Our 340 put out 392 hp on the dyno, and is destined for a '70 Swinger 340 that's still a l
Sitting in the corner decorating our workshop was a hot little street 340 destined for the space between the rails of our '70 Swinger 340. Built-up on a budget, initially to resto-stock 340 specs, the small-block delivered 281 hp on the engine dyno. With the addition of a set of Hooker 151/48 inch headers, an Edelbrock 800 carb, an RPM Air-Gap intake, and a Comp Xtreme Energy 268 Hydraulic cam, we had a stout little 392hp 340 on our hands (See December 2000-February 2001 Mopar Muscle). The initial buildup was done for just a touch over $1,500. Even with the addition of the ported heads, this engine could be built for under 2,500 bucks. What kind of horsepower was to be gained with some simple porting and a cam change? Only the Mad Scientist knows. We were quite satisfied finding 111 hp with four bolt-ons, however, resting on our bench was a fresh set of ported small-block heads. Hmmm... Wanna take a guess where we're going with this?
While the 340 "X" heads on our engine were the best-flowing stock small-block castings we have tested to date, the flow potential of any of the 340/360 heads are similar in ported form. Our ported heads were of humble origins, a common set of late '70s #051 smog castings from a 360. We treated them to a well-executed but basic street porting job. The list of mods began with a fresh Serdi machine-cut valve job, opening the intakes to the 340's 2.02-inch valve size. While machining the valvejob, the chamber walls adjacent to the valves were relieved with a deshrouding cut and the bowls were opened with a 75-degree cutter. The handwork consisted of pocket-porting the bowls, including reshaping the valve guide bosses; cutting a nice, smooth radius onto the ports' shortside turns; opening up the pushrod pinch point; gasket-matching the intake runners; and radiusing the hump in the exhaust port roof. We finished by polishing the combustion chambers. Nothing dramatic or exotic, but all nicely done, resulting in flow improvement (shown in Table 1). The heads picked up substantially in flow over their factory form, and handily exceeded the excellent flow of the stock 340 X heads.
Our plan was to swap the hydraulic cam and stock rockers for a solid setup and replace the
The chamber mods had increased the cc's of our 360 heads by a fairly substantial 7 cc-from the stock volume of 70 cc to 77 cc. This, in itself, would have cost upwards of a point of compression on our 340. Substituting a set of Milodon's Street Valves netted a reduction of chamber volume of over 3 cc's, owing to the flat face of the aftermarket valve versus the deep-dished stock tulip shape. From there, milling the heads .030 inches brought the volume down to the stock 340 spec of 68 cc. To compensate for the milling in terms of intake manifold fit, the intake face of the heads was milled .030 inches as well. To wrap up the cylinder-head package, we cut the valve guides for P/C-style Teflon valve seals, reducing the guide height at the same time by .100 inches to allow retainer clearance for high valve lift.
Lifting The Lift
With the fatter flow curve across the board and greater peak flow, our ported heads would be capable of delivering a respectable power increase on their own. In terms of power production, the higher port flow would respond favorably to more camshaft, and naturally encourage more rpm. The Comp XE268H hydraulic cam currently in the 340 was making peak power at near 6,000 rpm with the stock "X" heads. Adding ported heads and our plan for more cam would extend this well out into the mid- to upper 6s.
Pulling apart the top and front end, we extracted the hydraulic cam-a Comp XE268. Since th
Why would we tear off a perfectly good set of highly desirable 340 "X" heads? Because we h
The first step should always be getting the machinework done. A cutting machine like the S