Exhaust & Header Tech - Exhaustive Choices

Don’t diminish the importance of proper header selection. Pipes are not just pipes. Their sizing, arrangement, relationship to engine size and rpm and the ability to function as a tuning tool are all critical issues when matching engine components to track length and conditions. Choices are commonly made based on misleading or incorrect information.

One successful approach to making proper decisions is to understand the exhaust process and apply that knowledge to application-specific situations. Simply stated, that is the purpose of this story.

What Is “Blowdown” And How Does It Affect Power?

Let’s define “blowdown” as a relationship between cylinder pressure and exhaust system pressure, beginning when the exhaust valve opens and ending when the cylinder pressure equals the exhaust-system pressure. When the blowdown period begins, residual combustion pressure overcomes exhaust-system pressure and forces gas out into the atmosphere. When this period ends, the remaining exhaust gas must be pumped out by piston action or any pressure excursions (tuning) remaining in the exhaust path.

Whatever can be done to reduce blowdown pressure tends to increase the amount of work done on pistons to produce power. For example, if blowdown pressure is high, this indicates an unnecessary loss of power into the exhaust system.

What affects blowdown pressure? In particular, combustion efficiency or how much fuel is being converted into power. An engine that produces a slow burn rate tends to exhibit high blowdown pressure and a loss of power. The faster the burn, the lower the tendency of power loss through excessive blowdown pressure. Mechanical compression ratio is also a factor in blowdown pressure, as is timing of the exhaust valve opening, ignition spark timing and heat losses to combustion-exposed surfaces.

How does this relate to header design? Consider the following: As blowdown pressure decreases, there is an increased dependency on the exhaust system’s tuning characteristics to rid cylinders of unwanted gases. Therefore, the importance of selecting proper headers, collectors and mufflers increases if power is to be optimized. In other words, increased combustion efficiency and power output requires a critical examination of all exhaust system components.

The Practical Side Of The Exhaust Process

Even though it’s possible to document “pulsating” flow in an exhaust system, the total process may be more easily understood by considering the exhaust gases “flow” from the cylinders to the atmosphere. If we do this, then it’s reasonable to accept that the volume of exhaust gas an engine must expel is a function of rpm and piston displacement. Even though friction horsepower and pumping losses are part of the equation, it is interesting how accurately headers can be sized based upon engine rpm and piston displacement.

In fact, there is a significant body of information supporting the notion that peak torque (volumetric efficiency or “cylinder filling”) can be associated with a specific flow rate in an exhaust passage. Stated in previous Circle Track tech articles, this value is in the range of 240-260 feet/second. Since our assumed exhaust “flow” passes through primary header pipes, its rate is influenced by the pipe cross-section area. This phenomenon then becomes a “tool” for relating torque output to rpm to piston displacement. By first determining the rpm span in which a torque boost is required and knowing a value for piston displacement, primary pipes can be sized. (See sidebar on primary pipe sizing.)

Headers As A Tuning Tool

In a fashion quite similar to the influence intake systems can have on how an engine makes power, exhaust systems can also be a tuning tool. Let’s consider the interior of an engine’s cylinders to be an environment of continually changing pressure conditions. Atmospheric pressure, existing on both “sides” of these cylinders, operates at a comparatively constant pressure. Depending upon cylinder pressure conditions, atmospheric pressure can cause flow into the cylinders as well as resist flow exiting the engine. These so-called “pressure excursions” are forms of energy that, if properly addressed, can improve both cylinder filling and exhausting.

Today, engine builders and tuners are finding ways to optimize individual cylinder power to the benefit of net output. Header design is not exempt from investigation in this regard. Even though primary pipes are typically joined in some fashion at their exits, resulting in a sharing of pressure pulses among cylinders (sometimes called “cross talk”), primary pipe sizing does not always need to be equal.

As pointed out elsewhere in this story, “stepped” pipes and multiple pipe sizes can be used to pinpoint rpm points at which torque boosts can be created. (See sidebar.) Plus, from time to time, chassis or rules restrictions cause header modifications (or allow changes) that may be disguised as opportunities to the creative engine builder. Just because headers are normally of all the same primary pipe size or step location, pipe to pipe there’s nothing that suggests this is cast in stone. Progressive engine builders are discovering ways to optimize individual cylinder power beyond the scope of special ignition systems, mixed camshaft lobe profiles and multiple rocker arm ratios on or in the same engine.

Including a variety of collector lengths and primary pipe extensions in your trackside trailer toolbox is not far removed from the necessity of having a timing light and handful of carburetor jets. The possibilities are exhausting.