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.