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The manufacturer does this – they will pick an exhaust size that is somewhere in the middle, optimized for the rpm when the engine develops it’s maximum torque and optimised to deliver the greatest area under the power curve. For a fixed exhaust pipe size then you need to pick a compromise. So ideally you’d have a variable sized exhaust pipe that increases in size as rpm and fuel injection quantity increases. Exhaust volume increases with engine rpm and fuel delivery. The greater the exhaust volume, the greater the optimal exhaust size. The optimal exhaust size is a function of exhaust volume. Manufactures don’t increase exhaust size because it doesn’t universally improve performance or efficiency. Why else wouldn’t original manufacturers increase exhaust size? Imagine how much extra metal is involved in a slightly bigger pipe. Even without any understanding of exhaust physics, a basic understanding of design compromise will tell you there must be compromises involved. The manufacturer’s design is based on this optimum. It’s a compromise between resistance to flow and flow velocity. So smaller isn’t necessarily better either. Smaller exhaust pipes present a greater resistance to steady state flow. This translates to less energy required accelerating the mass of gas in the exhaust pipe with each exhaust stroke. Further, smaller exhaust pipes means the mass of gas held within is less. This reduces the energy required to clear the cylinder of exhaust and re-accelerate the gas residing in the exhaust pipe already, thus reducing the energy wasted pushing out exhaust and improving the efficiency of the engine. As the gas continues to flow between exhaust strokes, it creates a low pressure at the exhaust manifold which helps to suck out exhaust gases on the next exhaust stroke. High flow velocity increases inertia of the flowing gas and smooths out the stop / start nature of gas pushed out by the engine. Exhaust gases are emitted in pulses – one pulse every exhaust stroke. Why wouldn’t the manufacturer exploit such a cheap and easy optimisation opportunity?įlow velocity is important in exhaust pipe design. Why? Because bigger isn’t better! The exhaust diameter coming out of the factory is already optimised for that engine. Unless it didn’t work.Įven in the final run of a particular engine, manufacturers do not increase exhaust size.
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If you were the engineer would you exploit this solution? I know I would. So it seems a bigger exhaust would be an attractive solution for manufacturers trying to squeeze extra power and economy out of engines. A complete new redesign is a huge expense. A bit of extra metal in the larger diameter pipe? Bugger all. How much would a bigger exhaust cost relative to a completely redesigned engine? I’d suggest that once a bigger exhaust is integrated into a production line, the extra cost would be almost nothing. Have you run out of options? What about a bigger exhaust? You’ve already de-bottlenecked it several times to squeeze out more power. It’s reaching the end of its life and the upcoming model is the last model before the engine is retired and the all new engine supersedes it. However this engine is already several models old. You’ve been tasked with increasing the power output of an engine and squeeze out a little more fuel economy too. It’s the same engine as previous model, but for the new model which needs a bit more power and economy for marketing reasons. Imagine you’re a design engineer for a vehicle manufacturer. Why You Shouldn’t Support Mandatory Vaccinations