Selection of proper pipe diameter is something that is critical for performance. Engines are air pumps and the more air they can move, the more power they can make. Most of us are taken up by the engine’s ability to draw in air but being able to push the air out does have a considerable impact. How do you improve the ability of an engine to help air leave the combustion chamber? By adding bigger exhaust diameter pipes; sometimes.
On most vehicles, the exhaust system’s sole job is to lead gases out of the engine.
Proper exhaust-pipe size is a function of both engine output and displacement. The higher the engine’s output, the larger the pipes should be. Likewise, larger engine displacements call for bigger pipes. But you don’t want to become a victim of “More’s Law,” either: While up to a point reducing exhaust restriction generally helps both power and gas mileage, going too large can over-scavenge an engine and actually decrease engine output as well as fuel efficiency.
To avoid becoming a restriction, all muffler and catalytic converter inlet and outlet sizes should correspond to the exhaust pipe sizes and design your exhaust system to use the minimum number of bends. But if you drive a performance vehicle, your exhaust system will play a bigger role in determining your engine’s performance and power. To set up optimal exhaust system, you need consider your engine’s power band, its maximum usable RPM, as well as back pressure.
But back pressure should not be eliminated altogether – you really need it!
Back pressure – Is it good or bad?
Back pressure exists all around us.
The term refers to the atmosphere which pushes down on your exhaust system. And back pressure generally has an extremely negative effect on your engine’s power output. When your engine works at the top of its power band, it creates a lot of spent exhaust gases. If the exhaust pipe diameter is too small and it generates too much back pressure, these exhaust gases will be heavily restricted from exiting the engine. The amount of generated exhaust gases will be greater than the amount of exhaust gases that are able to leave the engine. This “surplus” of exhaust gas will contaminate the cool and fresh air-to-fuel mixture on the next intake stroke.
Engine power and performance will be reduced.
If it reduces power – why do I need it?
If back pressure reduces your engine’s power and performance. Why do you need it?!
Flow Velocity
If you equip your ride with a super large exhaust diameter pipe to eliminate back pressure, you will also reduce the flow velocity of exhaust gases.
And you need higher flow velocity.
A high flow velocity aids the complete expulsion of burnt gases. And it increases the amount of the air-to-fuel mixture being drawn into the combustion chamber.
When exhaust gases exit the engine with a higher flow velocity, they create low pressure behind them. And this low pressure continues to suck out exhaust gases at an extremely high rate, creating a valuable cycle.
Waste of Air/Fuel Mixture
Your engine’s exhaust port opens to expel exhaust gases from the combustion chamber. And it remains open for a short time once they are out. During that time, unburned air/fuel mixture can escape directly into your exhaust. This wastes fuel, and needlessly creates pollution. The only way to prevent this waste is if the back pressure at the opened exhaust port is greater than the pressure in the cylinder. It becomes clear that too much back pressure is bad. And too little back pressure is bad.
You need just the right amount.
What exhaust diameter size makes the most sense?
Don’t be fooled into thinking that a bigger pipe is always better for power and performance for your classic car.
To strike a balance between low back pressure and high gas flow velocity, you need the correct pipe size. The industry has developed a table to be used as a general guide. It tells you the proper pipe diameter based on your vehicle’s engine size and horsepower.
But what if your engine is putting out more than the numbers shown in the table?
Imagine you are a design engineer for a vehicle manufacturer. You have been tasked with increasing the power output of an engine and squeeze out a little more fuel economy too. It is 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 is 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 would 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 completely 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 did not work.
Even in the final run of a particular engine, manufacturers do not increase exhaust size. Why? Because bigger is not better! The exhaust diameter coming out of the factory is already optimized for that engine. It is common sense. Why wouldn’t the manufacturer exploit such a cheap and easy optimization 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 mean 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 is not necessarily better either. It is 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 would not original manufacturers increase exhaust size? Imagine how much extra metal is involved in a slightly bigger pipe. Not much, and steel is cheap. Manufactures do not increase exhaust size because it does not 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 would 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 optimized to deliver the greatest area under the power curve. This will provide the greatest possible torque whilst maximizing average power over the entire power curve. Maximum torque and low rpm performance are favored because this is what makes the biggest impact for normal day to day driving.
A larger exhaust diameter changes the shape of the power curve. It makes it peakier, 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”. Back pressure is a bad thing. It should be minimized. Engine’s do not 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 is not necessarily better (which is true).
Despite what seems like simple logic, there is a limit to the bigger-is-better mentality. Remember, you want some back-pressure to help with the velocity of the exhaust that is leaving the combustion chamber. Factory exhaust systems often create too much backpressure, resulting in a high amount of exhaust gases in the chamber, which hurts power. But too little back-pressure can produce the same result, by slowing down the velocity of the gases.
Therefore, you need to install the right size pipes to help find that sweet spot of flow.
I truly love your blog.. Excellent colors & theme. Did you create this amazing site yourself?
Please reply back as I’m trying to create my own personal blog and would love to learn where you got this
from or just what the theme is named. Cheers!
Hello; the blog was set up in Bluehost using WordPress with the plug ins provided by Bluehost.
Thanks for asking.
thank you for the information
Hello classicautoadvisors.com administrator, Thanks for the informative and well-written post!
Hi classicautoadvisors.com webmaster, Thanks for the comprehensive post!
Hello classicautoadvisors.com admin, Your posts are always well-delivered and engaging.
Hello classicautoadvisors.com owner, Your posts are always well-written and easy to understand.
Dear classicautoadvisors.com webmaster, Thanks for the well-researched post!
Hello classicautoadvisors.com owner, Thanks for the in-depth post!
Hi classicautoadvisors.com webmaster, Your posts are always well-received by the community.
Dear classicautoadvisors.com admin, Thanks for the well-presented post!
Dear classicautoadvisors.com admin, You always provide useful information.
Hi classicautoadvisors.com admin, You always provide key takeaways and summaries.
Hello classicautoadvisors.com admin, Great post!
Hello classicautoadvisors.com administrator, Your posts are always well-referenced and credible.