Loaded onto the Westech dyno, we bolted on the 1 5/8-inch headers. The changes to our prev
Would a single-plane intake prove better? We bolted on a modified Torker II single-plane to find out. This particular intake had been a proven power maker for us in the past, especially in the upper rpm range on similarly powerful small-blocks. Not this time. Output was down over 20 lb-ft at the start of the pull, and widened to as much as 30 lb-ft. The gap narrowed approaching peak horsepower, but even peak horsepower was down by a solid 15. It was totally unexpected, and we still can't explain it, but it was real. The combination of cam timing, heads, and headers just became unhappy with this intake. Quite peculiar. An interesting note was that the idle vacuum was down a significant 1-1.5 inches Hg with the single-plane, and idle quality deteriorated noticeably. We reinstalled the Air-Gap to find vacuum back up, the idle smoothed out, and power right back in line at 477 hp.
Were we happy? Frankly, we considered the milder version of this engine a fine street mill with 392 hp on tap. The Comp XE268 hydraulic cam used in that combo is a nice street grind, and driving one daily for 60,000 miles in my 318 '69 Dart has been convincing evidence of that. This combo, with the solid cam, produced 1.38 hp per cube from our 340 on Unocal 91 octane pump gas-a remarkable output through a set of mildly ported Cordoba smog heads. Getting that done while maintaining reasonably good idle quality, 9 inches of vacuum, with only 346 cubic inches, tells us these new MM solid grinds are pretty extraordinary.
Hmmm.... It will bolt right in place of that 318 in the '69 Dart while the '70 Swinger gets built.... Wanna guess where we're going with this?
We used one of Westech's dyno carbs-a reliable 750 mechanical secondary Speed Demon.
We were going to run a set of ported heads, and as part of the porting program, the intake port openings were matched to standard 340/360 gasket size. Production intake manifolds are generally finished somewhat undersized at the port exits. A mismatch wherein the intake is smaller than the heads has little negative effect, while a mismatch where the intake is larger than the head will leave a stepped obstruction in the direction of flow at the joint. Since head port openings are typically cast somewhat undersized, the manifold is likewise made undersized to lessen the risk of an undesirable mismatch. A port match will increase the area available to flow and offer the smoothest transition from the intake into the heads. Here's how we did ours, as easy as 1...2...3.
Although the porting job on our smog heads was basic, it really was the culmination of a great deal of experience with these ports. The mods were described in the text. The experience and form are best shown in a few pictures here. We limited polishing efforts to where it does the most good, namely at the shortside turn, and left the rest of the port in the raw-cut state. Actually, much of the runner was left untouched by the cutter.
After running in the cam, the valve lash was reset, the timing adjusted to 36-degrees tota
The heads and cam added substantially to output, with the 340 now delivering 458 hp. The s
A set of Denso Iridium plugs were tried, and we found an increase in low rpm torque. Peak
A 1-inch open space robbed some low-end torque, but moved peak power to 465.
A set of big-tube 1 7/8-inch Hooker Super Competition headers also took away from the bott
We tried a modified Torker II single-plane intake believing it would let the engine scream