Back Pressure on engine.

"BACK-PRESSURE" .. A highly misunderstood concept

What is Back Pressure?

Every exhaust system is designed to evacuate gases from the combustion chamber quickly and efficiently at a given RPM . Exhaust gases are not produced in a smooth stream; exhaust gases originate in pulses. The more pulses that are produced, the more continuous the exhaust flow. And this process of removal of exhaust gases from the engine is also called scavenging.

A fluid flowing through any system would always have resistance for the flow which can be offered from various sources and because of various reason. In case of 4 Stroke IC (internal combustion) engines, the exhaust gases flowing out of the exhaust valve and then to the exhaust pipes get this resistance for the flow from the inner pipe walls, from the exhaust pulses, due to temperature difference etc. So basically the back pressure is nothing but in this case, the resistance to positive flow of the exhaust stream and no system is designed to have the exhaust gases remain in the system or have them flow slowly out of the system.

If exhaust gases remains in the system that only translate to the dilution of the fresh charge that has come in to the engine in the intake stroke which in turn leads to bad combustion and then bad power. Not only that too much or even a little bit of pressure means our piston has to work against it to push the exhaust gases out of the cylinder which means some of the power is getting lost in working against it (visualize it this way: exhaust gases pushing the piston down while engine is trying to push them out) OR you can see it this way: since now the engine is working against the exhaust gases to push them out, some of the power at crank has been shared which results in loss of power at the crank and then ultimately at rear wheel.

So ideally any amount of back-pressure is not good but since IC engine is a complex set of inter-dependent components and processes hence it is an unavoidable evil which in perfect world would never be desired of.

Now the exhaust gases do not move at random, they move in pulses and when a pulse moves ahead it creates a vacuum before it and then that vacuum draws the pulse behind it with more velocity. (Exactly the way we are drawn towards the vacuum created behind by a fast moving train. You guys must have read in fluid mechanics in physics, a high velocity fluid would create a less pressure area or vacuum and a slow moving fluid would create a high pressure are. One more example is a fast moving car would always draw more dust behind it than a slow moving car because of this phenomenon).

Having understood the above concept now understand this.

The exhaust gases has to be spelled out from the system with the highest possible velocity so as to create less back pressure. Now at low RPM range of the engine, the pulses are not continuous rather a little in parts, so every pulse would create a vacuum behind it causing the pulse behind it move with greater velocity (as the vacuum would pull the pulse behind into it) hence quicker evacuation of gases. But as the RPM increases the engine is producing more and more exhaust gases, hence the exhaust pulses are no more distinct now rather continuous and the volume is more hence the scope of vacuum has apparently become zero. So a narrower Silencer would always result in faster exhaust velocity at low RPM range (as we have exhaust pulses) but the same would cause lot of back pressure at higher RPM range. So you see how the dynamics changes at different RPM range, so it is always good to have a little narrower exhaust system if your bikes power-band is located at low RPM range 2000-4000 RPM and a wider exhaust system if your bikes power-band is located at high RPM range 7000-9000 RPM.

But if exhaust gas velocity is all we want to be high at low RPM range then wouldn't a soft drink drinking straw's diameter exhaust system gives the highest velocity? The answer is, too narrow exhaust system means lot of back pressure because too much resistance to the flow of gases. It would have been true, have been the case if exhaust gas volume is very very less. So there should be optimization between exhaust system diameter and velocity.

Similarly too large exhaust diameter at high RPM range means a very less exhaust velocity and also quicker cooling of exhaust gases and colder the exhaust gases, slower they move out of the system, creating lot of back pressure.

Idle scenario: The ideal scenario would be that by some magic our exhaust system adjust it's diameter automatically. At lower RPM it remains, less and as soon as the RPM increase it expands. The most advanced is Ferrari's which consists of two exhaust paths after the header - at low RPM only one path is open to maintain exhaust velocity, but as RPM climbs and exhaust volume increases, the second path is opened to curb back-pressure - since there is greater exhaust volume there is no loss in flow velocity.

Not only that, the velocity with which exhaust gases moves out also depends upon various other factors like, engine CC (which would decide volume of gases), bent pipe and silencer length/diameter and material, temperature drops, RPMs and few other factors.

Therefore, exhaust design is a complex compromise involving engine displacement, number of cylinders, gas temperature, rpm, and a host of other considerations. Each design is optimum for one set of conditions. When you remove or drill the baffles, allow a leak to exist, or cause any other change in the design or operating conditions and you will affect performance.

One example: Someone here in sans page said that after using titanium wrap he feels the bull has got more power and mileage also became better. And few guys just rubbished him, few astonished and few perplexed. I was perplexed. Later after reading many articles, I understood he wasn't wrong completely at least for this set of IC engine (RE 350 Cc). By using the titanium wrap what he assured is that, the heat is not dissipated easily in the environment, in turn it makes sure that the exhaust gases remain hot, which in turn means faster exhaust velocity and hence a better scavenging. (Ever wondered, the technology has become so advanced then why not provide more cooling to exhaust? for the above explained reasons)

Now below are commonly asked questions:

They why we say less back pressure means bad low end torque?

Now this one is the favorite among those who love to talk about back pressure but unfortunately has misunderstood it but is this completely a wrong statement? NO. Then? True statement with carburetor engines. A less back pressure means a good evacuation of exhaust gases that, in turn would result in a higher air flow, and if you did not adjust the fuel delivery (that is why many ask, do I need to upjet or retune the carb after installing this Silencer? ) it would result in a higher (leaner) air-to-fuel ratio and a leaner air-to-fuel ratio would always produce less power than ideal mixture which is 14.7 parts of oxygen by mass to 1 part of gasoline (again, by mass). Hence the loss in low end torque.

Next question is a free-flow silencer(which means less back pressure) would burn the engine valves?

For the explanation given above, the leaner the mixture because of very less back pressure (and not adjusting the air-fuel mixture) the hotter the gases after combustion and hence chances of valves getting burnt is more.I hope I could clear some doubt of you guys related to back pressure and the highly misunderstood notions and concepts about it.

Cheers!!

Written by Gyanendra Singh