Exhaust Back Pressure: The Facts

Ok, I’m going to delve deep into the known subject of exhaust back pressure, when, where, and sometimes why it’s needed. It is going to be a long article, but I assure you in the end you will have a better understanding of it, when to retain it, and when not to.
As described in the title, this is the FACTS!

“Ok, deep breath”… here we go,

The common belief is that by reducing any amount of back pressure in the exhaust system you will automatically gain horsepower. Unfortunately this is not the case. With proper fuel management or re-jetting a carbureted engine however the maximum horsepower can be improved.
Without proper tuning the more free flowing aftermarket exhaust and or modified will generally cause your engine to run lean and promptly lose 5-20% of the rear wheel/s horsepower depending on how drastic of a change is made.

There is a difference between usable power and maximum power. The maximum power of two engines may be similar, but the power torque curves may be different. The area under the power and torque curves defines the "Available power" the engine produces. The more area or sharp rise that is under the curve (the curve graph commonly seen on dyno results), the more power your engine produces.

A typical non-baffled "aftermarket" exhaust (tuned for mid to high power ranges) produces a power curve that initially rises very slowly. As the RPMs start to rise above the mid-range power, the curve begins to rise at a more rapidly increasing rate until maximum horsepower is achieved. Once RPMs have passed maximum horsepower, the curve drops off extremely rapid and power will begin to fade.
So what is exhaust backpressure? Basically, the whole exhaust system is one giant flow restrictor, with things like curving bends, collectors, baffles and a muffler impeding the airflow, and this causes the upstream pressure to increase.

These pressures will be higher at the valve, and fade in pressure the further out the pipe gets from the valve. This pressure eventually acts on the back of the exhaust valve, and this is bad. Why? Air flows best from hot to cold, and high pressure to low pressure, and the willingness of the air to flow is roughly proportional to the pressure differences in the overall system.
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" (also known as VE I have covered this vaguely in other posts) and minimum combustion gasses remaining in the cylinder. This means you want the maximum pressure differential, and hence the least backpressure possible in most cases.
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 mixture).
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 therefore make less power.
Two, we leave some products of the previous combustion event gasses in the cylinder, which causes problems with our subsequent next cycle of (fresh intake mixture) combustion. In an exhaust system, we often route multiple down tubes (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" of a (low density) pressure wave is formed, which travels upstream back to the exhaust valves,(similar to a sound wave)decreasing pressure at the valves.
Depending on the length of the runners (down tubes to collector or muffler whichever is first), this will cause peak performance to shift to different RPM ranges and drastically affect the power/torque curve. You can also use the cylinder pulses from the other cylinders in unison to help each other out and improve your scavenging performance. This is where you run into "Patented" exhaust designs, where the exhaust is constructed in a way that lets the firing order fall into a symbiotic chain and contributing to the scavenging speed and performance, known as a “Tuned Exhaust”. This allows consistent maintained exhaust valve wave length pressures, with almost no change in pressure across all cylinders.
All OEM exhaust systems are essentially a “tuned” system.
Backpressure can be seen as helping?

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 engine, and then go and reduce the downstream backpressure, we change the expansion properties and shift where the "volumetric efficiency" peaks are on that engine altering the power/torque curve at tuned RPM ranges.
Without re-tuning, this will cause a performance drop across the board (all rpm ranges), Velocities often come into play too. Often people who have fitted a massive diameter muffler or larger diameter down tubes 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. (Larger than OEM will slow velocities, where as smaller than OEM will increase velocities. Slower exhaust velocities = equals slower intake speeds, resulting in less air/fuel mixture and loss of power.

Pressure wave scavenging is far less important on turbo engines, as the turbo boost is a far more significant of a contribution to total performance and intake velocities, so you typically optimize the exhaust output for maximum spool times (speed at which the turbo reaches maximum output RPM). You still want the scavenging to be reasonable al low rpm’s, and you can do this by dropping the pressure of the whole exhaust system. Most professionally tuned race engines that run turbo’s, will have a significantly low exhaust velocity at rpm ranges below the desired boost level.
So what’s the final understanding?

1. Turbo or modified race engines, drop that post exhaust pressure as low as you can!
2. Factory/OEM engines will require some retained back pressure to maintain the power/torque curve, as
nothing else in the engine has changed, but still try to achieve as low as possible.

Hope this help everyone to understand when it is, and is not needed.
I can post an article on "VE" later on if enough people want/request the info.

Thanks for reading,
Mechanic1978