Our heads boasted fairly extensive pocket porting, opening the bowls and reducing the mass
At this point we moved to a solid lifter profile. We had recently obtained a special cam and were eager to try it out. Previously, we introduced a series of fast-rate Competition Cams solid lifter lobe profiles. These are adaptations of the technology used in the design of Comp's successful Xtreme Energy lobes, but designed to take advantage of the even quicker action possible with Mopar's .904-inch tappet diameter. These quick lobes are Comp's MM-Series solids, which are not even in the current catalog. We had run a large version of the MM-Series solid lifter profile in another engine test, and were impressed with the performance.
The hallmark of the MM-Series is the fast valve action, which obtains high lift for a given duration. We were intrigued with the idea of having a cam ground using the smallest versions of the MM lobes. The expectation was that the relatively short overall duration (seat) would allow for reasonably good street manners and idle, while the super-aggressive lobes would provide the high-lift and big upper-lift duration numbers for serious punch. These cams are all custom grinds, which requires a call to the Comp tech line for ordering assistance.
The shortside turn, the area where the port turns in from the floor to the valve seat, is
We asked for lobes #6581/6582 int/ext ground on a 110-degree lobe separation with 4 degrees of advance. These are the smallest two lobes in the MM series. With a short 265/269 advertised seat duration (checked at .020-inches), a moderate (for a solid) 239/243 int/exh duration at .050 inches, a healthy 157/161 duration at .200 inches, and a fairly serious .573/.586 gross lift with the 1.6 rockers we were planning on running, the fast lobes make the cam seem small at the bottom of the lift curve, but come on to catch a fairly big cam by max lift. In fact, conventional solids typically need to start opening the valves 15 degrees earlier to reach the same lift, with advertised seat durations (also rated at .020 inches) in the mid 290s. Would we find the power of a race cam and the manners of a hot street cam? The specs would certainly indicate an excellent trade-off between the two.
To make the change from a hydraulic to a solid, an upgrade in valvetrain components was required. Our 340 still carried its stock stamped-steel nonadjustable valvetrain and solid pushrods. The stock setup belies its simplicity and low cost by offering good performance and rpm capabilities with moderate hydraulic cams. With a solid, an adjustable valvetrain is, of course, required, and we opted for a set of billet aluminum beauties from Probe Industries. In keeping with our quick valve-action rationale, the rockers were ordered in the quicker 1.6:1 ratio rather than the stock 1.5:1. For pushrods, we ordered a set of Comp's Magnum 5/16-inch ball-cup pushrods (PN 7692) for the solid-lifter application.
Intake And Exhaust Tract Tweaking
As part of our bolt-on plan last time around, Edelbrock's Performer RPM Air-Gap manifold was mated to the 340's heads. In numerous tests, the Air-Gap has proven to be an outstanding intake for the small-block Mopar, producing terrific torque down low and belting out big power up top. Although an Edelbrock carb was used in our last configuration, it had been borrowed from the 340 several months ago, and has seen regular street duty since. This time around we bolted on one of Westech's dyno carbs-a 750 cfm mechanical secondary Speed Demon.
Finally, as part of our previous program, we had installed a set of Hooker's 1 5/8-inch headers-a common size for general-purpose street applications. The 340 already made 392 hp, and we were certainly expecting more with our beefed-up combo. With this in mind, it was likely that the 340 would benefit in output with a larger header. The same 1 5/8-inch street headers were bolted to the reconfigured engine, but we were packing a pair of big-tube Hooker 1 7/8-inch headers for the dyno session to come.
The intake runner was opened to gasket size, and the pushrod pinch point was opened up to
The exhaust runner was cleaned up lightly, focusing on reducing the hump found at mid-roof
We flowed our heads before and after on Westech's SuperFlow 600 and found the gains worthw