Does a New Exhaust Increase Motorcycle Horsepower?
Is upgrading to a new exhaust system on your motorcycle really going to boost its horsepower? This is a question that many motorcyclists often ask, with answers varying greatly based on the make, model, and modifications already done to their bikes.
My larger motorcycles, for instance, have seen aftermarket exhaust installations paired with adjustments in carburetion or fuel injection (FI) systems to ensure optimal performance. Specifically, my fuel-injected (FI) Kawasaki ZX-9R experienced a significant increase in rear wheel (r/w) horsepower from 119 to 140 with an Akrapovic 4-1 system. Kawasaki claims this bike can reach 143 bhp at the crankshaft. Similarly, my Honda Blackbird 11 saw an increase from a stock r/w of 121 to 142 with a BlueFlame exhaust and a PowerCommander black box, with Honda claiming 162 bhp at the crankshaft.
Exhaust upgrades can make a noticeable difference, even if the improvement isn't vastly significant. For example, swapping out the can muffler on my 1200 Bandit after rejetting provided a modest but worthwhile improvement. However, installing a full-race exhaust system dramatically transformed the motorcycle's performance, forcing me to relearn riding techniques after 55 years and a 18-year racing career.
It's important to note that the exhaust system alone does not add much horsepower. However, when combined with air intakes and performance chips, significant improvements can be observed in motorcycle performance. Additionally, you might see a slight increase in fuel economy. So, while the exhaust may not directly contribute to horsepower, it can enhance the overall engine efficiency and tuning potential.
Upgrading an Old Motorcycle
When upgrading an old motorcycle, changing the exhaust could be part of a broader bike renovation. However, fitting a so-called "performance" exhaust on a modern motorcycle is likely to alter the sound and possibly harm performance. Many original equipment manufacturer (OEM) exhaust systems are incredibly efficient due to extensive tuning. To truly increase power and performance, you would need to consider additional modifications such as chip tuning and altering the air intake. The most significant gains from exhaust upgrades often come from engine sound modifications rather than horsepower.
Mechanical Considerations
From a purely mechanical standpoint, exhaust systems do not add horsepower. Instead, they reduce pumping losses by allowing the engine to use less power to expel exhaust gases. This effect is similar to using a better lubricant to see a slight increase in horsepower due to reduced friction. In reality, the only way an engine can add horsepower is by increasing the amount of air that enters the engine.
However, there are exceptions to this rule. In the case of severely restricted exhaust systems, particularly those from pre-2000 vehicles, some of the burnt gases may not exit the cylinder on the exhaust stroke, leaving valuable space in the combustion chamber that could have been filled with fresh air/fuel. This issue is typically related more to camshaft design and timing than the exhaust system itself. For example, a 5.7L V8 engine trying to push exhaust gases through a tiny 2.1-inch exhaust pipe will generate backpressure, which is detrimental.
A similar exception applies to turbocharged engines, where an exhaust system that is too restrictive can prevent the turbocharger from efficiently making boost. A lack of power in the exhaust can limit the overall performance of the turbo system, affecting the engine’s ability to produce the desired boost.
Ultimately, while a new exhaust system on your motorcycle may not increase its horsepower directly, it can contribute to overall performance enhancement by reducing power losses and providing a broader range of performance benefits. Remember, significant modifications often require a combination of different components and careful tuning to achieve the desired results.