Exhaust Back Pressure: Is It Good Or Bad?

Newton’s Third Law Of Physics Rings True Here

Newton stated ‘every action has an equal and opposite reaction’. For example, if you exercise force on something, it will put equal force back at you. Now we will understand this law in context to your car’s exhaust system. The gases and fumes produced as a result of combustion reactions in the engine chamber are required to be released in the atmosphere through the system’s tailpipe. To make this happen, the engine forms exhaust gas pressure so that the hydraulic resistance can be vanquished. The hydraulic resistance emerges in the exhaust system to show the exit gate to those exhaust gases.

The Exhaust Gas Flow Mechanics

Motorists might consider the back pressure bad which is true in some cases where this pressure in due course functions on the back of the exhaust valve. Why? Because air tends to stream from high pressure to low pressure, and this tendency of air is directly proportional to pressure gap. On the other hand, it might be required to let the system work effectively.

Exhaust gases are toxic in nature and from a safety point of view, and as a vehicle operator one would want to expel them as successfully as possible. The reason behind this is to permit the maximum gauged competence and minimum burnt gases staying back in the cylinder. This actually means the pressure gap is needed at its maximum, and the back pressure at its minimum.

Is The Back Pressure Good Or Bad?

Too High

Excessive than usual back pressure is a sign of poor scavenge (the process of eliminating combustion products from an internal combustion engine cylinder on the return stroke of the piston. This event is quite problematic for two reasons, one – the volumetric efficiency has declined which does not allow sufficient intake of air to power the fuel. More air implies more power, therefore, even after pouring much fuel the horsepower would not raise. Two – some products of the prior reactions are left in the cylinder, which induces issues with the successive combustion.

Too Low

Although low back pressure means high horsepower, however, we cannot deny the fact that too low back pressure is bad too. To keep it simple, back pressure is actually not good for power output but exhaust scavenging and velocity are. That’s what conventional wisdom says.


Before we jump to the underlying effects of exhaust back pressure, let’s learn the reason why the topic attracts the considerations of exhaust system designers. Well, diesel engines are installed with diesel particulate filters (DPF) and the after-treatment systems it requires are quite complex. This DPF fitment gives rise to the issue of elevated exhaust back pressure. Under ideal conditions, exhaust muffler and a clean DPF show the same level of pressure drop. This simply means that the major cause behind the pressure dip is not a filter substrate but the excess amount of smut collected by the DPF. To overcome this problem the DPF must be cleaned at regular intervals and the process is called regeneration.

Let’s see how the high exhaust pressure adversely impacts the diesel engine:

  • Turbocharger issues

  • Multiplied pumping work

  • Cylinder scavenging and combustion consequences

  • Diminished intake boost pressure manifold

  • Choked or stalled engine

Engine And The Back Pressure Limits

To conclude, engine manufacturers design every engine with the capacity to bear permissible amount of engine back pressure. For mufflers, the maximum back pressure limit should lie within the range of 6 kPa. In addition, large-sized engines involve valve overlap, and greatly boost pressure considerations. Engine designers keep numerous factors in view and think moderately when setting back pressure limits for the engines. For the engines that are less than 50 kW in size carry 40 kPa as maximum acceptable back pressure. Similarly, for 50-500 kW engines, 20kPa back pressure is allowed and the engines going beyond 500 kW in size can tolerate 10 kPa back pressure.

Call Us For Exhaust Repair Newport

The exhaust system and its function sound simple but actually, they are not. When it starts malfunctioning, your car’s fuel economy and the engine both would be at stake. Hence, your bad exhaust system needs expert handling. Trade Price Tyres strives to live up its customer’s expectations and are happy to assist you with any Exhaust Repair Newport concerns or other car services.

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