Big-Block Chevy Engine Build - Legacy Power

Speaking of cylinder heads, it was the top side of the Mark II “Mystery Motor” that really made the looky-loos of the ’60s scratch their noggins when Junior Johnson and Johnny Rutherford popped their hoods for the first time. Gone were the trademark “W” shaped valve covers in favor of some massive aluminum boxes to cover up the valvetrain. Once the valve covers were popped off, passersby gawked at the rocker studs that were pointed every which way. By canting the valve angles so that they opened away from the cylinder walls and toward the center of the combustion chamber, Chevy was able to stuff huge valves in there for big flow numbers and minimal shrouding. Of course, all those watchers were able to spy was a bunch of studs sticking out in different directions like a bunch of porcupine quills. Hence its nickname, “porcupine.”

But back to our story. Mike chose a set of small-runner Brodix castings for this jewel. To get that air mass into the engine requires a certain amount of velocity, and it is a combination of cross-sectional area and airflow that determines the velocity. The runners were about 290 cc, and figuring an average port length of 6 inches would yield a mean cross-sectional area (MCSA) of 2.95 square inches. These little babies were flowing a killer 384 cfm at .700-inch lift, so they were hauling the mail with an average port velocity of about 315 feet per second. That is considered a really good target air speed for the rpm range in question (2,500 to 6,500 rpm). To make all that airflow mojo happen, Mike enlisted the help of the legendary Joe Patelle at HVH Cylinder Heads in Knoxville, Tennessee. HVH has been porting heads since the old porcupine had soft quills, and Mike says: “HVH has been doing all of our heads for probably the last four or five years.” Once he received the heads, he did the valve job in-house to finish them off. Mike mentioned that he’s played with different valve job angles in the past but for a relatively low-lift and low-rpm application like this he felt a standard 45-degree valve job would be the best bet.

The other part of the top end equation is the intake manifold. Since the heads had oval-shaped raised runners, there wasn’t a whole lot out there Mike had to choose from, so like any good machinist, he made something else fit. Starting with an Edelbrock rectangle-port single-plane intake, he added about a quarter inch of epoxy to the floors, then matched the shape of the cylinder head by grinding out the intake to an oval shape not much bigger than a golf ball. That didn’t mean they were a restriction though. “The manifold needs to flow about 5 percent more than the cylinder head,” Mike says. With the manifold flowing right, they like to stick the head back on the flow bench to see if there are any changes. “We actually like to flow the cylinder head with the manifold on it, and the numbers should come out the same with or without the manifold.” Typically intake manifolds like this end up with some sort of carb spacer since they are usually running a carb. So what happens when running something like FAST XFI fuel injection? Well, Mike tried a bunch of different carb spacers again to see what worked. He’s got about six versions ranging up to 3 inches thick, but settled on a simple 1-inch-thick open spacer to give a little more plenum volume and to straighten out the air as it passes through the FAST throttle body.

Speaking of EFI, this was Mike’s first foray into fuel injection and it was not without drama. He started out wiring up the system, as the XFI comes with a fully terminated harness that is labeled to keep it idiot-proof. Once he got it up and running, they had trouble on the dyno; going to wide open throttle, the TPS would show 100 percent then suddenly drop back to 75 percent on the computer. They chased wires and connections but finally went to the local auto parts store and got a replacement for every single sensor. Once back on the pump, it turned out it was simply a bad throttle position sensor that was giving out a bad signal. Just one of those things. That being said, Mike did praise the XFI: “The fuel injection is really neat to work with. It’ll do exactly what it says it does. If you tell it you want 13:1 air/fuel ratio, it’ll give you 13:1 air/fuel ratio.”

To run in the 2,500 to 6,500 rpm range that the engine was intended for and to do so with EFI and not a carburetor, Mike had to completely change the way he picked the cam for this Rat. “Our circle track engines use anything between a 106 and 108 lobe separation, and we run the engines in a certain rpm range, which is a lot higher than what this engine runs.” For a broad torque range and with fuel injection, Mike likes to really shrink up the duration and open the lobe separation to 110-112 degrees. Mike says he’s been running Jesel beltdrives and rocker arms for a number of years as the quality is top notch. The beltdrive has the advantage of being able to advance or retard the cam easily by just quickly removing the water pump to gain access to the adjustment bolts. Camshaft endplay on the solid roller is also set easily by adding or subtracting shims between the timing cover and an adapter mounted on the nose of the cam. Typical endplay is targeted at .002 to .004 inch. Tight endplay not only keeps the lifters lined up with the lobes, but since the timing gear on the distributor is helical where it meshes with the cam, it keeps the dizzy phased correctly. With a cam design finally in mind, Mike’s contact at COMP ground him a shiny new solid-roller cam to make the lumpy-lump noise at the right time.

With the cam beboppin’ along at the right pace and the MSD box burnin’ up a bunch of VP100 gas, the exhaust note through the Beyea headers and Flowmaster mufflers was just perfect. On the DTS dyno at the University of Northwestern Ohio, Mike’s own “Mystery Motor” lifted the power needle to the tune of 726 angry horses with 621 pounding feet of torque. No doubt that this Mark IV was one prickly pump gas performer that packed a preponderance of potent power.