Some information regarding Back pressure

Ofc. Dangle

Ofc. Dangle

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Taken from http://www.uucmotorwerks.com/html_product/sue462/backpressuretorquemyth.htm
Destroying a myth.


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. Anytime 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 gasoline (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 carburetion often could not adjust because of the way that backpressure caused air to flow backwards through the carburetor 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 BMWs don't have to worry about the effects described above, because the DME (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 DME'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.

- Adapted from Thomas V.
Click to expand...

Pretty cool info.
 
wow i cant believe i just read that whole thing. i want my 5 minutes back. thats a total load of shit. there is a lot more then they talk about and half of what they said is either not true, partially not true, or doesnt apply to us.

just erase this thread and look up some "real" exhaust gas theory. look for work written by actual SAE engineers. also look up some work written by Allan Lockheed Jr.(son of the guy that started the Lockheed airplane company)
 
so ...since you read the whole thing... can you light up the issue... do we need back presure if we have straight pipe ? lol... i too lazy to read.
 
ROFL these guys are retards. Backpressure is NOT good. You do NOT want backpressure. What you DO want is scavenging. You want the exhaust to travel as fast as possible down the pipe to make room for more exhaust. Turbulence and other changes in pressure or velocity slows down the exhaust, increasing backpressure and decreasing exhaust.

Tons of research goes into making sure the exhaust can do this as quickly as possible. When the exhaust valve opens, there is sort of a vacuum effect that helps to pull the gases out of the cylinder and into the header.

Now, when you make a bigass 3 inch pipe for your NA engine, sure, you have practically zero backpressure. BUT, now you have a huge space that isn't being utilized completely, and therefore you won't have the exhaust moving down the pipe, but rather exhaust that tends to linger or move slowly down the pipe.

Supposedly, the best exhaust is one that starts narrow at the beginning and slowly widens toward the end. High pressure moves toward areas of low pressure, and an exhaust setup like this is ideal. Subaru exhausts are shaped in this way, and is the one of the reasons they sound the way you do.
 
These are probably the same guys that would use a SAFC to tune a vehicle that cleary needs a aftermarket computer/standalone system to run their setup.. I don't think their info goes into much detail other than "Backpressure is a good thing for you to have so you don't burn valves" WTF is that all about? :laugh::laugh:
 
My personal opinion I don't think that's entirely correct and coconut you are not entirely correct either... Not necessarily a bigger cam but the reason for a tuned exhaust is fr scavenging, which, describing it simply, is when the exhaust pulses and the runners hot the collector, there is a wave of backpressure that turns and runs back into the head. The exhaust valve needs to be closed to not allow this pressure back into the combustion chamber, and because of overlap can be brought back into the open intake manifold, very bad. A tuned exhaust and matching cam, or cam and matching exhaust, actually has equal length exhaust runners set a certain distance to the collector so when the exhaust pulse returns, the valve is closed... Long story short, it was just explained to me not too long ago and proven on the dyno, pretty cool shit
 
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