In general, enthusiasts know that back pressure is bad. But there are some who believe a little back pressure is good for turbo. In this article, I want to cover things in detail and see whether that’s true or not.
If you are too lazy to read – Back pressure is not good for turbo (or for anything else). With back pressure, exhaust gases cannot easily flow in the exhaust system. Which means they will spool the turbo with reduced force – resulting in power loss and more turbo lag.
Now that’s out of the way, let’s get into the details of back pressure and turbos.
To understand the details, we must first cover turbos – what are they and how do they work? If you are already familiar with this, then skip ahead.
To generate power, your car creates combustion in the combustion chamber – this is done by using fuel and air (oxygen). Fuel is injected into the combustion chamber, whereas air is sucked from the surrounding environment.
Combustion is a controlled explosion that generates power and toxic exhaust gases as byproducts. These exhaust gases exit the combustion chamber into the exhaust system, where they eventually exit the car.
If you want more power, you need bigger combustion – which means more oxygen and fuel. Injecting more fuel is easy, getting additional air into the combustion chamber is the hard part. Every performance mod ultimately works by increasing the amount of air in the combustion chamber.
Turbo is the same. Turbo is an air compressor that sucks additional air from the surrounding, compresses it and delivers it to the combustion chamber. With this additional air, your car just injects more fuel and bigger combustion is created. Simple right?
Turbo can give a significant boost in horsepower – 100 horsepower is possible and quite common. What’s interesting though, is that turbo is not powered by your fuel. Instead, turbo contains a turbine that gets spun by exhaust gases as they flow inside the exhaust system.
This means turbo is super efficient – it provides a significant horsepower boost without consuming additional fuel. Awesome! The only downside is ‘turbo lag’. Turbo will only work when there’s enough exhaust gases flowing through it.
Without exhaust gases, the turbine won’t get spooled and the turbo won’t have power to do its work. This is why the additional horsepower from turbo will only kick in at higher RPM ranges – when more exhaust gases are generated and flowing.
Okay, that should be enough information for now. If you want to know more about turbo, give my other article a read. Which is better: Supercharger or Turbocharger?
Recap: What’s Back Pressure?
Before we get into the meat of this article, we also need to understand back pressure. I promise this is the last recap section! Again – if you are already familiar, just skip ahead.
In the section above, I mentioned that exhaust gases are produced during combustion and they exit the combustion chamber into the exhaust system – where they eventually exit the car.
These exhaust gases must exit quickly to provide room for fresh air for the next combustion. Otherwise, the next combustion will be weak because there is not enough air (oxygen).
What prevents the exhaust gases from leaving quickly? Back pressure!! With some science knowledge from high school, we know that air particles (exhaust gases) will flow into an area with lower air pressure.
If the air pressure in the exhaust system is lower than the combustion chamber, then the exhaust gases will flow out easily. Otherwise, the exhaust gases won’t flow out and occupy the combustion chamber!
Back pressure refers to the air pressure in the exhaust system. After the discussion above, we now understand that back pressure should be kept as low as possible, so exhaust gases can flow easily.
Is Back Pressure Good Or Bad?
So is back pressure always bad? The answer is yes. You want to keep back pressure as low as possible. High back pressure is directly correlated with a drop in performance.
In the later section I will cover why there’s a misconception about back pressure being good and how can we reduce back pressure. So read on!
Impact of Back Pressure On Turbo
Back pressure impacts turbo in two ways:
- Increases turbo lag. Now that back pressure is preventing exhaust gases from flowing properly, it will take much more exhaust to spool the turbo until it works. This means you have to go to an even higher speed before the turbo kicks in.
- Reduces performance boost provided by turbo. Even after the turbo kicks in, it won’t be able to provide as much horsepower anymore. The flow of exhaust gas will always be limited and so will the turbo.
Back Pressure Vs. Exhaust Scavenging Vs. Exhaust Velocity
If back pressure is so bad, why do some people think it’s good? I can’t be 100% sure but I think some people are confusing back pressure with exhaust scavenging or exhaust velocity. Let’s cover all of them so you won’t be confused as well 🙂
You should already know back pressure at this point. If you don’t, scroll up and read the recap section on back pressure!
Remember I said exhaust gases will want to flow into lower air pressure areas? Exhaust scavenging is an effect where exhaust flow themselves create lower air pressure areas around them.
What I did not tell you was that instead of a single flow, exhaust gases flow out of the combustion chamber in pulses. Combustion chamber has many cylinders that connect to the exhaust system.
They open and close for every combustion cycle. When a cylinder opens, exhaust gas will flow out in a pulse into the exhaust system.
Each exhaust pulse travels forward at a high speed. And as they travel, they push all the air particles on their way forward with them. This creates a trail of lower air pressure areas behind them. This is known as exhaust scavenging.
Which means as each exhaust pulse travels, they create lower air pressure areas behind them that will pull more exhaust gases out of the combustion chamber. Exhaust scavenging is good because it reduces back pressure.
I illustrate this in the labelled diagram below. Click on each labelled number to see the whole flow.
Exhaust velocity defines how quickly your exhaust gases are flowing. Higher exhaust velocity is always good. This means the exhaust gases are leaving the combustion chamber quickly.
Exhaust velocity can be increased with narrow piping. However, this must be done with a balance – because narrow piping means higher backpressure.
Aftermarket exhaust is usually wider to reduce back pressure but not too wide that it reduces exhaust velocity.
Common Cause Of Back Pressure
Many stock parts cause back pressure. This is because they are not designed to eliminate back pressure in mind. Let’s look at the list of common causes of back pressure.
1. Exhaust Manifold With Non-Uniformed Lengths
Exhaust manifold is the entrance to the exhaust system. Exhaust gases leave the combustion chamber and travel through the exhaust manifold before they reach the exhaust system.
Exhaust manifold collects exhaust gases from different openings (cylinders) in the combustion chamber and combines them into a single flow.
Back pressure is introduced if the exhaust manifold has non-uniformed piping lengths. You see, exhaust gases exit the combustion chamber in turns – only one cylinder will open at a time. Typical cars may have 4 or 6 cylinders.
If the exhaust manifold has uniform lengths, then each exhaust pulses from each cylinder will take the same time to travel. This means they will not collide with one another as they enter the exhaust system. This is good.
If the exhaust has non-uniform lengths, then each exhaust pulses will have different travel time. Some travel faster and some travel slower. They are more likely to collide as they enter the exhaust system – causing the undesired back pressure.
Refer to this image below. Notice the stock exhaust manifold has non-uniform lengths. Whereas the aftermarket one (designed for performance) has uniform length.
I cover more about exhaust header in this article Do headers make your car sound better?
2. Restrictive CAT
Catalytic Converter (CAT) is a device equipped in the exhaust system to convert toxic exhaust gases into non-toxic ones.
Stock CATs are usually very restrictive because they have very fine honeycomb openings (See diagram below). Although this works great for emission, it’s the biggest contributor for back pressure.
3. Restrictive Muffler
Muffler is a sound suppressing device found in exhaust systems. They work to reduce the noise from combustion because who else would enjoy loud cars besides car enthusiasts?
Stock muffler reduces sound by causing sound waves to bounce into each other and cancel out. For this to work, stock mufflers must have lots of walls and chambers – another great contributor to back pressure.
Refer to the diagram below to see how muffler cancels out sound waves. And then notice how much metal walls and chambers they need to do that (exhaust gases must flow through all these).
How To Reduce Back Pressure?
Here’s a quick list of mods you can look into if you want to reduce back pressure.
1. Aftermarket Up-Pipe / Down-Pipe
Up-pipe and down-pipe are the entrance and exit for a turbo. Exhaust gases leave the combustion chamber and enter the turbo through an up pipe and then leave the turbo through the down-pipe
Stock up-pipe and down-pipe are restrictive because they are quite narrow and contain a highly restrictive catalytic converter. Switch them out if you want more horsepower from your turbo. I expect an aftermarket downpipe to provide 50 additional horsepower if tuned.
I cover them in more detail here Up-pipe Vs Down-pipe (And Why Modify Them). Do note that only some cars have up-pipe. If your car doesn’t, then there’s nothing to modify!
2. Aftermarket Exhaust
Aftermarket exhausts can reduce back pressure, increase horsepower and provide great sound improvement.
An exhaust system contains a lot of parts like muffler, piping, catalytic converter and exhaust tip. All of them can be switched out or kept stock – depending on what you want. If you are looking for an improved performance with great sound, then I recommend you look into aftermarket cat-back exhausts – specifically, the Flowmaster exhausts.
I cover why Flowmaster exhausts is my go to in this article.
3. Aftermarket Muffler
Aftermarket muffler that’s designed with performance in mind will reduce back pressure and provide great sounds. The beauty about an aftermarket muffler is that you can pick exactly which one sounds the best and then go for it.
Aftermarket mufflers come with lesser chambers and walls to reduce back pressure and allow exhaust gases to leave quickly.
Personally, I prefer a muffler that sounds aggressive but not so loud to the point it’s deafening. If you feel the same, then look into Flowmaster Super 40. Here’s an article I wrote about it, if you are interested.
4. Muffler Delete
Muffler delete is where you take off the muffler on your car. Muffler delete will definitely reduce back pressure because exhaust gases can flow freely.
I don’t really recommend muffler delete because it makes your car extremely loud – especially during cold starts. Not only will it be deafening, it is most likely illegal if you are driving in strict states like California.
5. Aftermarket CAT
Aftermarket CAT has bigger openings when compared to stock CAT. This allows exhaust gases to enter and leave the CAT much more easily.
Though, I wouldn’t recommend touching the CAT because you risk failing the emissions test. Aftermarket CAT is great to relieve back pressure but they do a poor job in converting toxic exhaust gases into non-toxic gases.
Not to mention you also risk having the check engine light (CEL) always on. Look at the diagram below comparing stock CAT and aftermarket CAT. Notice how the openings are different.