Some hints on sheer power

Continuing this month with a bit more detail into the aspect of headers we start where we left of last discussing pulses.

As the pulse of high pressure, high energy gas leaves the end of the primary tube and is diffused in the larger diameter header collector, a reflected pulse of sound energy just like a musical note is generated, much like that of a organ. This is exhaust noise. This reflected sonic pulse travels down the exhaust pipe at sonic speed which is usually around 1100-1900 feet per second in thin hot exhaust gas, causing a slight rise in pressure at the valve. The wave is then reflected back down toward the open end of the primary pipe trailing a rarefaction behind it. If the pipe is of proper length and diameter, this reflected wave can be exploited to lengthen the amount of time that the condition of low pressure exists around the exhaust valve.

These phenomena are harnessed by the smart header designer to tune the pipe to help get the maximum amount of burnt gas out and to help pull the most fresh fuel in. Of course, because a header is tuned like a musical instrument, a header can only be optimized to produce the greatest scavenge improving vacuum in a power-band of several hundred rpm.

In a nutshell without going into a lot of math, some general guidelines for selecting a header are that shorter primary runners and or bigger in diameter primary runners are better for top end power. This has to do with the tuning of the pipes fundamental note for reflected wave tuning and the travel time of the main initial exhaust gas pulse. Just like a piccolo is a higher pitched instrument than a clarinet, a shorter, fatter primary pipe is better for higher rpm. Conversely a longer and or thinner in diameter primary tube is better it will work at lower rpm for the same reasons as above. Camshaft design and the opening and closing point of the exhaust cam are large factors influencing header design. Generally the later the closing point of the exhaust valve in degrees of crankshaft rotation, the shorter the header primary pipes must be.

The way the primary pipes that come from each cylinder gather together at their ends is an important factor also. This area of convergence or the collector as it is called is critical for proper header function. It must be of larger diameter than the primary tubes because it must be large enough to acoustically represent the end of the pipe for pulse tuning reasons. It must also be big enough to support the flow from all the cylinders without creating excessive backpressure. Usually the collector is just a junction where all of the pipes are stuffed and welded into a larger pipe that may or may not neck down into the final size of the exhaust pipe. A well-designed collector, pair's cylinders opposite in the firing order with each other so an exiting pulse from one cylinder will not hamper the evacuation of the next cylinder. Adjacent cylinders in the firing order are kept separate so that the exiting pulse of one cylinder cannot contaminate the next cylinder that may be on the overlap part of the power stroke. In a typical inline 4 cylinder with a 180 degree throw crankshaft, that would mean pairing cylinders 1 and 4 and 2 and 3.

The best collectors are called merged collectors. This is a collector where the two opposite cylinders are paired together in a smooth taper before being introduced to the flows of the other cylinders. Merged collectors usually produce a wider power and sometimes more top end power. Not too many production headers are merged due to the difficulty in fabrication but many headers found on real racecars are.

Many headers presently available for popular sport compacts are of the Tri-Y design. For street cars Tri-Y's are usually the best as they are forgiving to camshaft design and other tuning factors that the header builder has no control over, unlike a real race car designer who knows exactly what is in his engine. Tri-Y also promotes a wide power band. A Tri-Y design pairs the opposite cylinder in the firing order together in a short "Y" and then brings the two pairs of "Y's" into a single collector, hence the name Tri-Y.

Well that's it for this episode let me take this opportunity to wish all you fellow motoring enthusiasts Happy Holidays, Seasons Greetings and Peace in Sri Lanka! I will be adding more to this in the next installment so make sure you tune in! Feel free to e-mail me with your questions on plasmaim@yahoo.com and I will do my very best to answer your queries.