The best header discussion ever....

[garickman]

Any thoughts?

[SLO_Z28]

Originally Posted by garickman

I am trying to educate myself on vehicle exhaust systems, mainly in the header department. I have searched the internet and various threads but have not found all the answers I am looking for. Most threads are specific to one question and I have several questions. I was thinking a lot of people have some of the same questions I do so I thought I could start a discussion which might address multiple concerns.

How important is the primary tube size?
Very, it will affect torque and horsepower
How do you determine the primary tube size?
I saw a top NHRA engine builder use a flowjet to determine flow at every amount of valve lift, and enter the data into a engineering dynamics software suite and compare it to 3d CAD flow simulation he had modeled with a digital plotter and a lot of time. The average guy can determine the approximate size from the overall characteristics of the engine
How important is it that primary tubes are all the exact same length?
Despite popular belief (and according to GM) not at all
Are stainless steel headers really that much better than mild steel?
No
If stainless steel headers really are better, would ceramic coated mild steel headers be just as good?
Yes
Can you ceramic coat stainless steel headers?
Yes
How come it seems some headers use gaskets and some don't?
They should all use gaskets
Is it a good idea to safety wire header bolts?
That is an application specific question, sometimes yes sometimes no
Do headers effect you torque as well as your horsepower?

How important is the size and length of the merge collectors?
It is the second most important part of the exhaust system
Should the o2 sensors be located in merge collectors?
LONG story short it depends on the exact engine and O2 sensor type, there is a LOT of research on this subject.

One of the most important things, and why OEM performance car exhausts seem odd sometimes, is the overall system length. As an example the NASCAR Cup cars can find a great deal of horsepower by changing the overall exhaust system length. The flow "pulses" for lack of a better descriptor and you want to tune that flow, in the high end of racing anyways.

[IMPALA MAN]

Here are some GENERAL tips that should help clear the air. Not a pro but have done a lot of research and have been involved in a lot of testing. Just like when you throw a ball in the air and it always comes down, the same results seem to play out every time this issue is tested.
1. Primary tube size: The best comparison that makes sense is similar to the intake side of the engine. Example: On a street car, you decide to put a tunnel ram intake on the engine. After you install it, you realize the car seems very sluggish on take off. The reason, way too large of intake area for the fuel/air to enter the engine and no velocity. Put a regular I intake back on, and you get your low end, take off torque back. So the answer is, it depends on what your are going to use the car for. An all out drag car that's going to leave the line at 4000 rpm, a tunnel ram and large tube headers would probably work fine. A street car that leaves from a stoplight at 2000 rpm, the tunnel ram and large tube headers would probably not be a good idea. Remember this: the larger and or shorter you go on the primary tubes, the more you decrease low end to and increase high end hp. The opposite, does the opposite. Smaller and or longer tubes, give you more low end torque and less high end hp. The best advice, if your building a land speed car, go big and or short. A street car, be realistic.

Equal length vs non equal length tubes: Equal length tubes typically produce a different power curve than non equal length tubes. The difference. You will typically find that an equal,length tube header will "wrap" or build rpm quicker. It will climb to its peak power quicker. It will jerk you back in your seat quickly. A non equal length header will be more gradual and smooth. It will seem to hold you back in your seat longer. What is best for you. You decide. Are you going to race the car and have to get up to speed quickly, stay in the higher rpm ranges, and have enough traction to handle the quick wrap? Or are you street driving it, typically leave from the stoplight around 2-3000 rpm, and don't wish to be jerked back in your seat while you spin the tires for the next city block. Another thing to keep in mind, with an equal length header, somehow you have to fit all of the tubes into the car, usually a pretty large (if even possible) task.

Gaskets: This one is easy. With any header, before you buy, look at one thing. The flange surface that bolts to the head surface (where the gasket seals).
If the header is surfaced like an exhaust manifold, you are likely to have a 100,000 mile trouble free seal. Do you ever have to replace your manifold gaskets every 2500 miles? Do you ever have to keep re tightening your exhaust manifold bolts? Why, because a flat surface mated to a flat surface, will seal. Now let's talk about option two, where you have a "weld ring" around the port on the header. This "weld ring" sets higher than the flange surface where the bolt is. So now let's play out what happens. You bolt the header on and when you tighten the header bolt, you bend the flange to try to get the header surface (where the bolt is) to become flat with the head surface. Ultimately it is impossible to do so as the famous "weld ring" is preventing it.
So you now have a bent flange at every bolt hole. So you heat up the engine and let it cool. Then heat it up again and let it cool again and again and again. You then re tighten the bolts again, as for some reason they seem to keep loosening up. In the end you have a flange that has been bent, heated and cooled over and over again and then bent some more. Now do I have to explain the blown gaskets? Bottom line, get a header that has a surface like an exhaust manifold. "Weld rings" are great only if you are a gasket salesman. This should also answer the safety wire question. Do you safety wire your factory exhaust manifolds? Safety wire is to keep the bolt from loosening. Why are they loosening, because the flange is not mating flat to the head surface.

Do headers affect torque and horsepower? YES, YES and YES. Refer to the explanation above about tube sizes and lengths.

Lastly ceramic coating: This is a great example: If you take a sponge and lay it in your hand. Then begin to drip water on it. Eventually the sponge will soak up as much water as it can and then water will begin to transfer to your hand. The sponge has held all it can and any excess will transfer through the sponge. Metal is the same way. If you take a piece of steel 2" thick and lay a thermometer on top of it. Then hold a lit torch on the bottom side of the metal. Eventually the heat will make it's way through the metal and you will see evidence of this by watching the thermometer temperature rise.
That being said, ceramic coating, thermal wrap, etc, will keep heat from radiating out of the metal. Here is the key, you have to make sure no more heat can radiate in then can radiate out. Example: Headers coated on the outside only. The metal will be able to absorb more heat than it can radiate out. The result: Tipically the metal will bake and become very brittle. Then the header will typically crack, or at minimum break in various places. Moral of the story, Ceramic coating is good for stainless and mild steel as long as it is coated on the inside and out. If you get coating on the outside only, save up some cash for a replacement set of headers, that will be needed within the next year or so.

Ultimately, the decision is yours. However, just like buying a home, car, entertainment center, tools, etc, determine what you are going to use the item for and then decide. Bigger is NOT always better. A little homework, will help you a lot. As a last question, have you ever talked with someone that ported out a set of heads to the max? I mean made the ports as large as they could possibly get. Then after putting them on the car, realized the car was a dog off the line.

As an example, a header to fit a 69 Chevelle with an SBC, needs to make thousands of customers happy. A custom built set specifically for your car, may perform better. It's no different that an off the shelf racing seat or one built specifically around your body. If you want something specifically built for you, open up your wallet and be ready to dig deep.
Hope this helps clear the air.

[PTAddict]

Header design, like many aspects of engine design, is the subject of many misconceptions or over-simplifications. The most fundamental (and non-intuitive) thing to understand about headers is that they are not, primarily, intended to enhance exhaust flow. They are primarily intended to enhance intake flow.

The only article I've ever seen in the mainstream auto mags that actually explains this is the one by David Vizard for PHR:

http://www.popularhotrodding.com/eng...h/viewall.html

The one quibble I'd have with David's article is that he uses the term "velocity" in describing why excessively large tube diameters are bad. This makes people think that what is desired is high velocity in the exhaust flow itself - which is not what is wanted.

To give a simplified picture, there are two things moving through a header pipe: pressure waves, and exhaust gas. Think of a river flowing, with waves on the surface from a passing boat. The wave motion is mostly independent of the flow of the underlying water.

Same thing in a header - we have pressure waves moving rapidly through exhaust gas, which is also flowing out, but at a much slower rate. The pressure waves are what we use to perform the magical scavenging/intake flow trick, so we want them to be as intense as possible, which means we want to confine them into a tight space. But at the same time, we don't want the engine to have to do a lot of extra work to have to push the exhaust gas flow through the pipe. Balancing these two goals is the essential tradeoff when sizing primary pipes.