When fitting a tubular exhaust manifold to a highly tuned normally aspirated engine it's essential to get the size of the primary and secondary pipes correct for the expected power output of the engine. Overly small pipes will restrict peak power and pipes that are too large will hurt both low end torque and top end power. Pipe length is a whole different subject governed by extremely complex equations and will be covered in a separate article if I ever get round to it.
The ideal exact specification of an exhaust system is a function of both engine capacity, power output, and cam duration but we can get in the right ballpark just from consideration of the power output. What I'm going to do is set out a table of ideal pipe sizes for a four cylinder engine of various power outputs. You can extrapolate from this to other engine configurations by working back to power per cylinder. Manifolds will usually be either 4 into 1 or 4 into 2 into 1. In the latter case we also need to consider the size of the secondary pipes which obviously need to be larger than the primaries. As a rule of thumb secondaries that connect two primaries need to be about 12% to 15% larger in internal diameter than the primaries but it depends on cam duration and is less critical than getting the primary size correct. For most engines secondaries 1/4" (6.4mm) larger in diameter than the primaries will be about right.
The ideal size of the exhaust system running through to the back of the car will be considered in a separate article but is usually another 1/4" to 3/8" bigger than the secondary pipes.
Imperial exhaust tubing is normally available in 1/8" diameter increments with 1/16" (1.6mm) wall thickness so each size of tubing will just fit into or over the adjacent larger or smaller size. Metric tubing normally has a 1.5mm wall thickness and can be bought in just about any size. In all cases I specify the outside diameter (O/D) of the tubing so the I/D will be 1/8" (3.2mm) smaller for imperial tubing. If you want to build a system from metric tubing you'll have to work back to internal diameters to properly equate to my recommendations. In all cases the table assumes a single primary pipe per engine cylinder. Siamese port engines like the Leyland A and B series will follow different rules.
|Primary Pipe O/D
|Primary Pipe O/D
|Max Power Output BHP
4 cylinder engine
Please don't assume that the maximum power output figure represents an absolute bottleneck bhp for that size of pipe. What will happen in practice is that a system that is too small in diameter will become progressively more restrictive but there should be no restriction up to the bhp limits set out in the table.
The importance of primary pipe size, and in fact the motivation for actually writing this article now, can be seen in this forum thread about a 2 litre 8 valve Saab engine I'm involved with tuning.
Reducing the primaries of this 4:2:1 system from the overly large 1 3/4" o/d they started out at to the 1 1/2" o/d I specified as being theoretically correct for the 160 bhp it was producing netted another 6 bhp and 5 ft lbs. Definitely another area in engine design where bigger is not always better.