Whether you are building street rods, show cars, or even NASCAR or Indy cars, you'll want the exhaust package to be the best it can be. All of the components, from the exhaust tubes to the merge collector to the exhaust piping, needs to conform to the overall design quality of the total engine setup. The exhaust system has a direct influence on the dynamic performance of your engine. To build a quality exhaust for your car your must start at the very beginning. That's right at the point where the exhaust ports come out of your engine block. It's here that your header flanges, the beginning of your exhaust system, come into contact with your engine.
The header flange often turns out to be the “red-headed step-child” of the exhaust package installed on custom cars and race cars. It isn't given much thought because it is basically covered up by the exhaust manifold tubes as it is bolted to the block. But to overlook the header flange is to weaken, and cheapen, the entire setup. A poorly adapted header flange can cause headaches in the future due to the possibility of it warping and losing a good seal with the block or else causing undue stress on the other components in the exhaust system. What should you look for in a quality header flange?
First of all, be aware of the cheap products flooding the marketplace today. Foreign producers use every possible means to produce low cost products to improve their bottom line. There are header flanges on the market that are just stamped steel. Others are actually cast iron. The possibility of warping is significant with these. That is, if they aren't already warped right out of the box! And the stamping or casting process often leaves tolerances and fits that are less than perfect, causing you to have to modify the header flange just to get it to fit right on your engine. It's not uncommon to have to drill out or elongate bolt holes to get all the bolts to line up. Sometimes even the exhaust port cutouts have to be filed to allow complete clearance. Definitely not worth the few bucks saved buying these types of flanges.
To overcome the problems of these cheaper header flanges it is recommended you buy from a reliable U.S. manufacturer who uses state-of-the-art CNC laser-cutting techniques in the production of their header flanges. Some foreign makers, and even some in the U.S., use plasma cutting to fabricate their flanges. While better that stamping or casting, plasma cutting is still a less quality process. It often leaves rough edges and undersize bolts holes, especially in stainless steel. CNC laser-cutting will yield the best cuts, on size and within all necessary tolerances.
The other main consideration with your header flanges is the material they are made of. Cheaper flanges often come in 1/4” and 5/16” thickness material that is prone to warping when exposed to the extreme heat it will experience at the exhaust ports of your engine. Don't settle for less than 3/8” stock for your header flanges. If cost is a stumbling block, buy the flanges made from 1018 cold rolled steel. They will service your setup just fine if you are using mild steel for your exhaust manifold tubes and piping. The better choice, of course, is the 304 nonmagnetic stainless steel option. Although more expensive, it creates an exhaust setup second-to-none. In either case, invest in the 3/8” CNC laser-cut header flanges. You won't be sorry.
As a general rule, American manufacturers who supply quality made header flanges also provide the entire exhaust system setup to complete your total package with compatible parts. Usually, you'll find that they have header flanges, manifolds, merge collectors, turbo flanges... everything to make your setup first class. So, no matter what kind of car you are building, you won't be sorry and you can't go wrong by using quality parts from a reliable source!