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.
Using the collectors at the optimal outlet of 4-inch, the plan is to make a transition to 3.5-inch oval tubing (which is less than 2.5 inches tall, but flows just as much as 3.5-inch round tube). From here we'll have a cutout for a 3.5-inch oval competition exhaust with a cutout.
The other side of the cutout will continue the 3.5-inch oval into an oval X-shaped crossover and then transition back to 3-inch round with a pair of 3-inch mufflers and tailpipes. For street driving and road racing that will work great, and when there is a need for more horsepower, the cutouts will be opened and the exhaust will flow enough for max horsepower.
It is accepted that merge collectors are worth 5-to-25 hp and many tests in recent years have borne that out. However, the standard slip-fit merge collector in 304 stainless is an expensive piece, especially in a 2x4-inch size. Cone Engineering offers a great "do-it-yourself" merge collector kit that's easy on the checkbook, and we quickly added it to this plan.
Cone's testing has shown that there is no appreciable difference in the performance of its merge collector design and the traditional slip-fit merge. An additional advantage is that this collector can be welded in place and therefore will not leak like a slip-fit collector. Best of all, the Cone merge collectors sell for about one-quarter of the price, and the fabrication is easy to do.
Finally, the last question to settle is the primary tube length, and whether or not to worry about getting them to be the same. In general, the longer the tube, the more possible torque at lower rpm, and so we went with a fairly shorter tube length of 30 inches because, "We will have too much torque anyway, so let's kill some bottom-end."
We also decided to try and keep all the tubes within 2 inches of 30-inches in order to have all the cylinders "see" the same exhaust to help with tuning.
With all that design worked out, we called Cone Engineering and ordered the deluxe header kit, nickel-plated head flanges, 2x4-inch collectors, a merge collector kit, and all of the fabrication tools to help build these monster headers.
Let's have a look at how we turned that box of stainless steel tube into a pair of working headers for our 1,400-hp Project Unfair Camaro.
Why We Did It This Way
I originally wanted to build a set of tri-Y headers for Unfair, but without any real design rationale; it was mostly to see how to make them. I wanted to learn about how the tube diameters and lengths of each leg are derived. But when I talked to some exhaust experts around the country, I heard the same basic response: tri-Y headers are carefully tuned for engine size, exhaust valve size, and rpm, and nearly all the available setups are tuned for naturally aspirated engines.
Adding a supercharger to the mix throws all the design information out the window, and I'd be better off with a large diameter traditional design that wasn't as risky.
As far as figuring out how big to make the exhaust, the calculation is fairly simple, once you know a couple of basic engine constants:
An engine requires about 2.2 CFM per horsepower, and exhaust gas flows about 115 CFM per square inch.
Assuming 1,400 hp / 2 (since there are two exhaust pipes) means we have 700 hp per exhaust pipe. Multiply that by 2.2 cfm / horsepower and we see we need 1,440 cfm. Divide that by 115 cfm / square inch, and we need 12.5 square inches of pipe area. The area of a 4-inch round pipe is equal to (pi r (squared) = 3.14 x 2 (squared) = 12.6 square inches. So a 4-inch exhaust is just barely big enough to support 1,400 hp.
As an aside, 1,400 hp means each primary tube in the header needs to support 1400 / 8 hp = 175 hp. Doing the same calculation 175 x 2.2 / 115 = 3.3 square inches. The area of a 2-inch tube is 3.1 square inches, so the headers might be the limiting factor in ultimate horsepower in Unfair.
Working the calculation backwards, we see that the 2-inch headers will theoretically limit the engine to 1,300 hp. Stay tuned. We'll see how that engine turns out on the dyno pretty soon!-John Parsons.