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Some say that "an engine needs backpressure to work correctly." Is this true?

No. It would be more correct to say, "a perfectly stock engine that cannot adjust its fuel delivery needs backpressure to work correctly." This idea is a myth. As with all myths, however, there is a hint of fact with this one. Particularly, some people equate backpressure with torque, and others fear that too little backpressure will lead to valve burning.

The first reason why people say "backpressure is good" is because they believe that increased backpressure by itself will increase torque, particularly with a stock exhaust manifold. Granted, some stock manifolds act somewhat like performance headers at low RPM, but these manifolds will exhibit poor performance at higher RPM. This, however does not automatically lead to the conclusion that backpressure produces more torque. The increase in torque is not due to backpressure, but to the effects of changes in fuel/air mixture, which will be described in more detail below.

The other reason why people say "backpressure is good" is because they hear that cars (or motorcycles) that have had performance exhaust work done to them would then go on to burn exhaust valves. Now, it is true that such valve burning has occurred as a result of the exhaust mods, but it isn't due merely to a lack of backpressure.

The internal combustion engine is a complex, dynamic collection of different systems working together to convert the stored power in gasoline into mechanical energy to push a car down the road. Any time one of these systems are modified, that mod will also indirectly affect the other systems, as well.

Now, valve burning occurs as a result of a very lean-burning engine. In order to achieve a theoretical optimal combustion, an engine needs 14.7 parts of oxygen by mass to 1 part of petrol (again, by mass). This is referred to as a stochiometric (chemically correct) mixture, and is commonly referred to as a 14.7:1 mix. If an engine burns with less oxygen present (13:1, 12:1, etc...), it is said to run rich. Conversely, if the engine runs with more oxygen present (16:1, 17:1, etc...), it is said to run lean. Today's engines are designed to run at 14.7:1 for normally cruising, with rich mixtures on acceleration or warm-up, and lean mixtures while decelerating.

Getting back to the discussion, the reason that exhaust valves burn is because the engine is burning lean. Normal engines will tolerate lean burning for a little bit, but not for sustained periods of time. The reason why the engine is burning lean to begin with is that the reduction in backpressure is causing more air to be drawn into the combustion chamber than before. Earlier cars (and motorcycles) with carburettion often could not adjust because of the way that backpressure caused air to flow backwards through the carburettor after the air already got loaded down with fuel, and caused the air to receive a second load of fuel. While a bad design, it was nonetheless used in a lot of vehicles. Once these vehicles received performance mods that reduced backpressure, they no longer had that double-loading effect, and then tended to burn valves because of the resulting over-lean condition. This, incidentally, also provides a basis for the "torque increase" seen if backpressure is maintained. As the fuel/air mixture becomes leaner, the resultant combustion will produce progressively less and less of the force needed to produce torque.

Modern Civics don't have to worry about the effects described above, because the ECU (car's computer) that controls the engine will detect that the engine is burning leaner than before, and will adjust fuel injection to compensate. So, in effect, reducing backpressure really does two good things: The engine can use work otherwise spent pushing exhaust gas out the tailpipe to propel the car forward, and the engine breathes better. Of course, the ECU's ability to adjust fuel injection is limited by the physical parameters of the injection system (such as injector maximum flow rate and fuel system pressure), but with exhaust backpressure reduction, these limits won't be reached.

Source: UUC Motorwerks
 

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Too little backpressure on an NA will cause low speed power losses

Scavenging is affected by exhaust backpressure. When any given cylinder is at TDC the inlet and exhaust valves are open at the same time and as the exhaust gas flows out of the exhaust valve it creates a suction effect (scavenging) drawing air in from the inlet. Too litle scavenging hurts low speed power but too much scavenging sucks the air/fuel mixture out the exhaust before the valve closes.

Low Backpressure has an ill effect as those will know who have driven cars with no exhaust
 

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COPY and PASTED
i never got into this debate for the simple reason 4 stroke engines dont need back pressure .
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So far I’ve seen A LOT of exhaust questions and A LOT of misconceptions, so im typing this up to clear those up and hopefully some people will learn the truth about exhaust systems back pressure and velocity, I will try to keep this as brief as possible,

But first let me say I don’t care what your friend said or what the exhaust shop says or what the v8 drag racer said to you, trust me on this one:

First I will address back pressure:
its simple, back pressure is BAD always BAD! Forget the idea of some engines need back pressure it’s a LIE!! Back pressure means the engine has the waste its HP to push the exhaust out of the system (also known as pumping loss) so you will ALWAYS lose power with back pressure!

Now I will address velocity:
exhaust velocity is what’s important, increase in exhaust velocity is what will free up extra power. now if your wondering how then read on:

Why does velocity free up more power: exhaust velocity is when the exhaust flow is so smooth that each exhaust pulse leaves a vacuum behind to suck out the next pulse so instead of the engine pushing the exhaust out the exhaust itself is sucking out exhaust hence taking the load off the engine and reducing pumping losses. This is where exhaust piping is important, different size piping will give you ideal exhaust velocity at different RPM points. You can’t have perfect velocity throughout your entire rpm range. a smaller piping will give you better velocity down low but too small and your sacrificing top end power because your restricting flow, a large exhaust will give you that ideal velocity higher up in the power band but you may loose some power down low.

Why do you loose power down low with large exhaust: when I say large I mean like having a 3" piping on a stock low revving na 1.6 sohc, what this does is it takes longer for the exhaust to exit the tail pipe because it takes longer to fill up the system, so the engine has to constantly push the exhaust through the system until it gets to the tail pipe. Basically there are more pumping losses. Basically the exhaust has more space to fill before it exits the tailpipe and the engine has to work harder to do that.

So now you know BACK PRESSURE IS BAD! And VELOCITY IS GOOD. but your wondering why so many people think back pressure is good, ill explain that too:

Why people think back pressure is good: in the 1960's racers would put larger pipes on there cars but there cars felt slower sometimes because they where shifting there ideal velocity to a higher rpm, V8's most redlined at 4500-5000rpm in those days so they shifted the point past there engine operating range, then they put much smaller pipes on which gave them a great deal of velocity down low so they had a jump in hp and tq say from 1500-3000 rpm and they thought it was because of back pressure, but it was actually velocity that had increased. Basically it was all a misunderstanding.

How do I know what size to choose to have ideal velocity:
well it’s really complicated it depends on a lot of stuff like your driving style and mods you have etc... So I don’t feel like getting into it, plus everybody has there own preference so I can’t decide what is good for YOU, only you can.
 

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Excellent posts,

Just to note, i rang my local powerflow dealer last week to ask about having a 2.5" inch exhaust fitted to my civic (1.8 vtec) and he refused saying it would kill the engine as it wouldnt have enough back pressure. I tried to explain what you have written above and the guy wouldnt have it he was adament that because his 400hp cosworth only needed 3" that a 1.8 n/a would suffer from a 2.5" exhaust.

Im guessing its differnt with turbos as the air is being forced in there so it has to come out anyway and pumpming losses arent as much of a problem as it is with n/a cars?

So Is 2.5" to much for a 1.8 i thought not and going on what you guys are saying it shouldnt be but i couldnt explain it very well. Obv if it looses some low end power then thats fair enough but end of the day VTEC isnt low down and when you want to go fast then your not driving at low revs anyway.
 

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ignore the tosser. you are the customer. the customer is always right

tell him how far the car revs to and the fact it already has a 2 1/4" pipe as standard.
 

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2.5" is fine thats what i have on my 1.6 B16A2.... to be honest you dont even notice the "loss" in the low end of the rev counter.... to be very honest the car feels like it breaths alot better threw-out the hole rev rang but then agen i do have a induction kit aswell
 

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i was just reading this post and i wanted to no what effect a 4-2-1 has against a 4-1 exhaust would have. I was on the understanding from what i had learnt that a properly designed exhaust would casue pulses and suck the exhaust gasses out like what paul20v said. And that its velocity that counts. But a 4-2-1 exhaust system has its branches paired. These pairs are 1+4 2+3 so as the normal firing order 1342 doesnt affect the cylinder firing next ie send exhaust gas into the fresh charge of the next firing cylinder. The joints and lengths of 1+4 2+3 and the correct length for each exhaust stroke and its this timing which causes a suction which aids the flow of the exhaust. but with a 4-1 exhaust this timing affect cant be had. can it?

So my question is why am i being advised by everyone who has a honda to buy a 4-1 header as there les problematic. Can some explain as i dont see the benifits.
 

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EG6mad said:
i was just reading this post and i wanted to no what effect a 4-2-1 has against a 4-1 exhaust would have. I was on the understanding from what i had learnt that a properly designed exhaust would casue pulses and suck the exhaust gasses out like what paul20v said. And that its velocity that counts. But a 4-2-1 exhaust system has its branches paired. These pairs are 1+4 2+3 so as the normal firing order 1342 doesnt affect the cylinder firing next ie send exhaust gas into the fresh charge of the next firing cylinder. The joints and lengths of 1+4 2+3 and the correct length for each exhaust stroke and its this timing which causes a suction which aids the flow of the exhaust. but with a 4-1 exhaust this timing affect cant be had. can it?

So my question is why am i being advised by everyone who has a honda to buy a 4-1 header as there les problematic. Can some explain as i dont see the benifits.
the only thing i can think is that they maybe on that because there is less joins and welds in a 4-1 header so less chances of cracking ??
just taking a guess thou.
 

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R3DD3V1L said:
some sort of pressure does matter, if u put a stupidly massive exuast on ur car it will slow it down
Yeah, it's because you're stuck on a speedbump lol
 

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R3DD3V1L said:
SaintGrimm said:
R3DD3V1L said:
some sort of pressure does matter, if u put a stupidly massive exuast on ur car it will slow it down
Yeah, it's because you're stuck on a speedbump lol
haha lol lol lol nah u no what i mean
lol, aye, that's what he's saying; the engine has to push the air out rather than have it create a vaccum like a performance exhaust should (I think or I've read it all wrong)
 

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R3DD3V1L said:
some sort of pressure does matter, if u put a stupidly massive exuast on ur car it will slow it down
Have you not read any of this :rolleyes:

Just a little example of a car with no exhaust system doing quite well in the power department ,
its all about what you want from a car also .

[youtube:1z78armg]http://www.youtube.com/watch?v=lm5RkeuKC1Q[/youtube:1z78armg]
 

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In my book any restriction in an exhaust system is bad, I run 3" stainless straight through (no cat) (with turbo) I have made up several different baffles for the 4.5" tail pipe to try and quieten the noise a tad, if I so much as reduce it by an inch the difference is noticable.... I'd run headers if I could, as for the the 4 - 1, 4 - 2 - 1 manifold debated its not even worth thinking about the probable power gains / probable torque gains between the two and lots of people seem to run them with standard exhausts with only the back box upgraded and / or cats lol
 

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paul20v said:
Just a little example of a car with no exhaust system doing quite well in the power department .

Yeah, but how much more power could this car develop if it were fitted with an exhaust system to create some backpressure? You have to wonder if the guys that built it aren't missing a trick here?
 

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Alan-VTIS said:
Excellent posts,

Just to note, i rang my local powerflow dealer last week to ask about having a 2.5" inch exhaust fitted to my civic (1.8 vtec) and he refused saying it would kill the engine as it wouldnt have enough back pressure. I tried to explain what you have written above and the guy wouldnt have it he was adament that because his 400hp cosworth only needed 3" that a 1.8 n/a would suffer from a 2.5" exhaust.

Im guessing its differnt with turbos as the air is being forced in there so it has to come out anyway and pumpming losses arent as much of a problem as it is with n/a cars?

So Is 2.5" to much for a 1.8 i thought not and going on what you guys are saying it shouldnt be but i couldnt explain it very well. Obv if it looses some low end power then thats fair enough but end of the day VTEC isnt low down and when you want to go fast then your not driving at low revs anyway.

Ignore him.... Turbo cars have the ability to have say upto 5" exhaust out the front wing. as the turbo maintains the back pressure...... forget that. you have a N/A so 2.5 is perfect i have a 2.25 on my 1.5vtec and it opened up the power band alot. from 3600-7000 it pulls and pulls... only thing i have is that a fairly large accent on the motorway in 5th at 2500rpm it struggles to maintain 70/80 mph in ECO... but you have a 1.8 so youll be fine mate.
 

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+ whilst am here. Back Pressure is like Aero Dynamics a very fragile system, if you think back pressure as like and Air brake on your roof. your ok at slow speeds but at high speeds you lack that extra bit...... back pressure in an exhaust is basically turbulence within the system it collects in pockets upsetting gas flow more and more gasses are need to remove it but that produces more and more pockets its a never ending fight. IF you get the correct size exhaust say 2.5 as that's a common size, you are effectively removing those pockets off turbulence therefore making the gasses flow alot more smoothly as there is no turbulence (back pressure) to fight therefore more gasses come out. aswell a very technical way to also make you exhaust efficient is to RIFLE it. Rifling twist's the gasses alot like a bullet in a gun, creating its own vacuum which is very strong, it pulls its own gas out so the engine doesn't work to push the gasses out. making an extremely effective exhaust system !

Hope this helps you all a little better... tips from the rally team =]
 
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