Richard Holdener
November 1, 2009
Wanting to optimize valve train geometry, we installed a set of 1.73 aluminum roller rockers from Comp Cams. The stock stamped steel rockers have no business on a performance motor.

Other changes to the stock motor included running with long-tube headers, a Holley 500-cfm two-barrel carb and an electronic ignition. Basically we had a high(er) compression two-barrel 400M with headers and a larger two-barrel Holley carb.

Equipped with the stock two-barrel heads, cam, and two-barrel intake, the 400M managed to produce peak numbers of 265 hp at 4,100 rpm and 412 lb-ft of torque at 2,900 rpm. The 400M was obviously tuned for low-speed torque, as torque production from the 400M exceeded 400 lb-ft from 2,000 rpm (and likely lower) to 3,200 rpm. The stock cam, heads and intake manifold literally choked off power production above 4,500 rpm.

With our baseline out of the way, we installed the performance parts. On went the ported Pro Comp aluminum heads from Dr. J's, the street roller cam from Crower Cams, and the Performer RPM Air Gap/spacer-plate intake combination from MPG. The Pro Comp heads not only improved the airflow over the stock two-barrel heads, but also increased the static compression ratio by reducing the combustion chamber volume from 79 cc on the stock two-barrel heads to 75 cc.

The taller deck height of the 400M required intake spacers to allow use of a 351C intake manifold. MGP supplied the necessary spacer plates for use with the Pro Comp head ports (different than either factory two-barrel or four-barrel).

The roller cam from Crower was augmented with solid-roller lifters and a set of 1.73 roller rockers from Comp Cams. Why go to all the trouble of installing a good cam only to reduce its efficiency with the stock stamped steel rockers? This new combination also required a pushrod change, as the adjustable valve train required a 0.050-inch longer (hardened) pushrod than the stock 400M units. The new combination was fed by a 750hp Street carburetor equipped with Percy Adjust-a-Jets. This slick feature basically allowed external jet changes to dial in the air/fuel mixture.

After minor jetting and timing sweeps, we were rewarded with a jump in peak numbers from 265 hp and 412 lb-ft to 489 hp and 502 ft-lb of torque. As expected, the new combination produced these numbers higher in the rev range than the stock components. Where the stock combination produced peak power at 4,100 rpm and peak torque at 2,900 rpm, the wilder combination produced peak power at 5,900 rpm and peak torque at 4,100 rpm.

Though designed for use with the new Edelbrock Cleveland heads, we chose to run the Edelbrock Performer RPM Air Gap intake on the 400M. A little port matching of the spacer plates was necessary to facilitate the test, but we've always had good luck with the Air Gap intakes on other applications.

The next order of business was an intake swap. Though we suspected a dual-plane intake was the best choice for a street motor, we wanted to see how well the CHI single-plane performed. Off came the RPM Air gap and aluminum spacers plates, and on went the CHI. The CHI intake required use of the lower valley cover plate employed with the spacer plates, but the intake swap was very straightforward. The great thing about working on Cleveland motors is that intake swaps don't require removal of the distributor. Pop off the old intake, pop on the new one, and you're ready to run.

The CHI intake was equipped with a 4,500 carb flange, so we installed a two-circuit Holley 1150 Dominator carb. A tad overkill on this application, the huge carb looked pretty menacing on the aluminum-headed 400M.

Equipped with the CHI intake and Dominator carb, the peak power numbers jumped to 529 hp and 506 lb-ft of torque. Compared to the dual-plane intake, the single-plane CHI improved both peak power and torque, though the single-plane did lose out to the dual-plane below 4,300 rpm by as much as 35 lb-ft.

In exchange for the losses down low, the single-plane improved the power output by as much as 45 hp. For this combination, the choice would come down to the intended application. The dual-plane would work best for a daily driver, but the single-plane would offer much better track performance.