How does exhaust backpressure work?
Lots of people say you need backpressure, but is that really true?
The subject of backpressure is one of the most misunderstood in the automotive world. Lots of people have different opinions and explanations, varying from "backpressure is always bad" to "you need X backpressure to let Y work effectively". So today let's break down what backpressure is and what you want to do with it.
Many aftermarket turbo and stock naturally aspirated cars run what is known as a log manifold. These aren't great for scavenging at a particular point in the RPM range, but often provide a broad spread of torque at the expense of total performance.
So what is exhaust backpressure? Basically, the whole exhaust system is one giant flow restriction, with things like mufflers and catalytic converters impeding the airflow, and this causes the upstream pressure to increase.
This pressure eventually acts on the back of the exhaust valve, and this is bad. Why? Air likes to flow from high pressure to low pressure, and the willingness of the air to flow is roughly proportional to the pressure differential.
When you are on the exhaust stroke you want to be pushing out the exhaust gases as effectively as possible to allow more volumetric efficiency and minimum combustion products remaining in the cylinder. This means you want the maximum pressure differential, and hence the least backpressure.
So when we have high backpressure, we have poor scavenging (the process of venting the exhaust from the cylinder and drawing in a fresh intake charge). This causes two problems, first our volumetric efficiency has dropped, so we have less intake air being drawn in, therefore we can't inject as much fuel, and make less power. Two, we leave some products of the previous combustion event in the cylinder, which causes problems with our subsequent combustion.
Naturally aspirated F1 cars have beautifully tuned length manifolds, designed for peak VE nice and high in the RPM range.
Lets talk about the myth that you need to have backpressure for an engine to run well, as this is simply untrue. Backpressure is naturally created at various points in your RPM range because of the nature of exhaust design, but it is never desirable.
When we have an exhaust system, we often route multiple runners (one from each cylinder) into a collector where they all meet. This collector is effectively a rapid expansion in pipe diameter from a single pipe. When a cylinder exhaust pulse hits the collector a rarefaction (low density) pressure wave is formed, which travels upstream back to the exhaust valves, decreasing pressure at the valve.
Depending on the length of the runners, this will cause peak performance to shift to different RPM ranges. You can also use the cylinder pulses from the other cylinders to help each other out and improve your scavenging performance. If that sounds a little complicated, it's explained in more depth in the video down the bottom.
High end drag cars vent their headers straight to atmosphere, which would produce a strong scavenging pulse. The headers are also very large diameter, however these cars flow a lot and have big per-cylinder capacity.
So why would backpressure be seen as helping? Well, with the tuned length pressure wave scavenging, it helps performance in some RPM ranges, and makes it worse in others. So, if we have a stock car (even with a log manifold), and then go and reduce the downstream backpressure, we change the expansion properties and shift where the volumetric efficiency peaks are on that engine.
Without re-tuning, this could cause a performance drop across the board, or with re-tuning or a self learning ECU the torque curve shape would change, which could result in more or less peak power. Velocities often come into play too. Often people who have fitted a massive diameter exhaust or header and see a drop in power have dropped their exhaust velocities significantly, and as a result have lost some of the strength of the inertial effects of the exhaust due to the slower gas speed.
My personal race car has an extremely short, low backpressure exhaust. Its turbo spools significantly earlier than the same spec engine with a full length exhaust.
We should also quickly talk about turbo cars. Turbos are powered by two things - heat and pressure differential. If you increase the backpressure on a turbo, you are decreasing the turbo's effectiveness, and the only way to compensate for this is to raise the exhaust valve exit pressure, which will make your scavenging worse.
Pressure wave scavenging is far less important on turbo cars, as the turbo boost is a far more significant contribution to total performance, so you typically optimise the header/runners for maximum spool. However, you still want the scavenging to be reasonable, and you can do this by dropping the pressure of the whole exhaust system. Moral of the story; turbo car, drop that post turbine pressure as low as you can.
For more details check out the video below!
Kyle Forster is a qualified Aerodynamicist, race car engineer, and the man behind JKF Aero, a firm that offers a variety of aerodynamic consultancy services for racing purposes. If you have any questions for Kyle or have any suggestions for future videos, drop them in the comments below!