On the ZZ572/620's baseline...
On the ZZ572/620's baseline dyno run, the engine delivered 649.1 lb-ft of torque at 4,800 rpm and 647.9 hp at 5,700 rpm. For our evaluation, we focused on improving the torque numbers and the shape of the torque powerband.
Test I
As delivered from Burt Chevrolet, the GM tall-deck motor included a Demon 850-cfm carburetor, an MSD distributor, and a GM open-plenum aluminum intake manifold. To that we added headers with 2-inch-diameter primary pipes, and a Flowmaster 3-inch exhaust system with H-pipe and the Super 40 (PN 953046) mufflers. After bringing the motor (in delivered trim) up to operating temperature, the torque peaked at 649.1 lb-ft at 4,800 rpm and the horsepower was 647.9 at 5,700 rpm.
Test II
We knew by the baseline numbers that we could improve upon the torque reading. First, we axed the open-plenum intake (largely designed for peak power numbers) in favor of a dual-plane Edelbrock Air Gap Performer RPM. We especially like the Edelbrock piece because it works well in the midrange rpm scale. But because the ZZ572 is a tall-deck design, we needed Moroso intake manifold spacers to facilitate the installation. We fired the motor to stabilize temperatures and ran the engine up to speed to record the data. Yow! The Edelbrock manifold raised and widened the midrange torque band considerably. Peak torque was 641.7 at 4,400 rpm, but more importantly, the entire torque curve with this manifold improved by as much as 60 lb-ft at 2,000 rpm over the existing GM open-plenum intake.
To broaden the torque curve,...
To broaden the torque curve, we removed the existing open-plenum aluminum GM intake and added an Edelbrock Performer RPM Air Gap. Because this engine is a tall-deck design, the cylinder heads are spaced farther apart, which required Moroso intake-manifold spacers.
Test III
For our next attack, we wanted the benefits of a shorter-duration camshaft. From the Burt Chevrolet parts bins we selected the 450/502 H.O. 450hp hydraulic roller (PN 24502611) that features 211/230 degrees of duration at 0.050-inch lift and 0.510/0.540 lift. To make certain that we had our camshaft data correct, McClelland degreed-in the bumpstick and found that it installed at a 110.25 intake lobe centerline. Then we installed the remaining components and fired up the King Rat. McClelland set total timing at 33 degrees. With the engine making all the right sounds, torque improved to 663.4 lb-ft at 3,700 rpm. What's more, the torque at 2,000 rpm climbed to 595.2 lb-ft compared to our baseline plot of 468.7 lb-ft at 2,000 rpm. Even better, the engine now had 19 inches of manifold vacuum at 800 rpm up from 8 inches at 1,000 rpm with the factory engine configuration.
Conclusion
By making a few simple changes to an already powerful package, we now had an engine that would idle easily in traffic and produce tremendous midrange power--just what street cars thrive on. For our testing, we ran 91-octane unleaded. Because a street engine is operated below peak power numbers, raising the torque curve across the rpm range improves performance. These modifications are easy to do and the parts are readily available. The same principles can be applied to smaller engines too. Keep the camshaft duration to a moderate level and choose a good street carburetor and pair it with a divided-plenum intake manifold. The results will keep you in the curve.