Drag Race 101: Headers, Part III
In past segments, we took a long hard look at what happens when the exhaust valve opens in an engine. Getting a handle on exhaust speed is extremely important (exhaust speed is the determined by the diameter and length of the primary pipe). But it’s the not only critical component in a header.
As far as the collector is concerned, the major objective is to take advantage of the “secondary tuning impulses” from the other pipes in the system. The influence of the collector is generally restricted to the RPM level below the torque peak of the engine in question. Collector sizing is more critical in engines that spend the majority of their time in the lower RPM ranges.
With a series of both exhaust waves and sonic waves entering the collector, it’s easy to see that there is probably a bunch more to collector shape than first meets the eye. For example, a merge collector minimizes the reflected wave, so it has less effect upon the inertia pulses. The actual hourglass shape of this collector design serves to regulate the high and low pressures. It also tends to speed up the velocity of the gas flow. Additionally, the shape (particularly in the case of a merge collector) helps equalize the transition of the individual exhaust tube gases.
The overall length of the merge collector is important. So is the angularity of the tubes that physically “collect” in the collector. You’ll also find a “spear” or “pyramid” inside a merge collector. The purpose of this pyramid is to control the exhaust gases so that they do not re-enter the primary tube and travel backwards into the engine by way of the low-pressure area (exhaust gas dilution). The neck of the hourglass is important too. By reducing the size of the neck, the exhaust gas velocity will increase as it is carried into and out of the secondary tube (collector). This form of collector is so efficient; some manufacturers have found there is an actual increase in the EGT (Exhaust Gas Temperature). The reason is the header assembly is extracting more of the air fuel mixture out of the cylinder and burning it in the collector. Because of this phenomenon, there’s chance the camshaft timing events may have to be revised. And obviously, the same applies to the carburetor jetting.
There are other types of collectors out there too: Some pair the cylinders based upon firing order. Essentially these collectors simulate a Tri-Y header configuration, and for a few applications, there could be more performance with the setup. And aside from their unique configuration, it’s interesting to note these collectors usually range in length from 23 to 25 inches – which is well beyond the length of most merge collectors.
In the end, each and every header collector design in each and every engine for each and every car on each and every track can be different. Honestly, there is no magical formula, chart or graph that you can use to determine collector diameter. What it boils down to is good old-fashioned testing (and most likely a wee bit of cutting, welding and testing again).
In the next segment, we’ll take a look at the simple formulas used to establish a header size baseline and we’ll also take a look at the practical side of header tuning. Watch for it.
So You Want to Be A Drag Racer: Buying, Building, Wrenching and Racing
The allure of the drag strip is easy to understand – a place where it takes less than 10 seconds to make a stand, prove your skill, speed, and nerve. But the road to the races can be intimidating. The Burnout wants to make that road a lot smoother for aspiring racers, whether it’s through building a new car, modding a used one, or taking that ride all the way up to the burnout box and beyond. This series is a work in progress, an ever expanding comprehensive guide to all the things that take drag racing from concept to reality.
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