at 0.600-inch lift. The cam with the 1.65:1 rockers lifted the valve to around 0.550 inch. If we average the airflow between those two lift points, we come up with around 243 cfm, which then calculates to 499 hp to our 492 hp actual reading. Extremely close, especially since we are not taking into consideration any flow loss from the intake manifold.
As an aside, it proves that the 6X cylinder heads and the potential horsepower are at their maximum with no room to improve the VE. We could potentially find more power with a higher compression ratio and a more aggressive cam profile, but the gains would be minimal since the engine can only ingest so much air through the cylinder heads. If we wanted to make more power, The Mule required better-flowing cylinder heads.
When Edelbrock introduced its first Pontiac cylinder head in the early ’90s, it was with a round exhaust port (laid-over D in appearance in Edelbrock’s case) and not the more commonly employed D-port design with the Siamesed center ports. There is no denying that it was an excellent casting with the ability to make huge power, but that came with a complication— either Round-Port exhaust manifolds or less-common-than-D-Port Round-Port headers were required. This added expense, and in some cases, required custom headers to be made for cars that weren’t originally offered with Round-Port heads. Recognizing that the Pontiac community had varied needs, Edelbrock created a replacement D-port cylinder head a couple of years ago. This is the design we wanted to try on The Mule: D-port versus D-port.
The Edelbrock Advantage
When choosing which cylinder head route to go with for your Pontiac, factors beyond airflow need to be considered.
 The Edelbrock cylinder head is a direct bolt on, but does require different head bolts
If your 6X cylinder heads are in good shape and are not cracked then RaceKrafters can duplicate what they did to The Mule for a charge of $2,500 including parts and labor. This entails cleaning, surfacing, porting, valve job, new 2.11/1.77 stainless steel valves (larger than stock exhaust valves), 7⁄16 studs, and new valvesprings, retainers, and locks.
The Edelbrock D-port fully assembled castings can be purchased from many retailers. Summit Racing has them under part number EDL-61575 for hydraulic roller cam for $2,319 or EDL-61579 for hydraulic flat-tappet cam for $2,199 per pair. They come with 87cc chambers, 2.11/1.66 stainless steel valves, valvesprings, retainers, locks, 7⁄16-inch studs, and pushrod guide plates. Bare 87cc chamber castings and bare or fully assembled 72cc chamber castings are also available.
It’s impressive that a docile street Pontiac makes this kind of power
When examining a cylinder head, it is very important to study the combustion chamber design and spark plug orientation. By all standards, the Pontiac factory casting employs a combustion chamber that undeniably works well, but is about 50 years old in design. We all know that engine technology has advanced quite a bit during that time. Also, the 6X head has the spark plug biased toward the intake valve. That is in contrast to the modern trend.
In theory, the plug should be in the center of the bore, which is the most turbulent. If it cannot reside there then the next best location is facing the exhaust valve. This is the hottest part of the combustion chamber and will allow for the reaction zone to be more efficient. This describes the heat transfer from the burned to unburned mixture, which is required to speed up the flame travel across the bore.
 Prior to installation the valvesprings were checked to ensure that they were up to tas
A benefit of the Edelbrock casting is that it employs a modern combustion chamber design and has the spark plug biased toward the exhaust valve instead of the intake valve like a Pontiac factory casting does. In some vintage Pontiac literature, the division claimed that by placing the electrode near the intake valve a leaner mixture could be ignited. That may have been the theory then, but it is not the trend over the last 30 years.
Another consideration is that the Edelbrock aluminum cylinder heads will remove 42 pounds from the front of your Pontiac, since the fully dressed pair of 6X heads weighs 108 pounds and the E-heads weigh 66 pounds. This is a huge benefit to a drag car, since it allows for more weight transfer to the rear tires. On a street Pontiac, the weight reduction will allow for more nimble handling and better braking. That is something that cannot be seen on an engine dyno or flowbench. In addition, the Edelbrock castings are a better jumping off point, since they allow for improved flow when massaged, while our 6X heads are now at their maximum.
To assign hard numbers to the flow potential, RaceKrafters tested the out-of-the-box Edelbrock castings and this was what they found.
The Edelbrock cylinder head outflowed the modified 6X casting at every lift point on the intake side, but the Pontiac casting did a substantially better job on the exhaust side, which we found interesting. At 0.500-inch lift, the Edelbrock head had an intake to exhaust ratio of 63 percent, while at the same lift the 6X enjoyed a ratio of 77 percent. For years the industry has debated the value of the intake/exhaust ratio with convincing arguments on both sides of the isle and we are not going to get involved in that. Here HPP is only providing an observation from our test data.
 RaceKrafters set up the guide plates on the 6X heads for 3⁄8-inch-diameter pushrods, b
Employing our equation and averaging the flow on the Edelbrock heads between 0.500 inch and 0.600 inch, we come up with approximately 263 cfm. This would translate to a potential 540 hp in stock form. Saving you the trouble of referencing the dyno sheet, The Mule produced 502 hp with the new heads. Simply put, the engine was not using the newfound airflow potential to the maximum.