Log turbo manifold
Many debate the usefulness of simple log turbo manifolds versus a header combined with a turbo. It is true that a turbo motor can make lots of power with a log-type manifold. It is also true that a log manifold can spool faster than a tuned manifold. This is because a turbo is driven partly by heat energy and expansion of the hot exhaust gasses. The longer tubes in a tuned header tend to dissipate a lot of this heat energy before the turbo, which can result in more turbo lag. However, turbos can use the pulse energy for better breathing and to help spool the turbo faster as well.

In my experience, log manifolds and a properly engineered tuned manifold will have nearly the same boost onset rpm. The log manifold spools the turbo faster and more violently while the tuned manifold has a smoother, more gradual onset of boost that is more manageable and controllable with the throttle. The engine with a tuned manifold will be snappier off boost. A properly designed, tuned turbo manifold will have from 30-100 more horsepower than an untuned, or log manifold, at the same boost level. It's usually a good trade off. The lag and heat loss of a longer runner tuned turbo manifold can be minimized through the use of stainless steel in the manifolds construction. Stainless has poor thermal conductivity and keeps the heat in the pipes and thus transfers more heat to the turbine. By adding tricks to the manifold design like pulse conversion where the runners 180 degrees out of phase are matched with each other to let the turbine have evenly timed pulses hitting it, the lag can be reduced or even improved over a log manifold while retaining the advantages of a tuned system's good breathing. As an example in a 4-cylinder: you would pair cylinders 1 and 4 with 2 and 3, side-by-side right before the turbine. Doing this with a twin scroll turbine housing makes for a huge improvement in spool time in 4-cylinder and rotary engines.

Building materials
When taking the effort to build a custom header, it's a wise idea to use premium materials. Stainless steel is the material of choice; and a 304 alloy is the minimal grade you would want to use, while 321 is preferred because it is tougher and more heat resistant. For a turbo engine, 321 is the only choice. If you are going to the expense of a custom header, the cost to upgrade to 321 is not too much of an issue. If you are going to be running your turbo motor at wide open throttle for long periods of time, as you would in road racing, then the aerospace super alloy Inconel should be considered. It's a little known fact that stainless steel headers make more power than mild steel headers. This is because stainless steel has much poorer thermal conductivity than mild steel. Mild steel conducts heat 220 times better that your typical stainless alloy. Stainless keeps the heat in the tubes, subsequently keeping the exhaust energy higher. Higher energy equates to higher velocity and better scavenging. Sure you can do this with thermal barrier coatings or thermal wraps, but coating cost extra money and wraps can cause the headers to crack. Again, Burns Stainless is one of the best places to buy tight radius bends and tubes of 304,321 and Inconel to make your header.

Typically a custom stainless header might set you back from $1500 to $3000. Although this seems pricey, when you have a fully built engine, it's relatively easy free horsepower on the table and is still relatively low on the bang-for-your-buck scale.

Hopefully this has made you an expert on headers, enabling you to optimize your setup or make wise decisions when buying a header for your ride. Until next time...

TUNED
RPM 50 55 60 65 70 75 80 85 90
4000 46.0 47.0 48.0 49 50.2 51.2 52.0 53.3 54.4
4500 40.5 41.3 42.3 43.2 44.3 45.1 46.0 47.0 48.0
5000 36.2 37.0 37.7 38.6 39.5 40.4 41.0 42.0 43.0
5500 32.6 33.2 34.0 34.8 35.7 36.4 37.0 38.0 38.8
6000 29.6 30.3 32.0 31.7 32.5 33.1 33.8 34.5 35.4
6500 27.2 27.6 28.4 29.0 29.8 30.4 31.0 31.7 32.4
7000 25.0 25.5 26.0 26.7 27.5 28.0 28.6 29.2 29.8
7500 23.0 23.6 24.2 24.7 25.4 26.0 26.5 27.0 27.6
8000 21.5 22.0 22.5 23.0 23.6 24.0 24.6 25.2 25.8
8500 20.0 20.5 21.0 21.5 22.1 22.5 23.0 23.5 24.0
9000 18.7 19.2 19.6 20.1 20.6 21.0 21.5 22.0 22.5
9500 17.6 18.0 18.4 19.0 19.4 19.8 20.2 20.7 21.2
10000 16.5 17.0 17.4 17.8 18.3 18.6 19.0 19.6 20.0
10500 15.6 16.0 16.4 16.8 17.2 17.6 18.0 18.4 18.8
11000 14.8 15.0 15.5 16.0 16.3 16.7 17.0 17.4 17.8

This table is handy to roughly estimate primary length. The numbers in the table are primary length in inches.