Running The Numbers
Getting more power with an engine combination gets progressively more difficult as the outright power escalates. While we felt there was still a bit of room to the upside, the 340 already produced well over 1 hp per cubic inch, at 1.13. Pushing the envelope generally comes with a higher price tag. Although a set of prepped aftermarket race heads would be the quick ticket to radical power gains, our rebuilt smog-heads came at about the same tariff as a rebuild on the original 340 heads. Since the porting was done at home, the added cost was only in time. Ditto for the manifold mods.

The solid flat tappet cam and lifter package only marginally exceeded the cost of the hydraulic grind we had previously run. Our new valvetrain was the major additional expense associated with the change in configuration. Revisiting the 340 in the search for more power did not produce a radically more expensive combination. With the added constraints of keeping the compression ratio at pump-gas levels, maintaining reasonable drivability, and not outstripping the rpm capabilities of the stock-rebuild bottom end (forged SpeedPro pistons though it is), just how much more power would we wring from this combination?

Our baseline configuration included the ported factory 051 casting heads, the small 1 5/8-inch Hooker headers, Comp's solid cam installed at 104-degrees intake centerline, with a 750 Demon carb topping the Air-Gap intake. After cam break-in, dialing in the timing, and checking jetting, we were ready to run for the numbers. The engine responded favorably to our modification plan, belting out 427 lb-ft of torque at 4,600 rpm, and 458.9 hp at 6,300. Right out of the gate, our newly modified 340 was making 1.32 hp per cube, while delivering a steady 850 rpm idle with around 9 inches Hg of vacuum. Our mods increased output by 17 percent over our already hot 340 combo, with power up noticeably throughout the test range. The engine came on stronger and kept pulling well past where the old combo ran out of steam.

We were running a set of Autolite #63 spark plugs, and had some of Denso's Iridium rare-metal electrode sparkplugs. Designed to fire-through under extremely demanding ignition requirements such as high-boost blown or fuel applications, they are extravagant in a normally aspirated street engine such as our 340, and certainly not cheap in comparison to a conventional plug. We were curious, however, as to whether they would benefit performance. We found the Iridium plugs added about one horsepower to the peak number, but more interestingly, they added a 5-8 lb-ft (over three runs) advantage in the first few hundred rpm of our test range. We left them in for the rest of our test session.

In numerous tests with the Performer RPM Air-Gap intakes, we have often seen a peak power gain with the addition of a 1-inch open spacer under the carb, particularly with higher horsepower engines. We figured adding one to our 340 would nab a few extra ponies for little effort.

The carb spacer paid off, raising peak output to 433.8 lb-ft at 5,000, and 464.9 hp at 6,400. Power was up noticeably in the upper end of the power band, at the expense of some torque lower in the range. Most striking was the way the spacer helped the engine carry power past peak. At 6,600 rpm, the spacer was worth 25 hp, and carried usably to 6,800, while without it power dropped more sharply.

Up to this point we were still running the 1 5/8-inch street headers, and at 465 hp they seemed kind of small. We bolted on our 1 7/8-inch Hooker Super Competition headers to see if the small tubes were, indeed, costing some output. The result was a trade-off, with power added to the top with a similar amount deducted off the lower end of the range. Peak output was up impressively to 477 hp at 6,500 rpm, with torque reading 431 lb-ft at 5,100. Looking at the averages over the entire curve from 3,500-6,800, it was an exact trade, with the data reading within 1/10 of a horsepower between the two tube sizes over the full rev range.