BREAKING: new 4 in 4 exhaust

[Ninemeister_imp]

Exhaust design is just about science in its own right. What makes it easier for an exhaust manufacturer is that most make (I'll not say design) their systems to suit standard engines and thereby eliminate a whole lot of variables that might affect the performance of the exhaust on the engine.

On a 4 cylinder engine, the parameters that an exhaust manufacturer has to consider include:
Header pipe diameter(s) & lengths
Collector merge angle & diameter(s)
Secondary pipe diameter
Secondary collector merge angle & diameter(s) (on a 4-2-1 system)
Silencer cone angle & length
Silencer outlet diameter

In general, the header pipe diameter and length is one of the most important decisions to make as this dictates the resonant response of the pipe and the reflected pressure waves coming back from the collector. Very simply, larger pipes and shorter lengths favour top end power, whereas smaller pipes and longer lengths favour mid range torque.

The 4-1 vs 4-2-1 conundrum is down to the expectation of the tuner. The 4-1 will provide the strongest peak resonant response from a single collector over a narrow rpm range, whereas a 4-2-1 will spread the response which may lose a little peak power but have the advantage of spreading the torque.

Then there is velocity to consider. The smaller the pipe, the higher the velocity of the exhaust gas. The higher the velocity, the lower the pressure in the pipe. Low pressure is good for scavenging the cylinder (extracting the most exhaust gas and initiating intake port flow) but the smaller pipe limits the mass flow down the pipe, hence why stepped or tapered headers are becoming more common as they give the best of both worlds.

Then there is the collector. The ideal geometry causes the flow to maintain or increase velocity through the small diameter merge point, then expand the gas away from the merge in a controlled manner. It's this high velocity through the collector that matters the most. Anyone interested should look up Burns Stainless Steel and play with their collector X design program.

As I said at the start, there are a lot of exhaust manufacturers & fabricators who are really good at bending and welding pipe to make beautiful works of art. Then there are exhaust designers who would not do anything without building a computer model of the complete engine first. Then there are a handful of gifted folk in between who have learned their craft through years of empirical testing and understand what it takes to make an exhaust "in the ballpark" of optimal. The secret, should you want a special exhaust system that actually works, is to find one of these guys who really knows what he is doing and commission them to make one from scratch, then spend a week or two on a dyno to fine tune the result.

Does this help?