VR: Angles between 5 and 15 degrees are optimal and the 15 degree gives us a shorter collector.
What are some of the main differences between mild steel and stainless steel when dealing with temperatures?
JB: Stainless steel has a better high-temperature strength than mild steel. In a normal header situation you can go to lighter gauge stainless then mild steel so you can save weight. In a turbo application, where you have much higher temperatures than a naturally aspirated application, same deal applies.
VR: The other good deal with stainless is you have a lower coefficient of thermal conductivity so it tends to insulate, whereas mild steel tends to let the heat travel through the metal quickly. Also, a lot of people will use a ceramic coating to reduce the heat transfer but stainless does this inherently.
JB: Then there may be long term durability problems with some coatings. As a matter of fact, one of the main drawbacks with carbon steel is that once it's overheated, it will start to flake and fall into the turbo. The same goes with the coating. This will also flake. I would say coating isn't useless but if you want to lower your radiant temperature, then stainless is the way to go. It can reach the same surface temperature as mild steel, but stainless will stay hotter on the inside and transfer the heat.
What about rumors that stainless steel cracks easily?
JB: The material isn't the cause of the cracking. In most cases in a turbo application, it's the manifold design that causes cracks. Most people don't allow for the thermal expansion with stainless, which is 50 percent greater than mild steel.
VR: What you have to keep in mind is most people will have a manifold that has the primaries fairly straight and then you add support between the head flange and turbo. This doesn't allow the stainless to expand and it causes stress on the tube. Supporting the turbo is a good thing, but you must compensate for expansion of tube by adding a bend area in each primary equally so you allow flexing and stretching to occur.
JB: With a lot of the manifolds you see on the imports, the turbo is mounted very close to the head. When the bends come in at different directions this will cause the manifold to crack at the collector because everything is expanding in different directions.
What about camshaft specs in conjunction with header design?
JB: Oh yeah, this is very important. We have even gone as far as redesigning a completely different header for customers that have changed out their camshaft(s). We've even had customers who say they are going to change over from two different types of camshafts constantly so they want their header to work with both. What we end up doing is giving them some adjustability to the header.
On a turbo application, how important is wastegate plumbing?
VR: Most people think there is just a constant pressure inside the manifold so you can just bleed off the excess exhaust pressure in the manifold from any point. In reality, inside the manifold are waves of pressure pulsing inside. There are delays between the pressure getting in and how far it has to travel in the manifold. If you interrupt these pulses by bleeding the exhaust out from a wastegate, you affect the performance of the manifold and the turbo system.