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

Auto Decompressor and Hydraulic Tappets

hello guys another write up regarding hydraulic tappets and auto decompression
please note this is not specifically for the UCE engine but a general auto decompression and hydraulic cam lifter theory
as we all know that the bullet is a pushrod type valve train system and in the older CI models the valves were operated via rockers arms that were pushed down by push rods that were further operated by timing wheels fixed with cam lobes, this system required frequent setting of the pushrod and rocker arm clearance and would cause the tik tik sound and, this also had a lot of metal to metal contact and had more wear within the parts itself , mechanics would refer to it as tappet setting what he was actually doing is removing free play between the push rod and rocker arm thats all
the UCE also sports the same arrangement on here because of the hydraulic tappets this free play never has to be set, the difference here is the push rods have a hydraulic lifer, its small cylinder which within itself comprises of a spring, a valve, oil seals and two ports (design may vary from company to company) basic funda is when the valve is closed this hydraulic lifer has its oil ports open and there is no pressure in it, as the cam lobe pushes this lifter oil fills up and closes the outlet port, since a liquid cannot be compressed the hydraulic lifer moves upward pushing the pushrod that pushes the rocker arm that opens the valve, simple enough? there is much more deeper insight to this but this is the basic working, since the hydraulic lifers are always filled with oil when being pushed there is never any clearance or free play between the pushrod and rocker arm,thats why this is a maintenance free system, however these hydraulic lifters do have a life (depends on company schedule) when the spring inside the lifter gets slack or if the lifter itself develops a leak then this needs to be changed
the auto decompression is fitted on the exhaust cam of the bike, it is fitted with a small pin and connected to a small flywheel like mechanism at one end with a small spring, the flywheel moves out due to centrifugal force only at a low RPM, since the flywheel move upward it moves the pin which in turn pushes the exhaust valve slightly so as to have a slight leak within the cylinder to ease starting, this mostly cannot be set as it is preset from the manufacturer, a bad decompression system can cause hard starting (with kick) and a noisy engine as the pin is running loose in there,now i do not know how this arrangement is in the UCE so i cannot pin point everything
the above explanation is a general theory and not specifically for the enfield,i have never seen the inside of a UCE so it's hard to comment as of now, my friend has offered to lend me his UCE to do all my experiments in exchange for a full tank of fuel, a polished bike and mainly a fully working and functional bike 

Posted by Yorik

All about Silencers and Exhausts - For those who want to know how their Royal Enfield bikes will be affected if they change their silencers

hi guys since exhausts systems are such a hit on our forum and everybody seems to have their own take on the matter I decided to write abt it
everybody is familiar with the four strokes of an engine (is hope) our beloved bullet is a four stroke engine, during the last stroke of the piston the burnt fumes or gases have to escape the cylinder barrel fast to clear space for the next batch of fresh air fuel mix to enter the cylinder namely the suction stroke, on a conventional type exhaust system (O.E) muffler the exhaust gases do not escape quickly as there is a certain amount of back pressure caused in the cylinder due to the baffles in the muffler and inlet manifold, also one must also realise that exhaust gases are hot and need to disperse, a free flow exhaust normally has no baffles, this in turn allows the gases to flow our without any restrictions and the cylinder is rid of all the exhaust fumes faster as compared to a stock exhaust........now is am sure you are wondering y I have been discouraging everyone using free flow gases and causing damage to your engine, well the answer is fitting a free flow exhaust causes the exhaust gases to go out freely this also has an effect to the intake of the engine, the vacuum at the inlet manifold will also increase thereby demanding more fuel into the cylinder via a carburettor or a fuel pump, one must understand the ration of petrol and air that goes into the cylinder has to be correct if it has an imbalance it can affect your engine, say for e.g. u fit a free flow exhaust with no baffles on ur bike, you will definitely get an good acceleration and u will feel the difference (apart from the noise) but the mix that is going into the cylinder was calculated with the back pressure caused by the O.E exhaust now that u have fitted the free flow the mix is to lean, this will get the exhaust valve to turn hot and I mean real hot this in turn can damage ur valve seat and eventually the valve ,to counter attack this problem u need to increase the air fuel mix going to the engine and u can only do this by re jetting the carb and using a cold air intake filter,i was avoiding a lengthy explanation on this topic but after I guess it was needed .only a qualified mechanic who has some experience working with rally spec vehicles will know this ,most of them won't even agree to my explanation.
this is for our bike now with more than one cylinders in play the ball game is a bit different, a stock exhaust manifold ( also known as a header manifold) is usually made of a consolidated type which means all four cylinders (in case of a four cylinder engine) combine into a single manifold that connects to a bend pipe that will either houses a turbo charger or if not turbo then a catalytic converter then a stock muffler and then a tail pipe
so HEADER-TURBO-CATCON-MUFFLER-TAILPIPE ( the cat con is always present irrespective of the turbo being there or not)
So what does the cat con do? the cat cons job is to reduce pollution by a neutralising the exhaust impurities, but the cat con also increases back pressure of around 1 to3 psi,this is not harmful but only reduces the efficiency of the engine where it could have performed better, then the muffler, the stock muffler is a series of baffles that let the exhaust travel within itself to reduce sound.the cat con by itself is another subject and i am not exaggerating on it here
for a multicylinder engine to perform u need to change the exhaust header, a performance header will be tuned to suit the vehicle it is built for, pipe diameter, bends etc are very crucial to the design, a sport header will have individual pipe outlets for each cylinder, so in a four cylinder vehicle u will have 4 pipes sticking out of the head, as per firing order 1-3-4-2,pipe no 1 and pipe no 3 will go a little distance and join so will pipe no 2 and 4 so u are left with 2 outlets coming from a 4 cylinder engine, this then goes to a free flow muffler also calculated for the specific vehicle or engine it is designed for, both these together complement a free flow exhaust, by just changing the muffler makes no sense, again only noise is achieved ,u can immediately tell if a car has a sport header or a normal header by listening to the exhaust, a sport header exhaust will have a very grumbling sound as u can hear each cylinder firing (music to my ears) and a stock header will only make ONE loud sound

so after reading this I hope u have some idea abt sport exhausts and free flows, there are many more topics related to exhausts,Turbochargers,EGR systems,heatshrink wrapping on exhaust pipe, if anybody wants to know anything more pls message me thanks

i am attaching pics of stock mufflers,free flow mufflers and both types of exhaust headers. 

Posted by my friend Yorik