When HPP left Racekrafters' shop the last time, The Mule performed quite admirably with the out-of-the-box Edelbrock cylinder heads installed. It produced 502.8 hp and 570.7 lb-ft of torque.
As always, it did nothing wrong but make power like a freight train. But as stated in the previous installment of this series, all of us were a little dumbfounded by the stroked and bored 455's reluctance to spin higher in rpm and still move the dyno needle.
Going into the test, we all believed that the better-flowing and larger intake runner Edelbrock D-port heads would not only give us power, but also expand the engine speed range.
Craig Wise installs a more aggressive Comp Cams hydraulic roller cam for testing in The Mu
By all accounts, we are nit-picking though. Any Pontiac enthusiast who would bolt on a set of these heads in place of stock 6X castings would think that a nitrous bottle was mysteriously added in the trunk. The Mule, however, enjoyed fully worked 6X head castings from the skilled hands of Corey Porter under the tutelage of Bob Wise and his airflow expertise. So our expectations were jaded going into the original test, since our jumping off point was higher than most. After discussing the results, we decided to swap in a more aggressive cam and matching valvesprings to see what would happen.
When RaceKrafters designed the engine combination that became our Mule, the goal was for a street-friendly, drivable, and octane-tolerant powerplant. Over the past year, we have proven time and again that every goal was not only met but also exceeded.
As with any engine and especially a true Pontiac, the camshaft design is going to be paramount in the personality of the mill. The camshaft can make or break the engine combination. With this established, RaceKrafters chose the popular Comp Cams hydraulic roller (PN 51-423-9). This cam has duration at 0.050-inch lift of 224/230 degrees, 0.552/0.561 lift, and a lobe separation of 110 degrees.
 The new higher-pressure Comp Cams valvesprings used the same steel seat that the other
No one could argue with the results it provided. But Comp Cams part number 51-433-9 offered both more duration (236/242 degrees) and lift (0.572/0.594) while still working with the same basic valve events. In many ways, one could think of it as our original cam placed on a copy machine and blown up in size. There are other subtle differences in the two grinds once the cam cards are reviewed.
If you recall as the author, I believed that the valve opening and closing points would possibly need to be modified to work better with the Edelbrock cylinder heads. And when a cam profile is tweaked it is almost impossible to alter the duration while not impacting some other aspects of the grind. This was the case with our two Comp Cams bumpsticks.
The original cam (PN 51-423-9) had the intake valve open at 6 degrees BTDC and close at 38 degrees ABDC, while the exhaust valve opened at 49 degrees BBDC and closed 1 degree ATDC. In contrast, the new cam (PN 51-433-9) opened the intake valve at 12 degrees BTDC and closed it at 44 degrees ABDC. The exhaust lobe opened the valve at 55 degrees BBDC and closed it at 7 degrees ATDC.
 To start the cam swap, Corey Porter removed the Edelbrock heads and checked the open a
From a theoretical standpoint, the intake valve would open when the piston is at TDC, beginning its intake stroke (not BTDC). It would then close when the piston reached BDC, not ABDC, so no intake mixture could escape when the piston started upward. The exhaust valve would open at BDC, after the expansion or power stroke (not BBDC), and close at TDC (not ATDC), after the exhaust stroke.
An engine with this cam timing, however, would not be very efficient for a number of reasons.
Intake mixture and valvetrain parts have inertia. This means that they do not like to get moving, and once in motion are difficult to slow down, stop, or have change direction. Due to this, the valve events are started earlier in the cycle, as depicted in the Comp Cams cam specs.
 Craig removed the Comp XR276HR cam and prepared the new XR288HR for installation. It d
The intake valve gets a head start and opens before the piston begins the intake stroke, and then it will open further once the stroke begins. The intake valve does not close until the piston passes BDC because the charge can still fill the bore due to inertia. The same principal applies to the exhaust valve. It opens before the piston reaches BDC and closes after the piston reaches TDC. Thus, the valves are opening sooner and closing later, all while staying open longer and being lifted slightly higher with the new cam grind.
The lobe lift on the original cam is 0.3350/0.3400 inch, intake and exhaust, respectively. In the same order, the new cam provided 0.3470/0.3600 inch lift. The total valve lift is these values multiplied by the rocker ratio employed.
The following is a comparison of the different valvesprings we used. The heads are still virgin but the more aggressive camshaft and valvesprings are now in place.
||Comp Cams Springs
|Installed height (in.)
Since the entire premise of The Mule is research and education, HPP felt that this would be a prime opportunity to provide our readers with some camshaft theory via a brief tutorial.