Editor's note: This month's installment for Project Unfair (from Prodigy Customs and II Much Fabrication) deals with a familiar subject, but with an Unfair twist. We promised lots of tech and insight into the decisions and trade-offs of building this car, and headers are at once familiar, but also full of design choices. Read on to see how John Parsons and Frank Serafine decided on this particular header design.
Project Unfair, our '69 uber Camaro needs a set of headers that can support 1,400 hp. That's a lot of horsepower, and as you might suspect, there isn't any ready-made solution, which means author Parsons had to make a custom set to meet Unfair's unusual requirements.
There is a bewildering array of design choices when making custom headers. We consulted the experts at Cone Engineering, Specialty Products Design, Burns Stainless, along with Jim Bell at Kenne-Bell (who manufactures the 3.6L supercharger that will assist in making all that power) and engine builder Pat Musi at Pat Musi Performance. We had originally planned on building tri-Y headers (see sidebar), but the consensus was that a traditional four-into-one header design would work better with the big huffer.
With that major decision done, a whole new bunch of design decisions came into play: What primary tube diameter? Should the diameter be the same, or should it step up in size along the tube? How long should the primary tubes be? How important is getting all the tubes the same length? What size should the collector be? Is a merge collector important? What size should the exhaust beyond the collector be? And finally, what material should be used?
We quickly settled the material question by going with 304 stainless steel. It's an excellent alloy for headers as it resists corrosion at typical header temperatures, is easy to weld, and can be cut and fitted with the right tools, though the right saw is needed.
With the material chosen, the next question is the header's primary tube diameter. There is a lot of information around about the importance of this parameter, and some of it is contradictory. Bell had sized some headers with his big supercharger and found that 2 1/8-inch primaries made the most power, but only about 20 hp more than 2-inch designs.
However, given the dual mode of the car (1,200-1,400 hp for drag racing and top-speed runs, 800 hp for road racing and auto-crossing), you also have to consider performance when operating at the lower horsepower. While too small of a primary tube restricts airflow, too large of a primary tube causes a drop in exhaust gas velocity, and likewise a decrease in performance. By compromising on the tube diameter, you are trying to get the best performance possible in each configuration using the same header, even though you'll forfeit maximum performance in one mode.
Given that, we turned to Rich Craig at Cone Engineering and discovered something else: 2-inch mandrel-bent stainless tube was widely available and reasonably priced, while 2 1/8-inch tube is less common and therefore much more expensive. Not only did the decision to go with 2-inch versus 2 1/8-inch tube work for performance, it also saved some cash-never a bad thing, even in a high-end build.
Now comes the daunting task of trying to get 1,400 hp under the car and through a full exhaust. Basic airflow calculations (see sidebar) quickly show that a 4-inch dual exhaust is optimal for 1,400 hp, though a 3-inch exhaust is good enough for 800 hp. Not only is there a dual-mode constraint with the primary tube diameter in the headers, but now there's another one: how does one fit 4-inch tubing under a car that doesn't even have room for 3.5 inches? Compromise and ingenuity is how.