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Old 09-15-2009, 09:30 PM   #1
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Exhaust Back Pressure

As a non-mechanic I have been trying to wrap my mind around the phenomenon of back pressure vs. torque for some time now. Today I got online and googled "why is back pressure good?". The article at the top of the list went very in-depth about how this works but the really interesting part was the last paragraph which spoke about modern BMWs not needing back pressure to operate properly. I don't want to start a civil war among the E46 know-it-alls but I think many people here, including myself, would benefit from the discussion about to ensue. Here is a link to the article but I will copy and paste the article below in case the link ever dies. Enjoy!

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.


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Last edited by RangerDave; 09-15-2009 at 10:19 PM.
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Old 09-15-2009, 11:37 PM   #2
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great... this debate again...
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Old 09-15-2009, 11:46 PM   #3
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That article from UUC is correct. Backpressure does not equal more torque. Clean air and fresh fuel with the correct mixture = torque.

Exhaust fumes and incorrect fuel mixture = loss of turque.
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Old 09-15-2009, 11:50 PM   #4
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This feels like a repost.. we've covered this too many times and its off a major sponsors website
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Old 09-16-2009, 02:39 AM   #5
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it's not back pressure, rather perfect volumetric efficiency... if the pipe is too big, the engine has to push to fill it, if it's too small it has to push to fill it. what you want is for it to have zero back pressure, yet if it was any smaller it would have back pressure so the exhaust will just naturally flow out and promote better scavaging.

Just like valves, the engine breathes better at particular RPM, so it all depends on how you tune it... btw, vtec engines have 4 valves, but only use 2 of them at low rpm based on the principal that the engine does have to fill the exhaust ports, and uses all 4 at high rpm so it doesn't have to push it out.

I've also heard that Ferrari uses different diameter dual exhaust with a valve to use one or the other depending on the RPM and load
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