Building Your Own Headers - Part 3

Building Your Own Headers - Part 3
By O'Darrell Poole (c)

In our first two articles (1) (2) we discussed the characteristics of a properly designed and tuned header. In this month's article we will discuss some theory on the gas flow in a tube and also the difference between mandrel bent and welded headers.

Tube Bending
So that a piece of tubing can be bent and maintain consistent wall thickness as well as cross sectional area a mandrel type bender is used. When headers are made with mandrel bent tubing that use 1 piece of tubing per primary cylinder, bend radius is limited to 1 � times the tubing diameter. In designing a header of this type and merging multiple primary tubes into one collector variations in tubing length are inevitable. Due to limited room and available radiuses header design must be compromised and a under performing header is the usual outcome. Some will disagree and say "when I installed my set of brand X headers to replace my stock manifolds my performance increased and � mile et and mph improved." Unfortunately what had really happened is that the stock design manifold was produced for mass production and was designed to meet many different operating conditions and its not performance, so that bolting almost anything to the head to replace the stock manifolds will produce more power because of reduced restriction. You will also find for example in a small block Chevy that a poorly designed header of unequal length primary tubes will produce fouled plugs usually in the front and rear cylinders and nicely tuned centre cylinders. This is because the tubing in the middle cylinders is pretty close to the properly tuned length for the conditions of the engine and the front and rear cylinders are to long and to short (in tubing length ) and cause exhaust to be forced back into the cylinders (inert gas) mixed with fresh gas and a misfire results. I would never recommend using unequal length or one piece primary headers on ANY ENGINE EVER!!! The potential to damage your engine is far more expensive than buying a set of headers that are properly sized and tuned for your engine. Now while custom made headers or welded headers are much more expensive than the ones we have just discussed the advantages are well worth it. Custom made or welded headers will still use mandrel bent tubing but because they can be cut and rewelded at any spot and position necessary the lengths and design of the header will not be compromised. Although welded headers are sometimes extremely difficult to build because the primaries may have to snake their way through and around each other to achieve equal length primaries the performance gains are well worth the effort and at plug check time all the cylinders will be burning the same. I image my e-mail box will be full of comments from those who will say that the welds cause gas flow problems because of the weld intrusion into the primary pipe.

Gas Flow in Pipes
When gas or a liquid travels down a pipe there is what is called a gradient of flow. This means that the gas or liquid in the centre or middle of the pipe moves faster and speed diminishes as it gets closer to the outer wall of the tube and is actually almost no movement on the wall itself. As an example take a walk to the nearest quick running stream in your area. Notice out in the centre of the stream that the current is carrying the water quiet quickly now walk to the edge of the stream and notice the water at the shore line it's barely moving!! This is the same way that a header tube works. The gas in the centre of the primary pipe is traveling extremely fast (1,700ft/sec) but at the tubing wall is almost at a stand still therefore while a poor weld and gobs of weld protruding into the primary pipe from a weld will adversely affect gas flow, a properly cleaned and applied weld fillet with proper penetration will have no effect on gas flow.

Remember we welcome your questions on intake and exhaust systems as well as any comments you may have. Stay tuned for more articles on intake and exhaust systems in upcoming articles.

O'Darrell Poole
Autocraft Dynamic Design Inc.
E-mail: autocraftdynamicdesign@live.ca