The Anatomy of A Motorcycle – Headers and Exhaust Pipes Part One
Kicking Out the Gas
No combination of header and exhaust pipe systems is the perfect solution for all applications, so you’re facing a dilemma when you choose one or build it yourself.
Depending on the design and purpose intended, all exhaust systems are a compromise. You give some in one arena to get some in another, so before performing header or exhaust modifications to increase performance, you need to to know what kind of performance you’re looking for and at what portion of the power band you want it to happen. If you want maximum,low RPM, stump-pulling power your modifications will be very different from those you want to make to attain maximum mid-range power.
If you want peak horsepower when you roll your bike onto a dyno at the show, you’re probably leaving some of the power you actually want on the table.
Here’s how it works: For a vehicle to cover distance as quickly as possible, it’s not the highest peak horsepower generated by the engine that makes the difference, it’s the highest average power generated across that distance which typically produces the quickest times.
When you compare two power curves on a dynamometer and assuming all other factors remain constant, the curve which demonstrates the greatest average power is the one which could be said to represent the ideal range for covering distance in the minimum time. It’s this fact which makes the reality of the power band important. If you’re looking for actual overall performance, you don’t want to give up powerful mid-range output just to have the highest peak power numbers. If you go for the highest peak power, you’ll knock down your acceleration. The majority of the time you spend accelerating occurs when the engine is outputting at it’s mid-range, not at peak-power RPM.
You want both maximum mid-range power and maximum peak power, and that’s where the compromise comes in.
The reality is that an exhaust system cannot produce more power from an engine on its own and the potential power of an engine is determined by the amount of fuel available for combustion. More fuel has to be introduced into the combustion chamber to increase potential power, period. The efficiency of the combustion processes is influenced by the exhaust system and a properly designed exhaust system can reduce engine output losses, but it can’t add horsepower. It can prevent those horses from being wasted, so the primary design objective for a high performance exhaust should be to reduce engine-pumping losses.
Increasing volumetric efficiency is the ticket, and the result of reduced pumping losses is more power available to move your bike.
You’ve probably heard people talk about back-pressure in relation to exhaust systems. Even some professionals believe the old saw that some back-pressure is necessary for top performance.
That’s just, well, wrong. In damn near any scenario where you’re tuning for high performance throughout the power band, back-pressure in an exhaust system increases engine output losses and decreases maximum engine power. It’s not a point of contention or a preference, it’s just the laws of physics.
In a normally aspirated tuning which doesn’t included without special fuels or oxygen-rich additives, an engine’s maximum power potential is directly proportionate to the volume of air you can introduce into the system. Here’s a shocker, what it means it that an engine with a displacement of 750 cc has the same maximum power potential as an engine of 1000 cc – as long as they both use the same volume of air. The power band characteristics of the two engines? They might be different, but the peak power you could theoretically attain is essentially the same. You can resist the idea of the truth of this all you want, but that won’t change reality or the laws of physics.
So what are you really looking for and how can you make it happen?
How about some overall design guidelines to start the discussion.
- Longer header tubes tend to increase power below the engine’s torque peak and shorter header tubes tend to increase power above the torque peak.
- Large diameter headers and collectors will limit low-range power and increase high range power, and small diameter headers and collectors will increase low-range power and limit high-range power.
- Balancers or equalizer tubes between the header tubes will flatten the torque peak and widen the power band.
- Stainless steel headers don’t transfer heat to the air around them as fast as headers constructed of mild steel. That’s an important distinction as retaining more of that heat inside the header pipes aids exhaust flow. Hot exhaust gases are more energetic and reduce the amount of heat flowing across the engine.
Your goal when you’re making engine modifications should be to maximize air and fuel flow into your mill and exhaust flow out of the powerplant. The inflow of the air and fuel mixture is a separate issue, but it’s directly influenced by exhaust flow. The critical segment of that timeline happens during valve overlap when both valves are open for a given number of degrees of crankshaft rotation.
Made simple, the reason for that is that gasoline requires oxygen to burn, and by volume, dry, ambient air at sea level contains about 21% oxygen, 78% Nitrogen and trace amounts of other mostly inert gases. Given that oxygen is made up of only about a fifth of the air’s volume, an engine needs to take in 80 percent useless volume to get the oxygen it needs to create the combustion of fuel. Of course, if you inject an oxygen-bearing additive such as nitrous oxide, or use an oxygen-rich fuel like nitro methane, you can make much more power from the same displacement. It stands to reason. Both additives bring more oxygen to the combustion chamber to support the combustion of fuel. If you add a supercharger or turbocharger? The result is the same, you get more power as more oxygen is forced into the combustion chamber.
Where does that leave us with the exhaust tuning problem?
The single most critical factor which determines the efficiency of exhaust flow is the relationship between flow volume vs. flow velocity, and the same goes for engine intake events.
Engines need the highest possible flow velocity to provide quick throttle response and torque throughout the low-to-mid range portion of the power band, and the same engine also needs the highest flow volume possible throughout the mid-to-high range portion of the power band for maximum output.
Yep, it’s here where a crucial conflict pops up…