GM Performance's ZZ4 350 small-block is one of the most popular crate engines out there. The ZZ4 has endeared itself to hot-rodders and mechanics alike with its advertised 355 horsepower and 400 pound-feet of torque, premium-gas compatibility and factory-solid reliability. Although most installers will install this engine as-delivered with GM's recommended intake and carburetor, exhaust header selection is a bit more difficult since headers have to clear the chassis.
GM based the ZZ4 crate engine on its seminal L98 small-block, which powered almost a full generation of Corvettes through the 1980s and 90s. General consensus has it that the L98 was GM's most sophisticated pre-LT-Series small-block V-8, making it a perfect fit for most street rods.
The ZZ4's L98 heritage means that it uses angled-plug aluminum cylinder heads. You don't necessarily need to use angle-plug-specific headers, but if you opt not to then you'll need to install sheet metal heat shields between the spark plug boots and the header tubes. Even then there's no guarantee that you won't burn the boots, so L98-specific headers are a good idea.
For chassis that have the clearance, 1 5/8-inch primary diameter, full-length race headers will extract the best horsepower from your engine. The 1 3/4-inch primary header is another popular option, but High Performance Chevy Magazine found these a bit too large for its ZZ4. HPC Magazine's ZZ4 test showed a 6 horsepower and 20 pound-feet loss from the stock engine's advertised horsepower and torque ratings. However, these headers worked well when the magazine installed a Comp Cams 268H hydraulic flat-tappet cam and a Performer RPM intake manifold. The net result: 380 horsepower at 5,400 rpm and 402 pound-feet of torque at 4,100 rpm.
Material thickness is one very important but often overlooked factor in header selection. Manufacturers rate their headers according to the primary tubes' outside diameter, but not all manufacturers use the same gauge (thickness) of tubing; gauge can vary between 14 (0.166 inch thick) and 18 (0.098 inch thick). On a 1 5/8-inch outside diameter header, the thicker tubes give up about 0.427 inch of cross-sectional surface area to the thinner ones. Assuming about 80 cubic feet of air per minute airflow per inch, the thicker headers lose about 34 cfm per tube. Multiply that by eight (the number of header tubes on the engine), and the thicker tubes can cost you a fairly whopping 273 cfm off of the thinner tubes' 2,871-cfm airflow.
This oft-overlooked discrepancy may help to explain why some ZZ4s work better with 1-3/4 inch tubes and some do better with 1-5/8 tubes. A 1-5/8 inch diameter/20 gauge tube has an inner diameter of 1.429-inch, and a thick-wall 1-3/4 inch/14 gauge tube has an inner diameter of 1.418. This makes the narrower-but-thinner 1-5/8 header tube about 0.01-inch larger internally than its wider-but-thicker 1-3/4 inch equivalent.
