Last updated 22/07/2019
Imagine you’re a design engineer for a vehicle manufacturer. 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. However this engine is already several models old. 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. You’ve already de-bottlenecked it several times to squeeze out more power. Have you run out of options? What about a bigger exhaust?
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. A bit of extra metal in the larger diameter pipe? Bugger all. A complete new redesign is a huge expense.
So it seems a bigger exhaust would be an attractive solution for manufacturers trying to squeeze extra power and economy out of engines. If you were the engineer would you exploit this solution? I know I would. Unless it didn’t work.
Even in the final run of a particular engine, manufacturers do not increase exhaust size. Why? Because bigger isn’t better! The exhaust diameter coming out of the factory is already optimised for that engine. It’s common sense. Why wouldn’t the manufacturer exploit such a cheap and easy optimisation opportunity?
Flow velocity is important in exhaust pipe design. Exhaust gases are emitted in pulses – one pulse every exhaust stroke. Small pipes yield a high flow velocity. High flow velocity increases inertia of the flowing gas and smooths out the stop / start nature of gas pushed out by 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. 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. Further, smaller exhaust pipes means the mass of gas held within is less. This translates to less energy required accelerating the mass of gas in the exhaust pipe with each exhaust stroke. Again engine efficiency is improved.
Smaller exhaust pipes present a greater resistance to steady state flow. So smaller isn’t necessarily better either. It’s a compromise between resistance to flow and flow velocity. There is a sweet spot. An optimal compromise. The manufacturer’s design is based on this optimum. Even without any understanding of exhaust physics, a basic understanding of design compromise will tell you there must be compromises involved. Why else wouldn’t original manufacturers increase exhaust size? Imagine how much extra metal is involved in a slightly bigger pipe. Not much, and steel is cheap. Manufactures don’t increase exhaust size because it doesn’t universally improve performance or efficiency.
The optimal exhaust size is a function of exhaust volume. The greater the exhaust volume, the greater the optimal exhaust size. Exhaust volume increases with engine rpm and fuel delivery. So ideally you’d have a variable sized exhaust pipe that increases in size as rpm and fuel injection quantity increases. For a fixed exhaust pipe size then you need to pick a compromise. 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. This will provide the greatest possible torque whilst maximising average power over the entire power curve. Maximum torque and low rpm performance are favoured because this is what makes the biggest impact for normal day to day driving.
A larger exhaust changes the shape of the power curve. It makes it more peaky, but the area under the curve is reduced. Average power is less but peak power is more. This will present as an extra couple of kW of peak power on a dyno run as it provides lower resistance at peak exhaust volume. However, looking at the entire rev range, the larger exhaust has deviated from the optimal size and average power over the full rpm range will be reduced. Maximum torque may be less and low rpm will produce less power. Fuel efficiency will also be worse, since most of the time your engine operates in the bottom half of its rev range.
A common mistake in terminology that people make when describing the relationship between exhaust pipe diameter and performance is that the engine requires a certain “back pressure”. Actually back pressure is a bad thing. It should be minimised. Engine’s don’t require back pressure but installing an exhaust pipe that is too big does negatively impact engine performance as the paragraphs above explain. I guess the back pressure explanation is a simplified way of saying bigger isn’t necessarily better (which is true).
Often an argument for a larger exhaust improving fuel economy is that if it improves power by a few kW then you should be able to get the same power as with the factory exhaust by using a bit less fuel. This isn’t correct. The increase in power output is occurring at high rpm only. At low to medium rpm the engine is less efficient and will use more fuel. Do not get a larger exhaust if you want to save fuel!
What about vehicles with turbos? Some people believe vehicles with turbos always benefit from larger exhaust pipes. A turbo is not a magical device that perfectly and completely isolates the upstream pipe from the downstream. It’s just one pipe with an obstruction in it, no different to the affect of a catalytic converter or muffler. What comes into the turbo must come out of it. The fitting of a turbo does not circumvent the explanations above. A larger exhaust downstream of the turbo will have slower moving gas, reducing scavenging at the exhaust manifold and increasing the energy required to accelerate the exhaust gas with each exhaust stroke. It will still yield less exhaust velocity and therefore less ability to continue to suck out exhaust between exhaust strokes. There MUST still be compromises involved otherwise the larger pipe would be there already. Do you honestly believe that vehicle manufacturers, with their hundred million dollar development budgets, years of cumulative experience, access to the best people in automotive design and an extremely competitive and mature automotive industry, would produce turbo charged vehicles with exhaust pipes that are always too small? If yes I have some property to sell you in the desert of Western Australia, it has great development potential.
Optimising a turbocharged reciprocating engine is different to optimising the energy extracted from a turbine. They are two different design problems. In a turbocharged engine the turbo is a parasitic load that wastes engine power. That waste needs to be minimised so that overall engine efficiency is optimised. You want the turbo to consume the minimum possible energy required to give you exactly the desired quantity of air (and no more) whilst simultaneously designing the exhaust system to minimise the energy required to clear the exhaust gas. This maximises engine efficiency. In a turbine engine, like on aircraft, you want to maximise energy transfer to the turbine. This confuses some people into thinking a bigger exhaust pipe is better on turbocharged engines.
Some people have indicated that with their turbo charged vehicles low end performance and fuel economy suffer when they installed a larger exhaust pipe. This is a more reliable finding than those that confirm a larger exhaust pipe is better in every possible way due to the effect of placebo and confirmation bias.
What if you have a diesel performance chip? Chips dump more fuel in the engine to get more power. This translates to an increase in exhaust volume across the entire rev range. The optimal exhaust size will be larger than the optimal size for less exhaust volume. So if you have a chip you may benefit slightly from a larger exhaust. Same applies to any other performance enhancement that dumps more fuel and air into the engine.
Some power gains (of maybe a few kW) can be had across the entire rpm range through less restrictive catalytic converters and mufflers / silencers and less bends in the exhaust pipe. The power gains are due to simply putting less stuff in the way of the exhaust flow and are unrelated to pipe size. Increased emissions and more noise are negative consequences to these types of modifications.
Replacing the catalytic converters and mufflers with high performance versions yields misleading dyno results. Firstly, if a bigger pipe was also installed, the vendor will have you believe that some of the improvement in power was due to the bigger pipe, even though at low rpm the bigger pipe actually made it worse. Secondly, the vendor will attribute part of the increase in power to the catalytic converters and mufflers being better than the old. Actually it may be simply that the old parts were clogged with soot. Installing new standard parts may have yielded a similar improvement in power. To correctly understand the impact of the high performance parts you’d have to compare the performance to brand new stock parts.
If you are always running your engine at high rpm (for example if you are racing) then you may benefit from a larger exhaust. You will get poorer fuel economy, lose low end power and your maximum torque may be reduced but you’ll gain high end power. High rpm yields high exhaust volume which benefits from the reduced resistance to flow of a larger exhaust. So if you’re interested in slightly more high end power at the expense of low end power and economy then get a bigger exhaust. For me, I need a tool that takes me camping and fishing at minimal expense. This maximises my camping and fishing capability. So my exhaust will be staying stock forever.